Hormones which enhance sodium excretion, i.e., natriuretic peptide hormones, are very important for the maintenance of extracellu/lar fluid volume within a narrow range, despite wide variations in dietary sodium intake. This regulation occurs through a complex interplay of the antinatriuretic renin–angiotensin–aldosterone system and the antinatriuretic renal sympathetic system, which help to conserve sodium when sodium intake is low, and the natriuretic hormones, which enhance sodium excretion whenever sodium excess occurs. Several of the cardiac natriuretic hormones (Figure 37.1) directly inhibit aldosterone secretion and/or indirectly inhibit aldosterone secretion by inhibiting renin release from the kidney to help regulate extracellular fluid volume. This chapter will concentrate on the natriuretic hormones (cardiac, intestinal, renal, and adrenal) in normal renal physiology, their synthesis, secretion, biologic effects, pathophysiological changes with hypertension and renal diseases, and potential for treating diseases such as acute renal failure.
Introduction
Cardiac, Renal, Intestinal, and Adrenal Hormones which Enhance Sodium Excretion
Hormones which enhance sodium excretion, i.e., natriuretic peptide hormones, are very important for the maintenance of extracellu/lar fluid volume within a narrow range, despite wide variations in dietary sodium intake. This regulation occurs through a complex interplay of the antinatriuretic renin–angiotensin–aldosterone system and the antinatriuretic renal sympathetic system, which help to conserve sodium when sodium intake is low, and the natriuretic hormones, which enhance sodium excretion whenever sodium excess occurs. Several of the cardiac natriuretic hormones ( Figure 37.1 ) directly inhibit aldosterone secretion and/or indirectly inhibit aldosterone secretion by inhibiting renin release from the kidney to help regulate extracellular fluid volume. This chapter will concentrate on the natriuretic hormones (cardiac, intestinal, renal, and adrenal) in normal renal physiology, their synthesis, secretion, biologic effects, pathophysiological changes with hypertension and renal diseases, and potential for treating diseases such as acute renal failure.
History of Atrial (Cardiac) Natriuretic Peptide Hormones
In 1628, Harvey first correctly described the heart as a pump or a muscular organ that contracts in rhythm, pushing blood first to the lungs for oxygenation and then through the peripheral vascular system, bringing oxygen and nutrients to every cell in the body. It was another 350 years before the heart was established as an endocrine gland with its main physiologic targets being the kidney and vasculature. The history of experimentation leading to defining the cardiac natriuretic peptide hormonal system (the first of the natriuretic hormonal systems) has followed two pathways: anatomical and physiological.
History of Cardiac Hormones: Anatomical Studies
Atrial Granule Structure
Shortly before Henry and colleagues reported their observation that balloon distention of atria caused a diuresis, in 1955 Kisch, utilizing electron microscopy, described dense granules that were located in the atria, but not in the ventricles of mammals. The presence of these dense granules in the cytoplasm of atrial cardiac myocytes, but not in the ventricles of the heart, was rapidly confirmed by others utilizing electron microscopy. Jamieson and Palade demonstrated that such granules are present in cardiocytes of the atria of all mammals, including humans, and were the first, in 1964, to suggest that these granules resemble other granules that release polypeptide hormones. These granules are usually adjacent to one or occasionally both poles of the nucleus, and are interspersed among the voluminous elements of the Golgi complex and within close proximity to the mitochondria, and are influenced by salt intake reduction.
Ultrastructural cytochemistry has shown that these granules consist of proteins. They incorporate both [ 3 H]-leucine and [ 3 H]-fructose in a pattern identical to other endocrine-secreting cells, with protein synthesis occurring in the Golgi complex. The ultrastructural features of the specific granules of different species are similar, in that they display an amorphous core and a limiting membrane, and generally measure 300–500 nm. The size and number of these granules vary among species, and generally are inversely related to size. Thus, atrial myocytes from large animals such as cows contain fewer and smaller granules than myocytes from small rodents such as rats. In the rat there are up to 600 spherical, electron-opaque granules per cell.
Atrial Extracts and Natriuresis
In 1922, Banting and Best utilized what is now considered a classic endocrinological technique in their discovery of insulin. They pulverized pancreas with buffer, filtered the crude tissue extract, and found that it produced hypoglycemia in an experimental dog. In 1981, deBold and colleagues, utilizing a similar approach, infused the supernatants of extracts of rat cardiac atria and rat ventricles into other rats, and found that the rat atria extracts, but not the extracts from the rat ventricles, caused dramatic diuresis and natriuresis, with urine flow increasing 10-fold, and sodium and chloride excretion increasing 30-fold. This simple but elegant experiment led to the discovery of atrial peptides that have the most potent endogenous natriuretic activity of any substance yet described. Atrial natriuretic peptide(s) isolated from these atrial extracts has been found to be a two-fold stronger natriuretic producing agent than furosemide (Lasix®, which is one of the most potent natriuretic producing drugs utilized in clinical medicine today.) Other investigators quickly confirmed this natriuretic action, as well as the ability of atrial extracts to cause vasodilation. It was rapidly demonstrated that these effects were at least partially due to a peptide(s). Further investigation revealed that the atrial extracts have significantly more natriuresis and diuresis than pure synthetic ANP, suggesting that other peptide hormones with natriuretic properties were in these atrial extracts.
History of Cardiac Hormones: Physiological Studies
Association of Heart and Renal Function
In 1847, Harthshorne suggested that the heart possessed volume receptors capable of sensing the “fullness of bloodstream” induced by whole-body immersion, which he clearly recognized had a diuretic effect. This observation received little further notice until 1935, when John Peters of Yale University made the same proposal that “the fullness of the bloodstream may provoke the diuretic response on the part of the kidney”. This concept then received experimental verification when it was shown that expansion of blood volume increases urine flow. Peters also suggested that the diuretic response was secondary to the ability of the heart, or something very near the heart, to “sense the fullness of the bloodstream”.
Balloon Distention of Atria
Experimental evidence of an association between cardiac atria and renal function was provided in 1956 by Henry et al., who observed that balloon distention of the left atrium in anesthetized dogs was associated with an increase in urine flow. Because the renal response to left atrial distention could not be elicited after the cervical vagi had been cooled to block nerve conduction, Henry and colleagues concluded that stretch receptors in the left atrium must be present. This finding was later extended to the right atrium. In their reports, Henry et al. noted the diuresis, but did not investigate whether it was associated with increased salt excretion (natriuresis). It is well-established now, however, that balloon distention of the cardiac atria causes natriuresis as well as diuresis. Evidence that animals with denervated hearts or denervated kidneys may also respond to an atrial pressure increase to produce diuresis suggests a hormonal pathway between the heart and the kidney. At least part of this hormonal pathway involves hormones made in the heart.
The “Third Factor”
With respect to a possible hormonal agent causing natriuresis and diuresis, de Wardener and colleagues demonstrated in 1961 that saline infusion produced an increase in urine flow and sodium excretion in anesthetized dogs independent of changes in glomerular filtration rate (GFR), which was decreased, and even in the presence of high circulating levels of aldosterone. These experiments gave rise to the popular concept of an unidentified “third factor,” a term coined by Levinsky and Lalone. The other two factors were aldosterone and GFR-affected sodium excretion. The search for this third factor soon focused on a possible hormonal mediator that came to be known as “natriuretic hormone.” Although this mediator (or mediators) from plasma or urine of volume-expanded humans or animals that causes natriuresis when injected into animals was never chemically identified, the evidence points toward this third factor having a peptide structure(s), because acid hydrolysis characteristically inactivated this substance. The “third factor” that was sought for decades now appears to be a family of peptide hormones termed “atrial natriuretic peptides” (ANPs), so named since they are found in their highest concentrations in the atria of the heart, have natriuretic properties, and are peptides. The third factor(s) also has the ability to inhibit Na + ,K + -ATPase in the kidney. Some of the natriuretic peptide hormones synthesized in the heart fill all of the criteria of being the “third factor(s).” Atrial natriuretic peptide (ANP) does not inhibit Na + ,K + -ATPase, so it would not fulfill the criteria of being the “third factor.” Three of the other peptide hormones synthesized by the ANP prohormone gene ( Figure 37.1 ), namely long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide, however, do inhibit renal Na + ,K + -ATPase and fill all of the criteria of being the “third factor(s)” that researchers have sought since the 1960s.
Family of Cardiac Natriuretic Peptide Hormones
At first it was thought that a single peptide was found in atrial extracts, but further investigation revealed a sophisticated endocrine system in the atria (and other tissues including the kidney) in which the atrial natriuretic peptide (ANP) prohormone gene synthesized four peptide hormones ( Figure 37.1 ), and two other genes were present, as reviewed below. The three other peptide hormones synthesized by the ANP prohormone gene – long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide – were first demonstrated to have biologic effects in 1987, and one of their mechanisms of action, via intracellular messenger cyclic GMP, was also elucidated in 1987. Fifth and sixth members of the natriuretic peptide family were identified in 1988, i.e., brain natriuretic peptide (BNP) isolated from a porcine brain cDNA library, and urodilatin, a peptide formed by differential processing of ANP prohormone in the kidney, which was first found in opossum urine. A seventh member of this family was identified in 1990 in brain tissue and termed C-type natriuretic peptide (CNP). A possible eighth member, DNP, was first described in 1992 from the venom of the green mamba snake.
Family of Cardiac Natriuretic Hormones: Synthesis of Three Prohormones
This family of cardiac peptide hormones has been designated atrial natriuretic peptides (ANPs), also known as atrial natriuretic hormones (ANHs). These peptide hormones are synthesized by three different genes, and then stored as three different prohormones (126 amino acid (a.a.) ANP, 108 a.a. BNP, and 103 a.a. CNP prohormones). In healthy adults, the main site of ANP prohormone synthesis is the atrial myocyte with its mRNA being 30–50-fold higher in the atria than that observed in the ventricle, but it is also synthesized in a variety of other tissues including the kidney. The different organs that synthesize the ANPs in the approximate order that they contribute to the synthesis of ANPs are listed in Table 37.1 .
| Molecular Weight (kDa) | Site of Synthesis | MAP | Diuresis | Natriuresis | |
|---|---|---|---|---|---|
| LANP | 3508 | Atria, ventricle, GI, lung, kidney, brain, adrenal | ↓ | ↑ | ↑ |
| Vessel dilator | 3878 | Atria, ventricle, GI, lung, kidney, brain, adrenal | ↓ | ↑ | ↑ |
| Kaliuretic peptide | 2184 | Atria, ventricle, GI, lung, brain, adrenal | ↓ | ↑ | – a |
| ANP | 3078 | Atria, ventricle, GI, lung, kidney, brain, adrenal | ↓ | ↑ | ↑ |
| Urodilatin | 3503 | Kidney | ↓ | ↑ | ↑ |
| BNP | 3462 | Atria, ventricle, brain, adrenal | ↓ | ↑ | ↑ |
| CNP | 2198 | Endothelium, CNS | ↓ | ↑ | – |
| DNP | 4191 | Atria, ventricle | ↓ | ↑ | ↑ |
| Adrenomedullin | 6029 | Adrenal, kidney | ↓ | ↑ | ↑ |
Cardiac Peptide Hormones Originating from Atrial Natriuretic Peptide Prohormone
Within the 126 a.a. ANP prohormone encoded by a single gene are four peptide hormones ( Figure 37.1 ) with blood pressure lowering, natriuretic, diuretic, and/or kaliuretic (i.e., potassium excreting) properties in both animals and humans. These peptide hormones, numbered by their a.a. sequences beginning at the N-terminal end of the ANP prohormone, consist of the first 30 a.a. of the prohormone (proANP 1–30, long-acting natriuretic peptide [LANP]); a.a. 31–67 (proANP 31–67, vessel dilator); a.a. 79–98 (proANP 79–98, kaliuretic peptide); and a.a. 99–126 (ANP) ( Figure 37.1 ). These peptide hormones which were each discovered before BNP and CNP were named for their most prominent biologic effects rather than the tissue they were first found in, because these peptides are synthesized in many tissues. Brain natriuretic peptide, so named because it was first found in porcine brain cDNA, for example, is actually present in the heart in 10-fold higher concentrations than in the brain. Each of the four peptide hormones from the ANP prohormone circulate in healthy humans, with LANP and vessel dilator concentrations in plasma being 15–20-fold higher than ANP and 100-fold higher than BNP.
BNP and CNP Prohormones
The BNP and CNP genes, on the other hand, appear to synthesize only one peptide hormone each within their respective prohormones, that is, BNP and CNP. The pro BNP gene and its regulation are reviewed in the section on BNP prohormone gene. The biologic effects of BNP and CNP are reviewed in sections on BNP, “Biologic Effects” and CNP, “Circulating Concentrations and Biologic Effects.”
Origination of Peptide Hormones from Prohormones
More than one peptide hormone originating from the same prohormone is common with respect to the synthesis of hormones. Adrenocorticotropin (ACTH), for example, is derived from a prohormone that contains four known peptide hormones. α-MSH, which has natriuretic properties, originates from this same prohormone. ACTH, similar to vessel dilator, originates from the middle of its prohormone. The middle of their respective prohormones is the most common origin of hormones with calcitonin, glucagon, vasoactive intestinal peptide, gastrin, cholecystokinin, and substance P, as well as ACTH and vessel dilator. Several hormones, such as vasopressin (antidiuretic hormone (ADH)), oxytocin, pancreatic polypeptide, angiotensin, and gastrin-releasing peptide, originate from the N-terminus of their respective prohormones, as does long-acting natriuretic peptide (proANF 1–30). The origin of hormones from the C-terminus of their respective prohormones like ANP, BNP, and CNP is less common, with somatostatin, inhibin, and parathyroid hormone (PTH) being the only known C-terminal prohormone-derived peptides. In the case of PTH, 84 of the 90 a.a. in its prohormone are considered to be the C-terminal “active” hormone; thus, it is not a small C-terminal-derived prohormone peptide, but rather nearly the intact PTH prohormone that serves as the actual peptide hormone.
Molecular Biology of the Cardiac Natriuretic Hormonal System
ProANP Gene
The gene encoding the synthesis of atrial natriuretic peptide prohormone (proANP) consists of three exon (coding) sequences separatedby two intron (intervening) sequences which encode for a mature mRNA transcript approximately 900 bases long ( Figure 37.1 ). Translation of human ANP prohormone mRNA results in a 151 a.a. preprohormone. Exon 1 encodes the 5′-untranslated region, the hydrophobic signal peptide (leader segment), and the first 16 a.a. of the ANP prohormone (first 16 a.a. of long-acting natriuretic peptide). The signal peptide, which is important for the translocation of the precursor peptide from the ribosome into the rough endoplasmic reticulum, is cleaved from the preprohormone (151 a.a.) in the endoplasmic reticulum ( Figure 37.1 ). The resulting 126 a.a. prohormone is the storage form for the four atrial natriuretic peptide hormones in tissues and the major constituent of the atrial granules. The first 16 a.a. of this prohormone encoded by exon 1 are, after proteolytic processing of the ANP prohormone, also the first 16 a.a. of long-acting natriuretic peptide (LANP) ( Figure 37.1 ). Exon 3 encodes for the terminal tyrosine (a.a. 126 of the ANP prohormone) in humans, and terminal three a.a. (Try-Arg-Arg) in rat, rabbit, cow, and mouse. Deletion of this terminal tyrosine residue encoded by exon 3 does slightly affect the binding of ANP, but does not appear to contribute to biologic activity, as there is no apparent decrease in biologic activity when this terminal tyrosine is not present. Exon 2 encodes for the rest of the prohormone (a.a. 17–125 in humans).
There is considerable homology in the proANP gene among species, particularly in the encoding and 5′ flanking sequences. The proANP gene has many features common to all eukaryotic genes, including a TATTA box (T=thymine; A=adenine), intervening sequences bounded by GT-AG splicing signals (G=guanine), and a consensus sequence found in promoted regions. An interesting feature of the human proANP gene is a consensus sequence for a putative glucocorticoid hormone regulatory element in the second intron.
The amino acid sequence of the whole ANP prohormone synthesized by the above gene is strikingly homologous among many species with differences clustered at the extreme carboxy terminal end of the prohormone, i.e., where ANP is formed. ,450 In each species, the C-terminus is distinguished from the rest of the prohormone by forming a 17 a.a. ring structure via the joining by a disulfide bond between two cysteine residues (105 and 121 of the prohormone), as schematically shown in Figure 37.2 . The ring structure originally was believed to be absolutely necessary for biologic activity, but linear forms (same amino acids in linear form) without a ring structure have since been shown also to have biologic activity. For full natriuretic and vasorelaxant activity, the Phe-Arg-Tyr (a.a. 124–126) at the COOH-terminus and a.a. 99–104 of the NH 2 -terminus of ANP are necessary. In the dog, it appears that deletion of a.a. 99–102 of prohormone does not affect natriuresis, but deletion of a.a. 103 and 104 decreases natriuretic activity 10-fold. Twenty of 30 a.a. in long-acting natriuretic peptide ( Figure 37.2 ) are exactly the same in the five above species, and another six of the remaining 10 amino acids are exactly the same in four out of the five species. Only three a.a. (33, 42, and 43 of the prohormone) are not the same in vessel dilator ( Figure 37.2 ) in the majority of the five species. Kaliuretic peptide has a highly-conserved sequence among the aforementioned five species, with 16 of its 20 a.a. ( Figure 37.2 ) being the same in all five.
BNP
This extraordinary conservation among species of LANP, vessel dilator, and kaliuretic peptide is not observed in the BNP prohormone, where there is a marked difference in amino acid sequence homology among species.
Tissue-Specific Expression of ProANP Gene
In healthy adult animals and humans, the atrial myocyte is the main site of the ANP prohormone synthesis, but it is also synthesized in a variety of other tissues. ProANP gene expression is 30–50 times higher in the atria of the heart than in extra-atrial tissues. The expression of this gene has been found in kidney, gastrointestinal tract (antrum of stomach, small and large intestine), lung, aorta, central nervous system, anterior pituitary, and hypothalamus. An example of where the proANP gene synthesized peptides localized in the kidney is illustrated in Figure 37.3 .
Mechanisms of Action of Gene Products (i.e., Cardiac Hormones and Urodilatin) of ProANP Gene
Part of the intracellular mechanism of action(s) of the four peptide hormones encoded by the proANP gene is that after they bind to their specific receptors they enhance membrane-bound guanylyl cyclase to cause an increase in the intracellular messenger cyclic GMP ( Figure 37.4 ). Cyclic GMP then stimulates a cyclic GMP-dependent, protein kinase that phosphorylates protein(s) in the cell, producing physiologic effects ( Figure 37.4 ). Cyclic GMP mediates the vasodilation of each of the cardiac hormones. The receptors for ANP that mediate ANPs biologic effects (e.g., ANP-A and -B receptors) are interesting, in that they contain guanylyl cyclase and a protein kinase in the receptors themselves ( Figure 37.5 ). The NPR-A receptor has a 441 a.a. extracellular portion that binds ANP which, in turn, activates the catalytic portion of guanylyl cyclase in the cell ( Figure 37.5 ). The protein kinase in this receptor has an inhibitory influence on guanylyl cyclase until this receptor is activated by ANP or BNP. There is a 21 a.a. portion of this receptor which attaches this receptor to the membrane ( Figure 37.5 ). The natriuresis secondary to the ANP is thought to also be mediated by cyclic GMP. Vessel dilator, LANP, and kaliuretic peptide’s mechanisms of action of producing a natriuresis is via enhancing the synthesis of prostaglandin E 2 , which in turn inhibits Na + K + -ATPase in the kidney, which ANP does not do. Vessel dilator and kaliuretic peptide’s homodynamic effects via vasodilation of blood vessels are, however, mediated by cyclic GNP.
Processing of Atrial Natriuretic Peptide Prohormone in Kidney
ANP prohormone processing is different in the kidney compared to other tissues, resulting in an additional four a.a. added to the N-terminus of ANP (proANP 95–126, urodilatin ( Figure 37.2 )), a peptide first identified in opossum urine. Thus, in the kidney, the identical four a.a. from the C-terminus of kaliuretic peptide are added to ANP to form the peptide urodilatin ( Figure 37.2 ). At first, urodilatin was thought not to circulate, and that it was not a hormone. To be defined as a hormone, a given protein has to be synthesized in a tissue or organ, circulate in the bloodstream, and have biologic effects in another tissue or organ. With a very sensitive radioimmunoassay, it appears that urodilatin does circulate, but in such low concentrations (9–12 pg/ml) that it may not be physiologically relevant. Since urodilatin constitutes less than 1% of the circulating natriuretic hormones, its physiologic importance as a circulating hormone is very limited, with over 99% of the physiologic natriuretic effects being from the other natriuretic hormones. Urodilatin, however, may have paracrine functions, and may mediate the effects of one of the other natriuretic hormones (ANP). Infusion of ANP increases the circulating concentration of urodilatin, suggesting that some ANP effects may be mediated by urodilatin. Infusion of long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide, on the other hand, do not affect the circulating concentration of urodilatin in healthy humans.
Regulation of Atrial Natriuretic Peptide Prohormone Gene
Enhancement of ProANP Gene Expression
Stretch
Mechanical stretch, or more specifically tension, delivered across the atrial wall is a potent activator of proANP gene expression and/or secretion. In animals, an increase of sodium intake results in an increased release of the ANP prohormone peptides.
Thyroid Hormones
Thyroid hormones thyroxine (T 4 ) and triiodothyronine (T 3 ) increase proANP gene expression. The increase in proANP mRNA in hypothyroidism when treated with thyroid hormone is paralleled by the increase in circulating concentrations of the gene products of this synthesis – vessel dilator, LANP, and ANP – in persons with hypothyroidism treated with thyroid hormone.
The changes in proANP mRNA in both hypothyroidism and hyperthyroidism parallel very closely the circulating concentrations of vessel dilator, LANP, and ANP in humans, which are decreased in hypothyroidism and increased in hyperthyroidism. When the hyperthyroid subjects were treated with the antithyroid drug propylthiouracil (PTU) the circulating concentrations of LANP, vessel dilator, and ANP decreased 50% after one week of treatment, with a simultaneous 50% decrease in serum triiodothyronine (T 3 ) levels.
Glucocorticoids
Dexamethasone, at a dose of 1 mg/day, increases proANP mRNA levels in both atria and ventricles of the rat approximately two-fold. There is negative feedback between cortisol and the gene products of proANP gene expression in that the cardiac hormones vessel dilator, LANP, kaliuretic hormone, and ANP decrease the circulating concentration of cortisol. This decrease in cortisol is due, at least in part, to these cardiac hormones decreasing the circulating concentration of the hypothalamic peptide corticotrophin-releasing hormone (CRH), with a resultant decrease in ACTH, which stimulates the production of cortisol.
Mineralocorticoids
Administration of mineralocorticoids to animals causes transient fluid and sodium retention. Despite continued administration of a mineralocorticoid, animals return to sodium balance within a few days, a phenomenon termed “mineralocorticoid escape.” To investigate the role of ANP in mineralocorticoid escape, Ballerman et al. administered DOCA to rats in sodium balance, and found plasma ANP levels and atrial proANP mRNA content increased in rats retaining sodium in response to DOCA. After “escape” from the mineralocorticoid-induced sodium retention, plasma ANP levels returned to baseline and relative atrial proANP mRNA content remained moderately elevated. This increase in proANP mRNA probably resulted from the secondary cardiovascular effects of the steroids (e.g., increased intravascular volume), rather than from a direct effect of the mineralocorticoids on the ANP-secreting cell, as DOCA has no direct effect on proANP mRNA in ANP-expressing neonatal cardiocytes.
Vasoconstrictive Peptides
Several vasoconstrictive peptides, including endothelin, norepinephrine, and angiotensin II, stimulate proANP transcription and secretion. The cardiac hormones, in turn, affect endothelin in a negative-feedback manner.
Calcium
Primary cultures of neonatal rat cardiocytes exposed for 24 hours to 2 mM CaCl 2 in the culture media increase proANP messenger RNA three-fold, and increase secretion of ANP prohormone into the media three-fold. When these cardiocytes were treated with the calcium channel-blocking agents diltiazem, nifedipine or verapamil, both proANP synthesis and secretion decreased to 25–40% of control values.
Transgenic Knockout and/or Mice Overexpressing Atrial Natriuretic Peptide Prohormone Gene
Transgenic mice with an 11 base-pair deletion in exon 2 of the proANP gene ( Figure 37.1 ) have increased blood pressure in homologous ( −/− ) mice of 8–23 mmHg compared to wild-type ( +/+ ) mice. Exon 2 of the proANP gene encodes for vessel dilator, kaliuretic peptide, and ANP ( Figure 37.2 ). Exon 2 homozygote mutants have no circulating ANP, and they become hypertensive when fed a standard diet. Heterozygotes ( +/− ) with this base pair deletion in exon 2 are salt-sensitive and become hypertensive (systolic blood pressure increases 27 mmHg) on a high-salt (8%) diet. Mice that overexpress the proANP gene, on the other hand, become hypotensive.
Human Diseases with Upregulation of Atrial Natriuretic Peptide Prohormone Gene
Cerebrovascular Disease (Stroke and Hypertension)
A genetic linkage study followed 22,071 male physicians, all of whom had no history of stroke, from 1982 to 1999. DNA extracted from peripheral white blood cells of those individuals who had a subsequent stroke revealed that, when compared to those without strokes, these individuals had a molecular variant in exon 1 of the proANP gene that was associated with a two-fold ( p <0.01) increased risk of stroke. The individuals who had cerebrovascular accident (stroke) had significantly ( p <0.001) higher systolic and diastolic blood pressures than the persons who did not have a stroke. This molecular variant of the proANP gene was found to be an independent risk factor (in addition to hypertension) for a cerebrovascular accident. This molecular variant was found to be responsible for a valine-to-methionine transposition in long-acting natriuretic peptide (LANP), i.e., the only peptide hormone synthesized by exon 1. (Exon 1 does not encode for ANP.) In the 16 a.a. of LANP encoded by exon 1 there is only one valine, which is at position 7 of LANP ( Figure 37.2 ). Residue #7 (amino acid #7 of the ANP prohormone) is highly-conserved among different species. In this human study there was no defect in the structure or expression of the proBNP gene. In humans, blood pressure and the cardiac hormones correlate throughout the 24-hour period in a circadian relationship. There is evidence to suggest that long-acting natriuretic peptide reflects salt sensitivity in hypertension-prone individuals, even before they develop hypertension.
LANP and Stroke
Long-acting natriuretic peptide (LANP) has potent vasodilatory properties in both animals and humans. Antisera to LANP (to block the biologic activity of this peptide hormone) results in a significant increase in mean arterial pressure from 112±12 mmHg to 131±9 mmHg in normotensive animals, and exacerbates hypertension in spontaneously hypertensive rats (SHR) from 140±10 mmHg to 159±9 mmHg. These antisera data indicate an important physiological role for long-acting natriuretic peptide in the regulation of arterial pressure. In the brain of stroke-prone rats, the expression ANP prohormone gene (which synthesizes LANP) is significantly reduced. There were no mutations in the BNP gene, and no differences in BNP gene expression between stroke-prone and stroke-resistant animals.
Further evidence of the importance of the peptide hormones synthesized by the ANP prohormone gene derives from studies in mice with the ANP prohormone gene knocked-out: all develop salt-sensitive hypertension within one week leading to stroke. The BNP gene does not upregulate to prevent hypertension and/or stroke when the proANP gene is knocked-out. Downregulation of the proANP gene in the brain in stroke-prone SHRs has further been found to co-segregate with the occurrence of early strokes in their F 2 descendants.
Natriuretic Peptide Hormones and Hypertension
The original hypothesis for hypertension was that there was a defect in the production of the blood pressure-lowering cardiac hormones. Experimental evidence revealed that, rather than being decreased, these blood pressure lowering cardiac hormones are elevated in the circulation in an apparent attempt to overcome the elevated blood pressure. ANP increases in essential hypertension and in persons with pheochromocytomas. The hypertension associated with pheochromocytomas is characterized by increased circulating concentrations of vessel dilator and long-acting natriuretic peptide (LANP), as well as ANP. Each of these blood pressure lowering hormones increase further with surgical manipulation-induced increases in blood pressure, and then these peptides return to normal after surgical removal of the pheochromocytomas and lowering of blood pressure. The hypertension of obesity also is associated with increased circulating concentrations of ANP which decreases into the normal range with weight reduction-induced decrease in high blood pressure.
In pregnancy, cardiac hormones increase in each trimester with the plasma volume expansion which accompanies a normal pregnancy. ANP, vessel dilator, and LANP increase dramatically with the hypertension of pre-eclampsia, compared to their circulating concentrations in healthy pregnant women. If one knocks-out the ANP prohormone gene that synthesizes the four cardiac hormones ( Figure 37.1 ), within one week the animals develop salt-sensitive hypertension while, on the other hand, transgenic mice overexpressing the ANP prohormone gene develop hypotension. In addition to directly vasodilating vasculature, kaliuretic peptide and ANP inhibit the release of the potent vasoconstrictive peptide endothelin which is produced by the vascular endothelium.
Congestive Heart Failure
In congestive heart failure (CHF), proANP gene expression is upregulated. The increase in proANP gene expression is, however, not in the atria of the heart, but rather in the ventricle of the heart. In persons with CHF, there is no defect in the production of the peptides from the ANP prohormone, but rather each are increased in the bloodstream in an attempt by the heart to overcome abnormal sodium and water retention by releasing more of these peptides that cause sodium and water excretion. Vessel dilator and LANP increase in direct proportion to the severity of CHF, as classified by the New York Heart Association (NYHA). The ANP-clearance receptor pathway is not linked to the avid sodium retention and/or to the renal ANP resistance observed in CHF.
Cirrhosis with Ascites
Another salt- and water-retaining state, cirrhosis with ascites, is characterized by increased circulating concentration of the cardiac hormones, and with a 4.1-fold ventricular (but not atrial) increased steady-state proANP messenger RNA. Although the liver expresses proANP messenger RNA, there is no upregulation of proANP gene expression in the liver when ascites develops. Rather, the upregulation of this gene is only in the ventricle of the heart.
Brain Natriuretic Peptide Prohormone Gene
The BNP gene is comprised of three exons separated by two introns, similar to the proANP gene in Figure 37.1 . Regulation of the BNP gene is controlled at the transcriptional level by several cis-acting regulatory elements in the proximal promoter, and the transcription factors that associate with them. The cardiac specific transcription factor GATA 4 plays a major role directing basal activity of the BNP gene promoter. GATA 4 is a nuclear mediator of mechanical stretch-activated BNP gene, and might function as a central integrator of regulatory activity controlled by other transcription factors, such as GATA 6, the neuron restricted silencer element-binding factor, and YY1 the embryonic development protein. Several of these factors interact synergistically with GATA 4, in both a physical and a functional sense, to stimulate BNP gene transcription. Both the proANP and proBNP genes are activated in cardiac hypertrophy. The GATA 4 transcription factor activates the proANP gene, as well as the proBNP gene.
In healthy animals, cardiac BNP mRNA is mainly of atrial origin, that is, 10–50-fold more abundant than in ventricles. In early left ventricular (LV) dysfunction, BNP mRNA markedly increases in the left atrium, but remains below or just barely at the level of detection in the ventricles. The majority of investigations have found no increase in BNP mRNA in the ventricles in congestive heart failure, which is exactly the opposite of ANP prohormone gene expression which increases in the ventricle but not in the atria in sodium and water-retaining states. Likewise, with streptotocin-induced diabetes BNP mRNA doubles in atria, without any change in ventricular myocardium BNP mRNA. BNP gene knockout mice do not develop hypertension or hypertrophy as ANP prohormone knockout mice do. BNP knockout mice exhibit cardiac fibrosis as the only known effect of the BNP gene being knocked-out. These knockout studies suggest that regulation of blood pressure is contributed to by the cardiac hormones synthesized by the proANP gene, but not by BNP.
Secretion of Cardiac Natriuretic Peptides
The main physiological stimulus to secretion of these peptide hormones to control blood volume appears to be an increase in pressure in the atria. An increase of 4 to 6 mmHg in the atria releases the four cardiac hormones from the ANP prohormone. These peptide hormones, in turn, decrease the volume returning to the heart secondary to their causing a diuresis and natriuresis. Rapid heart rates at 125 beats/min and higher release the cardiac hormones into the circulation. Both atrial and ventricular arrhythmias with heart rates of 125 beats/min and higher release these peptide hormones and increase the circulating concentrations of these cardiac hormones in humans. Hypoxia and a variety of humoral factors (endothelin, glucocorticoids, acetylcholine, adrenergic agonists) have been suggested as contributing to release, but the majority of these humoral factors’ effects are to increase the ANP prohormone gene synthesis, as outlined previously, rather than release per se . With respect to hyperosmolarity, the threshold for ANP release is as low as 10 mOsmol/kg H 2 O and this is regulated by a cross-talk between sarcolemmal L-type Ca 2 channel and the sarcoplasmic Ca 2 release. ANP, vessel dilator, LANP, and kaliuretic peptide have a feedback mechanism whereby they inhibit their own and each other’s release. CNP also inhibits ANP release.
Biologic Effects of the Cardiac Natriuretic Hormones and Their Mechanisms of Action
ANP
Vasodilation
The original report by deBold et al. where crude atrial extracts cause diuresis and natriuresis also indicated that these extracts could decrease mean arterial pressure. Crude atrial extracts were then shown to possess vasorelaxant activity in isolated aortic segments. When synthetic ANP became available, it was demonstrated that the pure form of ANP could also cause vasodilation in vitro . Both crude and synthetic ANP decrease total peripheral resistance. Large central arteries (aorta and renal) relax, whereas more distal (ear and basilar) arteries are refractory to nanomalor concentrations of ANP. Pulmonary, femoral, and iliac arteries are intermediate in their response to ANP. One exception to small arteries not responding well upon the addition of ANP is the carotid arteries, which respond well. In general, veins do not appear to vasodilate with the addition of ANP as well as arteries do, but ANP has been shown to relax peripheral veins in addition to aortic rings.
ANP produces a dose-dependent reduction in systemic blood pressure in humans. The immediate blood pressure lowering of 5.5 mmHg in 2 minutes with 100 μg ANP intravenously has been associated with a sensation of facial flushing in four of six human subjects, suggesting acute vasodilation of skin. ANP elicits greater blood pressure-lowering properties in spontaneously hypertensive rats than in normotensive rats. A greater response in hypertensive versus normotensive subjects is also true for most, if not all, antihypertensive agents.
Mechanism of Anp-Induced Vasodilation
The vasodilation observed with ANP is endothelium-independent. It is mediated by cyclic GMP, which is increased after enhancement of membrane-bound guanylyl cyclase by ANP. Cyclic GMP itself has been demonstrated to cause vasodilation. The vasorelaxation with ANP appears to be independent of calcium and cyclic AMP, with no change in cyclic AMP occurring during the same period of time that cyclic GMP increases. With respect to calcium, ANP can increase cyclic GMP and cause vasodilation with no calcium in the incubation media, but possible shifts of calcium already in the vasculature are still being debated. The ANP-induced relaxation of contracted vasculature is not blocked by adrenergic antagonists, cholinergic antagonists or indomethacin, the latter suggesting that this vasodilation is not mediated by prostaglandins.
Natriuresis
ANP-induced natriuresis appears to have a dependence on renal vasodilation, since ligation of the renal artery eliminates ANP-induced natriuresis. ANP has been shown to directly increase renal blood flow in dogs. Redistribution of renal blood flow to the proximal and distal tubules has been reported as contributing to the natriuretic effects. The proximal tubule contains guanylyl cyclase, and cyclic GMP produced by this enzyme increases amiloride-sensitive 22 Na uptake in the phosphorylated brush border membranes of the proximal tubule, suggesting that the ANP intracellular mediator, cyclic GMP, directly stimulates the Na–H antiporter in the proximal tubule.
ANP and Site of Action in Kidney
The renal actions of ANP are complex. Hemodynamic effects of pharmacologic doses of exogenous ANP constrict efferent and dilate afferent arterioles ; the resultant increase in glomerular capillary hydrostatic pressure could increase the glomerular filtration rate (GFR). Physiologic doses of ANP, however, do not increase GFR. The circulating physiological concentration of ANP is below the concentration of ANP that has been found necessary to increase GFR. Early micropuncture and microcatheterization studies suggested a late distal nephron site of action, but functional studies of ANP in the inner medullary collecting duct (IMCD) indicate that it is a major target site of ANP action in the tubule. Binding studies indicated specific binding sites for ANP on IMCD cells. ANP increases cGMP accumulation in isolated cells from this segment in a concentration-dependent manner. ANP also inhibits oxygen consumption in IMCD cells, indicative of inhibition of sodium transport. This inhibition occurs through a cGMP-mediated effect on an amiloride-sensitive sodium channel.
A proximal tubular site of action has been suggested from studies showing that ANP inhibits angiotensin II-stimulated proximal sodium reabsorption at very low concentrations (as low as 10 −12 mol/L). In the cortical collecting ducts, ANP inhibits tubular water transport by antagonizing the action of vasopressin. ANP has been localized by immunoperoxidase and immunofluorescence methods to the sub-brush border of the pars convuluta and pars recta proximal tubule, as well as the distal tubule. These studies indicate that ANP may have widespread actions on tubular function.
ANP Inhibits the Renin–Aldosterone System
Atrial natriuretic peptide has been found to be a potent in vivo and in vitro inhibitor of aldosterone secretion via a direct effect on aldosterone secretion from the zona glomerular cells of adrenal cortex, and indirectly through inhibition of renin release from the juxtaglomerular cells of the kidney. The mechanism of the inhibition of renin release by ANP appears to involve cyclic GMP, as this inhibition is duplicated by permeable analogs of cyclic GMP.
Cardiac Peptide Hormones: LANP, Vessel Dilator, and Kaliuretic Peptide
Vasodilation
LANP, vessel dilator, and kaliuretic peptide cause vasodilation of vasculature that is endothelium-independent, and similar to ANP endothelium-independent vasodilation of vasculature. The amount of vasodilation in vitro with these cardiac peptide hormones is similar to that observed with addition of ANP. When infused over 2 hours at the same 100 ng/kg of body weight/per minute concentration, vessel dilator and ANP were found to decrease blood pressure from an average of 145 to 124 mmHg ( p <0.05), and from 143 to 123 mmHg ( p <0.05), respectively. Long-acting natriuretic peptide lowered blood pressure from a mean of 138 to 122 mmHg ( p <0.05), whereas kaliuretic peptide decreased blood pressure from a mean of 155 to 138 mmHg ( p <0.05). Blood pressure did not change in the control animals throughout the 120-minute pre-experimental period or during the 120-minute experimental period. Similar to ANP, the mechanism of vasodilating vasculature for these hormones is mediated by cyclic GMP. The enhancement of guanylyl cyclase by the cardiac hormones is calcium-independent in vasculature.
Cardiac Hormones: LANP, Vessel Dilator, and Kaliuretic Peptide
Natriuresis Mechanism of Action
Comparison of the relative natriuretic and diuretic potencies of the same dose in 100 ng/kg of body weight per minute of vessel dilator, revealed that LANP and ANP have significant natriuretic properties in healthy humans, but kaliuretic peptide enhancement of the urinary sodium excretion rate was not significant. The natriuretic properties of vessel dilator are especially impressive in light of the fact that ANP has been found to be a more potent natriuretic and diuretic agent than furosemide, and that vessel dilator has equally potent natriuretic effects and circulates normally at a 17-fold higher concentration than ANP and 100-fold higher than BNP. This 17-fold higher circulating concentration is found both during unstimulated conditions and with release secondary to physiological stimuli, such as head-out water immersion where the atria are stretched releasing these peptides. Vessel dilator’s biologic effects also last significantly longer than ANP (greater than 6 hours versus 30 minutes). Vessel dilator and ANP, with nearly equal abilities to enhance sodium excretion, are markedly different, however, with respect to potassium excretion. Vessel dilator is the only one of the four cardiac peptide hormones from the ANP prohormone that does not significantly enhance potassium excretion. This potassium-sparing effect of vessel dilator makes it distinctly different from ANP, LANP, and kaliuretic peptide. Kaliuretic peptide does not significantly enhance the fractional excretion of sodium (FE Na ), but it is the only natriuretic peptide that significantly enhances the fractional excretion of potassium (FE k ) in healthy humans. (Fractional excretion of sodium or potassium is the percentage of glomerular-filtered sodium or potassium that is excreted into the urine. )
The natriuretic effects of long-acting natriuretic peptide, kaliuretic peptide, and vessel dilator have different mechanism(s) of action from ANP, in that they inhibit renal Na + ,K + -ATPase secondary to their ability to enhance the synthesis of prostaglandin E 2 , which ANP does not do. ANP, BNP, and CNP effects in the kidney are thought to be mediated by cyclic GMP.
Cardiac Hormones: Localization within the Kidney
Immunohistochemical studies have localized vessel dilator ( Figure 37.3 ) and long-acting natriuretic peptide as well as ANP to the sub-brush border of the pars convuluta and pars recta of the proximal tubules of animal and human kidneys. Immunofluorescent studies reveal that each of these peptides has a strong inclination for the perinuclear region in both the proximal and distal tubules.
LANP, Vessel Dilator, and Kaliuretic Peptide: Renin–Aldosterone System
Kaliuretic hormone and long-acting natriuretic peptide (LANP) are potent inhibitors of the circulating concentrations of aldosterone in healthy humans. Kaliuretic peptide and LANP effects on decreasing plasma aldosterone levels last for at least 3 hours after their infusions have stopped, while ANP no longer has any effect on plasma aldosterone concentrations within 30 minutes of infusion cessation. Vessel dilator does not appear to have direct effects on aldosterone synthesis, but is a potent inhibitor (66%) of plasma renin activity. Thus, the four cardiac hormones from the ANP prohormone gene act as endogenous antagonists of the vasoconstrictor and salt-and-water-retaining systems (e.g., the renin–angiotensin–aldosterone system, vasopressin, and endothelin ) in the body’s defense against blood pressure elevation and plasma volume expansion via direct vasodilator, diuretic, and natriuretic properties. These four cardiac hormones’ multiple biologic effects are illustrated in Figure 37.6 . It is important to note in this illustration with respect to the kidney, that these peptide hormones also increase protein excretion (albumin, B2 microglobulin) and phosphate reabsorption, as well as cause a natriuresis and diuresis.
Kidney Hormone: Urodilatin
Vasodilation
Urodilatin has vasodilatory effects similar to those of ANP. This would be expected, since the ANP prohormone post-translational processing in the kidney results in an additional four a.a. from kaliuretic peptide being added to the N-terminus of ANP (proANP 95–126, urodilatin) ( Figure 37.2 ). The rest of the amino acids in urodilatin are identical, and in the same sequence as those in ANP ( Figure 37.2 ). Urodilatin and ANP have identical ring structures formed with cysteine-to-cysteine bonding ( Figure 37.2 ). The four a.a. added to form urodilatin are the same four a.a. present in the C-terminus of kaliuretic peptide in other tissues, but in the kidney the ANP prohormone is cleaved between a.a. 94 and 95 rather than between a.a. 98 and 99 to form urodilatin. Urodilatin is not formed in the heart or in other tissues except the kidney. Urodilatin at first was thought not to be a hormone, in that it was thought that it did not circulate, but sensitive assays revealed that it circulates at very low concentrations (9–12 pg/ml). Infusion of ANP increases the circulating concentration of urodilatin, suggesting that some ANP effects may be mediated by urodilatin. Infusion of long-acting natriuretic peptide, vessel dilator, and kaliuretic peptide, on the other hand, do not affect the circulating concentration of urodilatin in healthy humans.
Urodilatin: Natriuresis
Urodilatin has natriuretic and diuretic effects similar to ANP, and since it has an identical amino acid sequence and ring structure as ANP one would expect it to bind to the same receptors and have similar biologic effects as ANP. It does bind to the same receptor and has similar biologic effects as ANP.
Localization of Urodilatin in Kidney
Immunohistochemical studies have localized urodilatin to the distal tubule with no evidence of urodilatin in the proximal tubule. ANP messenger-RNA studies have confirmed that the ANP prohormone is synthesized in the kidney. The amount of the ANP prohormone present in the kidney, however, is only 1/190th of that produced in the atria of the heart. These studies taken together suggest that since urodilatin is found mainly in the distal nephron, and since it is part of the ANP prohormone, synthesis of the ANP prohormone may take place in the distal nephron. The ANP prohormone gene is present and can be expressed in the kidney. This gene is upregulated within the kidney in early renal failure in diabetic animals, and in the remnant kidney of rats with 5/6 reduced renal mass. Within the kidney, in addition to urodilatin, the ANP prohormone gene synthesizes LANP, vessel dilator, and a shortened form of kaliuretic peptide, but not ANP per se .
Urodilatin and Renin–Aldosterone System
Urodilatin does not affect renin or aldosterone concentrations.
BNP and CNP
BNP: Biologic Effects
Brain natriuretic peptide (BNP) is a 32 a.a. peptide in humans (45 a.a. in rat) with similar diuretic and natriuretic effects and a short half-life as ANP ( Figure 37.2 ). BNP half-life is 100-fold shorter than the half-lives of vessel dilator and LANP. BNP has remarkable sequence homology to ANP with only four a.a. being different in the 17 a.a. ring structure formed by a disulfide bond common to both peptides ( Figure 37.2 ). Although BNP was named for where it was first isolated (porcine brain), the main source of its synthesis and secretion is the heart (10-fold greater than brain) ( Table 37.1 ). As with ANP, the highest levels of BNP are found in the atria of the heart. BNP levels in the atria, however, are less than 1% of ANP levels. The immunoreactive level of BNP in the ventricles is only 1% of BNP concentration in the atria; in brief, 99% of BNP is found in the atria. BNP, however, has been termed a “ventricular” peptide, based on ventricular BNP mRNA levels being similar to those in the atria with ventricles being much larger than the atria, but as above, 99% of BNP is in the atria rather than the ventricles.
The 108 a.a. BNP prohormone is processed in the heart to yield a biologic functioning BNP consisting of a.a. 77–108 of the BNP prohormone (in humans) and a biologically inactive N-terminus of the BNP prohormone (a.a. 1–76 of prohormone), both of which circulate. BNP circulating concentration is less than 20% of ANP. The sequence homology of BNP differs appreciably across species (both in size and a.a. sequence). The major circulating form varies substantially among species, being 26, 45, and 32 a.a. in pig, rat, and human, respectively. The marked sequence variability of BNP explains in part variations in its biologic activity in different species. The peptide hormones from the ANP prohormone, on the other hand, have remarkable homology across different species. Mice overexpressing the BNP gene, where the circulating concentration of BNP is 10- to 100-fold higher than in healthy mice, have less glomerular hypertrophy and mesangial expansion with intraglomerular cells than healthy mice 16 weeks after both received renal ablation. This mouse model of subtotal renal ablation, however, also has significantly increased ANP concentrations, which may also have contributed to the effects attributed to BNP in the BNP-gene overexpressing mice.
CNP: Circulating Concentrations and Biologic Effects
C-type natriuretic peptide (CNP) is a 22 a.a. peptide with remarkable similarity to ANP and BNP in its amino acid sequence, but lacks the carboxy-terminal tail of ANP and BNP ( Figure 37.2 ). CNP was found originally in the brain, but more recent studies suggest that it is also present in the heart and kidney. The amount of CNP in the heart, however, is very low and only small amounts are present in plasma. Two CNP molecules, 22 and 53 a.a. in length, have been identified in plasma. Each is derived from a single CNP prohormone, with the 22 a.a. form contained in the carboxy-terminal portion of the 53 a.a. form. The 22 a.a. form predominates in plasma, and is more potent than the 53 a.a. form in humans. The plasma concentration of CNP is very low, with some authors reporting that CNP is not normally detectable but becomes detectable only in renal failure and congestive heart failure. CNP is present in the human kidney. CNP has been found to have little effect on renal vasoconstriction. Although CNP has been reported to have natriuretic effects in some animals, when infused in humans at physiological concentrations and in concentrations that reached four- to ten-fold above those observed in disease states, CNP did not affect renal function. Thus, in healthy humans CNP had no effect on renal hemodynamics, systemic hemodynamics, intrarenal sodium handling, sodium excretion or plasma levels of renin and aldosterone. Even when CNP was increased 60-fold in human plasma there were no significant hemodynamic or natriuretic effects. The authors of this study concluded that it is unlikely that CNP has any endocrine role in circulatory physiology. There is one study in humans where infusion of CNP to increase CNP plasma levels 550-fold above normal caused a 1.5-fold increase in urine volume and sodium excretion. With this very high plasma concentration of CNP, both ANP and BNP also increased 2.4-fold, which may have been the cause of the natriuresis and diuresis observed that was not observed in any other study with CNP. Each of these studies suggests that CNP does not contribute physiologically to any natriuresis or diuresis in healthy humans. The main site of CNP synthesis is vascular endothelium, and CNP acts as a paracrine endothelium-derived hyperpolarizing factor (EDHF) via activation of NPR-C receptor and the opening of a G-protein-gated inwardly rectifying K channel (GIRK) in mesenteric resistance arteries to mediate vasodilation. In conduit vessels, on the other hand, CNP induces relaxation via a cyclic GMP-dependent mechanism.
Adrenal Natriuretic Peptides, Adrenomedullin and Proadrenomedullin N-Terminal 20 Peptide: Biologic Effects
Adrenomedullin (ADM), a 52 a.a. peptide with one intramolecular disulfide bond ( Figure 37.2 ) originally isolated from an extract of a pheochromocytoma, also has a range of biologic properties similar to the cardiac hormones, but these properties are less pronounced than those of the cardiac hormones ( Table 37.1 ). Infusion of ADM lowers blood pressure and produces diuresis and natriuresis. Adrenomedullin causes a long-lasting hypotension accompanied by increased heart rate as a side-effect. ANP, but not LANP, vessel dilator or kaliuretic hormone, increases the circulating concentration of adrenomedullin three- to four-fold, suggesting that some of the reported effects of ANP may be mediated via adrenomedullin. However, the natriuresis and diuresis secondary to ANP were much larger than has ever been observed with adrenomedullin, suggesting that ADM does not mediate all of the natriuretic and diuretic effects of ANP. Adrenomedullin is a larger peptide than any of the cardiac hormones, with its main site of synthesis being in the adrenal ( Table 37.1 ), but isolated renal cells also have the ability to synthesize adrenomedullin secondary to stimulation by vasopressin via V2 receptors. Since vasopressin (antidiuretic hormone, ADH) inhibits a diuresis these findings are opposed to findings that ADM causes a diuresis. Adrenomedullin is part of a peptide family that shares structural similarity with calcitonin gene-related peptides and amylin, which share biologic effects and some cross-reactivity between receptors. The adrenomedullin prohormone at its N-terminal end contains another biologically active peptide with vasodilating properties known as proadrenomedullin N-terminal 20 peptide (PAMP). Whether more PAMP or adrenomedullin is produced depends on alternate splicing of its prohormone by the enzyme peptidylglycine C-amidating monoxygenase. Adrenomedullin exerts its actions through G-protein-coupled membrane receptors linked to adenylyl cyclase, resulting in an increase in cellular cyclic AMP as opposed to the cardiac hormones (ANP, BNP, LANP, vessel dilator, and kaliuretic peptide) whose second messenger is cyclic GMP. Proadrenomedullin is thought not to act via either cyclic AMP or cyclic GMP, but rather via potassium channels, which eventually exert a presympathetic inhibition of sympathetic nerves innervating blood vessels.
Dendroaspis Natriuretic Peptide: Biologic Effects
Dendroaspis natriuretic peptide (DNP) is the newest of the natriuretic peptides ( Figure 37.2 ). This peptide was isolated from the venom of the green mamba snake Dendroaspis angusticeps . This venom also contains several polypeptide toxins that block cholinergic receptors to cause paralysis. DNP-like peptide has been reported to be present in human plasma and in heart atria. In plasma, DNP concentration is very low at 6 pg/ml, which is 0.5% of the circulating cardiac hormones. This peptide has a 17 a.a. disulfide ring structure similar to ANP, BNP, and CNP ( Figure 37.2 ), and causes a natriuresis and diuresis in dogs. Infusion of DNP does not cause any significant change in the circulating levels of ANP, BNP, or CNP.
Richards et al. have questioned whether DNP actually exists in humans and mammals, since it has not been characterized by high-pressure liquid chromatography linked to immunoassay, followed by purification and analysis to establish the human amino acid sequence, as has been done with the aforementioned cardiac hormones. The gene for DNP has not been cloned in the snake or in any mammal as has been done for each of the other natriuretic peptides. Richards et al. suggest that DNP may be “snake BNP,” since BNP varies markedly in amino acid sequence among species (and the BNP sequence in this snake is unknown). The peptides from the ANP prohormone are markedly conserved among species, and one would not suspect that DNP is one of these peptides as their amino acid sequences are markedly different from DNP. Further experimentation with the studies discussed previously suggested by Richards et al. should give one more insight with respect to this peptide.
Guanylin, Lymphoguanylin, Renoguanylin, and Uroguanylin: Biologic Effects
Guanylin, a 15 a.a. peptide, isolated from rat intestine, and uroguanylin, a 16 a.a. peptide originally isolated from opossum urine, are peptides which are structurally and functionally similar to bacterial heat-stable enterotoxins produced by strains of pathogenic Escherichia coli intestinal bacteria. Traveler’s diarrhea is the result of these enterotoxins interacting with a membrane-bound guanylyl cyclase-C (GC-C receptor) on the luminal surface of enterocytes. The resulting increase in cyclic GMP phosphorylates the cystic fibrosis transmembrane conductance regulator (CFTR), leading to an efflux of chloride into the intestinal lumen. Cyclic GMP in the intestine inhibits Na + absorption mediated by apical Na + /H + exchange, and activates protein kinase G II. Guanylin and uroguanylin, which have similar structures to this enterotoxin, have a similar mechanism of action in the intestine via the same GC-C receptor. These peptides have been identified in the intestine in different locations, with guanylin in the colon but not the proximal intestine, and uroguanylin expressed in the proximal intestine but not in the colon. These intestinal peptides have natriuretic properties. The observation of renal expression of guanylin and uroguanylin mRNA suggests renal synthesis and a local paracrine action of these peptides in a manner analogous to the ANP prohormone gene products. Wang et al. showed that intravenous and intraluminal administration of uroguanylin in the kidney affects tubuloglomerular feedback, but it failed to cause a natriuresis and diuresis in rats at up to 100 nmol/kg/h intravenously, which was less than other supraphysiologic concentrations used previously to cause a natriuresis. The uroguanylin dose used by Wang et al. was still, however, a pharmacologic dose which resulted in higher uroguanylin concentrations in rat blood and urine than physiological concentrations which are in the femtomolar range. Rats fed a high-salt diet had higher uroguanylin and cGMP concentrations in the urine; however, the plasma concentration of uroguanylin was not increased, which argues against uroguanylin being an endocrine hormone in the kidney.
Renoguanylin is a peptide hormone similar to guanylin and uroguanylin that has, thus far, been only found in Japanese eels. In the eel it has been proposed that renoguanylin may be involved in osmol regulation, but this has not been proven at present. Renoguanylin was not as prominent in the kidney and intestine of the eel as guanylin and uroguanylin. Renoguanylin has not been found in mammals or humans and may be unique to the Japanese eel and fish. Lymphoguanylin is a 109 a.a polypeptide expressed in spleen and lymphoid tissues of opossum. The 109 a.a polypeptide shares 84% and 40% of its residues with prouroguanylin and proguanylin, respectively. Lymphoguanylin is less potent than uroguanylin or guanylin in intestinal bioassay, and has reduced efficacy for activation of OK-GC receptor in the kidney. 100 μM of lymphoguanylin stimulates cGMP production in renal cells only five-fold, compared to 206-fold with uroguanylin and 88-fold with guanylin. A serine-7 analog of lymphoguanylin has natriuretic properties in ex vivo rat kidneys and increases cyclic GMP 1000-fold more than the native lymphoguanylin.
The inactive precursor of uroguanylin, i.e., prouroguanylin, is delivered to the kidney as an unprocessed propeptide, and is processed to its active natriuretic form exclusively within the renal tubules. The proximal convoluted tubule is thought to be the target for the uroguanylin natriuretic response. Renal uroguanylin messenger RNA expression is also highest in proximal tubules, while guanylin is expressed mainly in the collecting ducts. Salt-loading (1% NaCl in drinking water) for three days increases uroguanylin mRNA expression by 1.8-fold, but has no effect on guanylin expression. The synthesis of these peptides by renal tubule epithelium may contribute to local control of renal function and adaptation to dietary salt.
Both guanylin and uroguanylin elicit natriuretic responses from the kidney. Both guanylin and uroguanylin exist in conformationally distinct A and B type topoisomers. Topoisomer uroguanylin B has natriuretic activity in the kidney, while the uroguanylin A in high concentration antagonizes the natriuretic action of the B form. Uroguanylin knockout mice have an impaired ability to excrete an enteral load of NaCl, primarily due to an inappropriate increase in renal Na + reabsorption. Further, there appears to be an interaction between guanylin, uroguanylin, and the cardiac hormone natriuretic peptide system, in that pretreatment with ANP (0.03 nM) enhances guanylin and uroguanylin’s natriuretic activity when ANP is present in low dose. When pharmacological doses of ANP or urodilatin are utilized they clearly inhibit uroguanylin-induced natriuresis.
The GC-C receptor with which the heat-stable enterotoxin, uroguanylin, and guanylin interact in the intestine was cloned from intestinal cDNA libraries. It exhibits 55% identity to NPR-A and NPR-B receptors in the catalytic region, 39% identity in the protein kinase domain, but only 10% identity in the extracellular region. Within the kidneys, heat-stable enterotoxin, uroguanylin, and guanylin bind chiefly to apical membranes of proximal tubule cells, also a site of CFTR expression.
Two different guanylyl cyclase signaling receptors have been identified, one in kidney (OK-GC) and one in the intestine (GC-C), that are activated by the guanylin peptides. Uroguanylin and guanylin regulate transport in mouse renal cortical collecting ducts independent of guanylyl cyclase C receptor, and in guanylyl cyclase C-receptor deficient mice renal effects are retained, strongly suggesting that GC-C is not the mediator of uroguanylin or guanylin effects in the kidney.
ANP Prohormone System and Expression in Gastrointestinal Tract
Almost 37 years ago it was noted that an oral load of sodium resulted in a natriuresis that was greater than the same amount of sodium chloride given intravenously, suggesting that the gastrointestinal tract monitors and responds to oral sodium-load. Guanylin and uroguanylin may respond to this oral sodium-load in the colon and proximal intestine, respectively, but the stomach is an earlier monitor of this sodium-load. Immunoreactive cardiac hormones, ANP prohormone, and mRNA are present in the proximal stomach and antrum. ANP prohormone gene expression and gene products LANP and ANP have been localized to the enterochromaffin cells in the lower portion of antropyloric glands of the stomach. Fasting for 72 hours in adult rats results in a significant ( p <0.05) decrease in the levels of ANP prohormone messenger RNA, and a decrease in immunoreactive long-acting natriuretic peptide and ANP to <33% of that of fed rats. In humans, food intake increases the excretion of LANP, vessel dilator, and ANP into the urine, suggesting an interaction between the cardiac hormones synthesized in the gastrointestinal tract and the kidneys. A fluid load of Coca-Cola© rapidly (in 15 minutes) decreases the excretion of LANP, vessel dilator, and ANP into the urine, allowing more of these peptides which cause a diuresis to be present to respond to the fluid load. In the stomach, cholinergic neurons inhibit, and pituitary adenylate cyclase-activating polypeptide neurons stimulate, ANP secretion, suggesting that there is also neuronal control of their secretion from the stomach. ANPs are present not only in the stomach, but throughout the gastrointestinal tract (small intestine) and colon, as opposed to guanylin and uroguanylin, which are present only in specific portions of the gastrointestinal tract. Guanylyl cyclases A and B, which ANP, BNP, and CNP interact with, are present in the gastrointestinal tract, as well as guanylyl cyclase-C which guanylin and uroguanylin enhance. ANPs also appear to have effects within the gastrointestinal track itself. ANP increases the spontaneous phasic contractions of longitudinal music two- to four-fold over a concentration range of 10 pm to 1000 mM, which was associated with a three-fold increase in cyclic GMP. Vessel dilator and LANP also increase these spontaneous place contractions, which are additive with ANP. The ANPs appear to act as neurotransmitters in the gastrointestinal tract to move water and feces through the gastrointestinal tract via increased force of contraction of the longitudinal muscles until feces reaches the anal sphincter which ANP has been shown to relax to expel the contents of the gastrointestinal tract.
Melanocyte-Stimulating Hormones
Melanocyte-stimulating hormones (MSHs) are small peptides of three different primary sequences (α-, β-, and γ-MSH), derived from the precursor prohormone pro-opiomelanocortin (POMC), which also gives rise to ACTH. Thus, MSHs, like ANPs, are from a prohormone containing four peptide hormones when proteolytically processed. Each of the MSH peptides is natriuretic when infused in experimental animals. The mechanisms of action of the MSHs are different from those of the cardiac hormones, in that MSH works via intracellular cyclic AMP rather than cyclic GMP. The MSH-induced natriuresis does not appear to be a direct effect on the kidney, but rather via an interaction with renal nerves to inhibit sodium reabsorption, as prior renal denervation completely prevents the natriuresis secondary to MSH.
Ouabain-Like Factors
Ouabain-like factors (factors that circulate and by definition inhibit Na + ,K + -ATPase) have been sought for decades. Utilizing a very sensitive radioimmunoassay to ouabain, E. P. Gomez-Sanchez et al. determined that ouabain itself does not circulate in human or rat plasma, as a peak corresponding to ouabain was not found on high pressure liquid chromatography. In most samples, they found only very low levels of an ouabain-like substance was present. As outlined previously, vessel dilator, long-acting natriuretic peptide, and kaliuretic peptide are circulating peptide hormones that inhibit the ouabain site on renal Na + K + -ATPase. ANP does not inhibit renal Na + K + -ATPase, and therefore would not be the “third factor” or an ouabain-like factor. Since the other three peptide hormones fulfill all the characteristic of the “third factor,” they may actually be the “ouabain-like factors” that have been sought. Since LANP, vessel dilator, and kaliuretic peptide circulate at 100-fold higher levels than this substance, the volume-expanded substance(s) that do the majority of inhibiting of Na + -K + -ATPase at the ouabain site on the Na + -K + -ATPase are LANP, vessel dilator, and kaliuretic peptide, rather than some substance structurally similar to ouabain which, if it circulates, is present in extremely low levels in the circulation.
Natriuretic Peptide Receptors A, B, and C
Atrial natriuretic peptides, after moving via the circulation to their respective target tissues, mediate their action(s) at the cellular level by first binding to high-affinity specific receptors on the cell surface ( Figure 37.4 ), which results in the intracellular generation of cyclic GMP via activation of the enzyme guanylyl cyclase which resides in the cytosolic domain of these membrane receptors as an integral part of these receptors. Guanylyl cyclase (also termed guanylate cyclase) catalyzes the formation of the intracellular messenger cyclic 3′,5′-guanosine monophosphate (cyclic GMP).
The area in the kidney with the most ANP-binding sites is the glomeruli, followed by proximal tubules and then inner medullary collecting ducts. With respect to ANP, BNP, and CNP receptors, cDNA cloning has shown three types of natriuretic peptide receptors (NPR): NPR-A; NPR-B; and NPR-C. Only NPR-A and NPR-B exhibit the intracellular guanylyl cyclase (GC) catalytic domain ( Figure 37.4 ), whereas the third receptor, NPR-C, contains no guanylyl cyclase domain. NPR-A and NPR-B, which bind ANP, BNP, and CNP, are structurally similar and contain a ligand-binding extracellular domain, a protein kinase-like domain, and a guanylyl cyclase domain ( Figure 37.4 ). Upon ligand biding, a change in receptor conformation allows cytosolic factors to interact with the kinase-like domain, leading to activation of guanylyl cyclase and the consequent generation of cGMP, the second messenger of the cardiac hormones. The NPR-A receptor binds ANP, BNP, CNP, and urodilatin with a rank order of selectivity being ANP=urodilatin>BNP>CNP. The order is reversed for NPR-B receptor (CNP >> ANP>BNP). NPR-B is structurally similar to the NPR-A receptor, with 74% homology at the cytoplasmic domain, but only 44% homology in the extracellular-binding domain, which may explain the difference in ligand specificities of the two guanylyl cyclase receptors.
NPR-A mRNA is expressed mainly in the kidney, in the glomeruli, renal vasculature, proximal tubules, and in the IMCD. The distribution of NPR-B overlaps to some extent with that of the NPR-A, and is found in the kidney, vasculature, and brain. In vascular endothelium and smooth muscle, NPR-B is more abundant than NPR-A. Compared to the NPR-A receptor, low levels of the NPR-B receptor are present in the kidney.
Number of ANP Receptors per Cell
The number of ANP receptors per cell varies with the cell type. Smooth muscle vasculature appears to be the target cell most richly endowed with ANP receptors. The reported number of receptors in vascular smooth muscle cells has ranged from 18,400 binding sites per cell to 500,000. Comparison of a variety of cultured cells revealed 310,000, 80,000, 50,000, 14,000, and 3,000 sites per cell for vascular smooth muscle, lung fibroblasts, adrenal cortex, aortic endothelial cells, and Leydig cells of the testis, respectively. Twelve thousand ANP-binding sites per cell have been found in kidney glomerular mesangial cells; markedly less than in other vascular areas, but the receptors in the mesangial cells exhibited as high affinity as other vascular areas for ANP.
Inverse Relationship of Change in Number of ANP Receptors with Circulating ANP Concentrations
The number of ANP receptors varies with fluid status, and inversely with the circulating ANP concentration. Deprivation of water decreases the circulating ANP concentration and augments receptor number in both kidney and adrenal gland. Rats fed a low-salt diet for 2 weeks exhibit “upregulation” of glomerular ANP receptor density, whereas animals fed a high-salt diet have a decreased receptor density. The decrease in total ANP receptors, at least after salt-loading, is due exclusively to a decrease in the NPR-C receptor, rather than the NPR-A receptor.
Structure of NPR-A Receptor
The structure of the NPR-A receptor is illustrated in Figure 37.5 . This structure was elucidated using complimentary DNA (cDNA) encoding a 115 kDa human natriuretic peptide receptor (NPR)-active (A or functional) receptor that possesses guanylyl cyclase activity. The NPRA receptor has 1029 a.a., a 32 a.a. signal sequence followed by at 441 a.a. extracellular domain (i.e., projecting from the cell). This extracellular portion of the NPR-A receptor is 33% homologous to the 60 kDa NPR-C receptor. This extracellular portion of the receptor is the binding site for ANP, BNP, and CNP. A 21 a.a. transmembrane portion of the receptor “anchors” the receptor to the membrane. Inside the cell (intracellular domain) there is a 568 a.a. cytoplasmic portion of the receptor with homology (23%) to the protein kinase family (protein kinase domain) being next to the membrane, followed by a large guanylyl cyclase catalytic portion of the receptor ( Figure 37.5 ) that is 42% homologous to cytoplasmic guanylyl cyclase. The kinase domain binds ATP, but lacks true kinase activity. Rather, it functions to inhibit the guanylyl cyclase domain. If the kinase domain is “knocked-out,” guanylyl cyclase continuously functions. The molecular weight of this receptor is 114,426.
The guanylyl cyclase portion of human and rat NPR-A receptors are 90% identical throughout their sequences. The similar amino acid sequence between the NPR-C receptor and the extracellular portion of the NPR-A receptor reflect a common function shared between them – they both bind ANP, BNP, and CNP.
NPR-C (Clearance) Receptor
Cross-linking studies revealed that in addition to the high molecular weight receptors for ANP, BNP, and CNP there was also a low molecular weight 60 kDa receptor that appeared to be a subunit of the high molecular weight receptor. This 60 kDa receptor was found not to contain guanylyl cyclase or to mediate any of the known effects of ANP, such as natriuresis or diuresis.
Structure of NPR Clearance Receptor
The NPR-C receptor is similar structurally outside the cell to the NPR-A receptor, with 496 a.a. compared to the 441 a.a. projecting from the cell for the NPR-A receptor. They have a similar single short transmembrane-spanning region, but where the two receptors markedly differ is inside the cell. The NPR-C receptor has only a very short 37 a.a. tail into the cytoplasm of the cell as compared to the large 568 a.a. portion in the cell for the NPR-A receptor. Neither the protein kinase domain nor the guanylyl cyclase catalytic site is present in the NPR-C receptor. That the NPR-C receptor is not linked to a second messenger system explains its inability to cause vasodilation, diuresis or natriuresis. The order of binding to the NPR-C receptor is ANP>CNP>BNP. The NPR-C receptor is the most abundant receptor of the natriuretic receptors, accounting for more than 95% of the total receptor population, and is located at high density in kidney, vascular endothelium, smooth muscle cells, and the heart.
Vessel Dilator and LANP Receptors
Vessel dilator and long-acting natriuretic peptide do not bind to the NPR-A, B or C receptors, but rather have their own specific receptors. Vessel dilator, LANP, and kaliuretic peptide, on the other hand, are linear peptide hormones, and one would not expect binding to the above NPR-A, -B, and -C receptors which require a ring structure for binding which ANP, BNP, and CNP have.
LANP Receptor
Long-acting natriuretic peptide (LANP), as well as ANP, binds specifically to smooth muscle membranes, placental membranes, distal nephrons, proximal tubules, and renal cortical and medullary membranes. ANP and vessel dilator inhibit 125 I-labeled LANP-binding somewhat at concentrations above which these peptide hormones are known to circulate, i.e., 10 −6 M for ANP and 10 −7 M for vessel dilator. Scatchard analysis of the LANP-binding data resulted in a straight line, suggesting that these smooth muscle cells contain a single class of high-affinity binding sites for LANP with an equilibrium dissociation constant (K d ) of 0.11 nM. The binding capacity (maximal binding, B max ) for LANP was 2.57 fmol/10 6 cells, and the number of binding sites was calculated to be 1548 per cell.
Vessel Dilator Receptor
Vessel dilator also binds specifically to smooth muscle membranes, proximal tubules, distal nephrons, placental membranes, and renal cortical and medullary membranes at a site distinct from the binding of ANP to membranes. The binding of this peptide hormone could be inhibited by concentrations (10 −4 to 10 −7 M) of ANP, LANP, insulin, and ACTH, which are far in excess of their respective circulating concentrations. Scatchard analysis of the vessel dilator-binding data resulted in a straight line, suggesting that smooth muscles contain a single class of high-affinity binding sites for vessel dilator with an equilibrium dissociation constant (K d ) of 4 nM. B max for vessel dilator was 59.9 fmol/10 6 cells, and the number of binding sites was calculated to be 36,087 per cell.
Degradation of Natriuretic Peptides by Kidney
The inactivation of the ANP, BNP, and CNP occurs via two pathways: binding to clearance receptors and enzymatic degradation. The clearance receptor (NPR-C) clears ANP, BNP, and CNP through receptor-mediated uptake, internalization, and lysosomal hydrolysis with rapid and efficient recycling of internalized receptors to the cell surface. Enzymatic degradation of ANP, BNP, and CNP takes place in the lung, liver, and kidney, and the main enzyme responsible for this degradation is neutral endopeptidase (NEP-24.11). NEP, originally referred to as enkephalinase because of its ability to degrade opioid peptides in the brain, was subsequently shown to be identical to a well-characterized zinc metallopeptidase present in the kidney. This zinc metalloproteinase hydrolyzes internal peptide bonds of polypeptides, rather than those adjacent to their N- or C-terminal ends. NEP has a ubiquitous tissue distribution and multiple functions, sharing structural similarities with various metallopeptidases, including aminopeptidase ACE, and carboxypeptidases A, B, and E. NEP is most abundant in the brush borders of the proximal tubules of the kidney, where it rapidly degrades filtered ANP, thus preventing ANP from reaching more distal luminal receptors. In the case of ANP, NEP-24.11 cleaves the Cys 105 –Phe 106 bond to disrupt the ring structure and inactivate the peptide. NEP 24.11 is a nonspecific enzyme that also cleaves enkephalins, endothelin, substance P, kinins, neurotensin, insulin B chain, angiotensin, calcitonin gene-related peptide, and adrenomedullin, as well as ANP, BNP, and CNP. With respect to ANPs in humans, ANP and CNP are preferred substrates for NEP as opposed to BNP with the Cys–Phe bond of human BNP being relatively insensitive to enzymatic cleavage.
Influence of Acute Renal Failure on Circulating Concentration of Cardiac Hormones
Each of the cardiac hormones from the ANP prohormone (vessel dilator, ANP, LANP, and kaliuretic peptide), BNP, and CNP increase in the circulation (mainly from the heart ventricle ) in salt- and water-retaining states such as renal failure and congestive heart failure, compared to their concentrations in healthy individuals, in an apparent attempt to overcome the salt and water retention via their natriuretic and diuretic properties. The disease state associated with the highest circulating concentrations of the cardiac natriuretic peptides is renal failure. One would suspect that cardiac natriuretic peptides are higher in renal failure versus Class IV New York Heart Association congestive heart failure patients, because of the added pathophysiology of decreased degradation of these peptides with the decreased functioning renal parenchyma. Franz et al., however, have shown that there is an increased excretion of the cardiac natriuretic peptides in renal failure, and that the increase in vessel dilator excretion occurs even before serum creatinine levels begin to rise. The circulating concentrations of the cardiac hormones in chronic renal failure appear to reflect volume status. Despite increased circulating concentrations of cardiac hormones in sodium-retaining disease states, the kidney retains sodium and is hyporesponsive to ANP, LANP, and BNP. The mechanism for the attenuated renal response to these natriuretic peptides is multifactoral and includes renal hypoperfusion, activation of the renin–angiotensin–aldosterone and the sympathetic nervous systems.
Influence of Renal Failure on other Natriuretic Peptides
Adrenomedullin, guanylin, and uroguanylin increase in renal failure and/or experimental nephrotic syndrome.
Hemodialysis
Cardiac Hormones Synthesized by ANP Prohormone Gene
The circulating concentrations of the cardiac hormones have been suggested as possible indicators of when to perform dialysis in persons with chronic renal failure. Other data, however, suggest that ANPs are not useful to predict when hemodialysis is necessary. Hemodialysis lowers the circulating concentration of cardiac hormones by 34%–42%, with the amount of decrease appearing to be related to volume status of the patients. Hemodialysis does not return the levels of ANP to those of healthy adults, and it does not reduce circulating concentrations of vessel dilator and LANP. Part of the reason for the difference in hemodialysis effects on the cardiac hormones is that less than 1.5% of vessel dilator and LANP cross the dialysis membrane, compared to 15% to 25% of ANP crossing hemodialysis membranes. Hemodialysis using cellulose-triacetate dialyzers reduces plasma levels of these peptides in acute renal failure more than hemodialysis therapy with polysulfone dialyzers.
BNP
Hemodialysis has been reported both to lower, and to have no effect on circulating BNP levels. Before dialysis in persons with chronic renal failure (CRF), plasma BNP levels have no relationship to serum creatinine or mean blood pressure. In CRF patients whose plasma BNP levels decrease with dialysis, this decrease correlates with the degree of postural blood pressure drop, but there is no correlation with the fall in serum creatinine. In none of the studies of BNP and dialysis has BNP ever returned its circulating concentration to that of healthy individuals. With volume repletion after hemodialysis there is an exaggerated release of ANP, but changes in BNP are small and without any correlation with either atrial or ventricular volume.
Adrenomedullin and Proadrenomedullin N-Terminal 20 Peptide
Both adrenomedullin and proadrenomedullin N-terminal 20 peptide (PAMP) increase in chronic renal failure. Hemodialysis decreases these peptides to near-control levels, with PAMP being 2.17±0.18 fmol/ml versus 1.64±0.12 fmol/ml for controls.
Guanylin and Uroguanylin
Both guanylin and uroguanylin are increased in persons with impaired renal function. Hemodialysis with EVAL membranes decreases guanylin concentrations after one hour of dialysis, but the plasma levels after hemodialysis with PC membranes show no change.
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