The major physiologic determinants of single nephron glomerular filtration rate (GFR) are glomerular plasma flow (Q_A), glomerular transcapillary hydraulic pressure (P_GC), and the ultrafiltration coefficient (K_f).¹ These variables are determined, in part, by the contractile state of the afferent arteriole, efferent arteriole, and mesangial cells. K_f also varies with alterations in the hydraulic permeability of the capillary filtration barrier, which consists of endothelial cells, visceral epithelial cells, and the glomerular basement membrane. By binding to specific receptors on cellular and structural components of the glomerulus, circulating and locally produced hormones influence one or more of the physiologic determinants of GFR. Figure 8.1 summarizes the effects of different hormones on pregomerular, glomerular, and postglomerular contractility. Because the same substance can act at different sites in the glomerular unit, the net effect on GFR will depend on whether its actions are antagonistic or complementary. For example, atrial natriuretic peptide (ANP) decreases afferent arteriolar resistance, whereas increasing efferent arteriolar resistance results in an augmented P_GC and a rise in GFR.² Under certain conditions, ANP also increases K_f, which, in turn, contributes to the enhancement of GFR.³ On the other hand, angiotensin (Ang) II-mediated constriction of both afferent and efferent arterioles results in opposite effects on glomerular plasma flow and P_GC and, therefore, no change in single nephron GFR.⁴ Regulation of glomerular hemodynamics is further complicated by multiple interactions between hormones in the kidney. For example, infusion of Ang II along with a cyclooxygenase inhibitor causes a significant decrease in single nephron GFR, suggesting that endogenous prostaglandin production antagonizes the glomerular effects of Ang II.⁵ The net effects of renally relevant hormones on GFR are discussed in more detail later in this chapter.

Renal tubule cells express receptors for many circulating and locally synthesized hormones. The effects of a particular hormone on water or electrolyte transport are partly determined by the differential distribution of its receptor on functionally specialized segments of the renal tubule. For example, arginine vasopressin (AVP) binds almost exclusively to principal cells in the collecting tubule (CT), where it influences the physiologic functions of this segment, primarily water and urea absorption.⁶ Parathyroid hormone (PTH), on the other hand, exerts its biologic actions on renal tubule segments proximal to the collecting duct, where it modulates calcium, phosphate, magnesium, sodium, and bicarbonate transport.⁷ Table 8.1 summarizes the net effects of renally relevant hormones on water and solute handling in different tubule segments. Each hormone is discussed in detail in the sections that follow. As in the glomerulus, hormones may modulate their own actions by altering the production of counterregulatory hormones. For example, AVP induces local synthesis of prostaglandin E (PGE), which opposes the effect of AVP on water permeability in the CT.⁸

AVP is a 9-amino acid neuropeptide synthesized by magnocellular neurons in the paraventricular and supraoptic nuclei of the hypothalamus.¹¹ The AVP gene, located on chromosome 20, consists of three exons that encode the signal peptide, AVP, neurophysin II (NPII), and a glycopeptide.¹⁰ After cleavage of the signal peptide within the ER, the resulting prohormone is folded and packaged into secretory granules in neuronal bodies. These granules are then transported along the axons to nerve terminals in the posterior pituitary (neurohypophysis).¹² During axonal transport, further processing of the prohormone within secretory granules yields AVP, neurophysin II (NpII), and a glycopeptide. Interestingly, mutations of NpII impair AVP secretion, suggesting that NpII assists in the processing or secretion of AVP.¹³ In addition to the posterior pituitary, AVP synthesis has been detected in the pancreas, adrenal gland, ovary, testis, and regions of the brain.¹⁴ Its physiologic function in these sites, however, remains to be clarified.

The most sensitive stimulus for AVP secretion into the bloodstream is increased plasma osmolality. Osmosensitive neurons that respond to changes in plasma osmotic pressure by varying their intracellular water content have been identified in the anterior hypothalamus, specifically in the organum vasculosum of the lamina terminalis (OVLT) and the subfornical organ (SFO).¹⁵ When stimulated by osmosensitive neurons, the magnocellular neurons release the stored AVP into the posterior pituitary (an area that lacks a blood-brain barrier) and AVP enters the general circulation. As little as 1% change in plasma osmolality leads to a change in AVP concentration that is sufficient to modify renal water excretion.¹⁶ AVP secretion is almost completely suppressed when plasma osmolality decreases below an average of 280 mOsm per kg of water in humans.¹⁷

Secretion of AVP is also influenced by alterations in intravascular volume and blood pressure, sensed by baroreceptors located in the heart, aortic arch, and carotid sinus.¹⁸ These signals are transferred through the vagal nerves to the
nucleus solitarius in the brainstem, from which postsynaptic pathways project to the magnocellular neurons. Whereas a 5% to 8% decrease in blood volume or systemic arterial pressure has little effect, further hemodynamic compromise leads to a steep increase in circulating AVP levels. Significant reductions (10%-30%) in circulatory arterial volume or blood pressure can override osmoregulation and result in markedly increased AVP levels in the face of decreased plasma osmolality.¹⁹ Other less potent stimuli for AVP secretion include fever, emesis,²⁰ and oropharyngeal osmoreceptors.²¹

AVP circulates in the plasma nearly in an unbound form. Levels of circulating AVP depend on both the rate of AVP release from the posterior pituitary and the rate of AVP degradation. As discussed, the major factor controlling AVP release is plasma osmolality. The liver and the kidney both contribute to the breakdown of AVP and the decline in AVP levels when secretion ceases. In fact, the half-life of AVP in the circulation is 18 minutes due to rapid clearance by hepatic and renal vasopressinases.²² Under physiologic conditions, plasma vasopressin concentrations vary with serum osmolarity between 0 to 5 pg per mL.²²

It is noteworthy that AVP levels are difficult to measure in plasma because of the instability of this peptide and the low sensitivity of available AVP antibodies. Copeptin (or C-terminal proarginine vasopressin, CT-proAVP) is the C-terminal part of AVP, which is secreted stoichiometrically with AVP in a manner similar to C-peptide and endogenous insulin.²³ CT-proAVP provides a reliable means for estimating prevailing AVP levels in the circulation, thus facilitating the study of AVP in human diseases. A recent cohort study in renal transplant patients suggests that high CT-proAVP strongly correlates with a negative renal prognosis. In fact, in this study, the plasma concentrations of CT-proAVP predicted renal function loss over a 3.2-year follow-up.²⁴

AVP exerts its biologic actions through three specific cell-surface AVP receptors identified as V₁R (also called V_1aR), V₂R, and V₃R (also V_1bR).²⁵ These receptors belong to the G protein coupled receptor superfamily (Table 8.2). In the human kidney, mRNA for V₁Rs predominates in cortical collecting ducts (CCD), gradually decreasing as the collecting duct enters the medulla.²⁶ In addition, whereas V₁R mRNA is diffusely expressed in the CCD, it is restricted to the intercalated cells in OMCD. V₁Rs are responsible for mediating vascular smooth muscle cell vasoconstriction by activating G protein-dependent phospholipase C (PLC) and the downstream effectors, diacylglycerol (DAG), and inositol 1,4,5-triphosphate (IP₃). In turn, DAG stimulates protein kinase C (PKC), whereas IP₃ increases cytosolic Ca²⁺, thus
initiating the second-messenger cascade responsible for the cellular actions of AVP.²⁵ Stimulation of V₁R, although not directly involved in control of tubular water and electrolyte transport, increases sodium excretion because of the influences on blood pressure, effective arterial circulating volume, glomerular filtration rate, and circulation in the vasa recta system.²⁷,²⁸ Additional biologic effects of AVP mediated through V1Rs include platelet aggregation²⁹ and increased glycogenolysis and gluconeogenesis in the liver.³⁰

The V₃R (V_1bR), also coupled to PLC signaling, is present on neurons in the anterior pituitary (adenohypophysis) and is thought to mediate AVP-induced corticotropin secretion.²⁵,³¹,³² They are also found elsewhere in the brain, especially in the pyramidal neurons of the hippocampal CA2 field, in which they mediate fundamental physiologic actions such as memory and body temperature control as well as social behavior.³³ In addition, this receptor has been localized to pancreatic islet cells, modulating insulin secretion.³⁴ However, its presence and role at the level of the medulla in rat kidney remain unclear.

V₂Rs, on the other hand, are heavily expressed in the medullary TAL, macula densa (MD), connecting tubule, and cortical and medullary collecting duct, as well as weakly expressed in cortical thick ascending limb (TAL) and distal convoluted tubule.³⁵ These are the best characterized and studied vasopressin receptors. By binding to V₂R, AVP increases water reabsorption through multiple mechanisms.³¹ Activation of V₂R results in increased cyclic adenosine monophosphate (cAMP) levels and activation of PKA, which promotes insertion of water channels into the luminal suface of the epithelial tubular cells.³⁶ This ultimately mediates the antidiuretic effect of AVP by allowing back diffusion of water down its concentration gradient.³⁷ In addition, V₂Rs modulate sodium reabsorption through the epithelial Na⁺ channel (ENaC) across principal cells.³⁸,³⁹,⁴⁰,⁴¹ This facilitates free water reabsorption by supporting the axial corticomedullary hyperosmotic gradient. It was also recently shown in rats that AVP modulates sodium reabsorption even in the distal convoluted tubule by acting on the thiazide-sensitive Na⁺-Cl^– cotransporter (NCC).⁴² NCC is important in defining sodium delivery to the collecting duct,
which is necessary for ENaC activity. Finally, V₂Rs activate urea transporters, such as UTA1, in the distal nephron.⁴³,⁴⁴,⁴⁵ This increase in urea reabsorption and recycling maximizes sodium reabsorption in the TAL by supporting the axial hyperosmotic gradient drawing water from the distal nephron.⁴⁶ The clinical importance of the V₂R in water balance disorders is underlined by the current use of V₂R antagonists in a clinical setting (see later).

It is noteworthy that AVP also binds to two nonspecific receptors: oxytocin receptors and P2 purinoreceptors. Oxytocin receptors are found in the breast, ovary, uterus, and hypothalamus. Since the affinity of this receptor for vasopressin is relatively low, the clinical effects of this hormone are limited under physiologic conditions.⁴⁷ AVP may also act on P2 purinoreceptors in the heart, causing coronary vasoconstriction and contributing to the reduction in cardiac output.⁴⁸

AVP is implicated in major clinical syndromes of alterations in water metabolism, namely nephrogenic diabetes insipidus (NDI) and the syndrome of inappropriate secretion of antidiuretic hormone (SIADH). In addition to water metabolism, it has been recently suggested that AVP may play a role in the initiation and the progression of chronic kidney disease (CKD) and in the most prevalent form of hereditary renal disease, namely the adult polycystic kidney disease (Fig. 8.2).⁵⁹

NDI is characterized by impaired AVP-induced water reabsorption, resulting in polyuria and polydipsia.⁶⁰ If water intake is inappropriate, patients with NDI may fail to thrive, suffer from mental retardation, and die early. NDI can be acquired such as following lithium treatment, hypokalemia, hypercalcemia, or ureteral obstruction, all of which lead to downregulation of AQP2.⁶¹,⁶²,⁶³,⁶⁴ NDI can also be inherited (congenital).⁶⁵ In fact, two gene mutations have been linked to congenital NDI: one in the AVPR2 gene encoding V₂R (X-linked NDI) or mutations in the AQP2 gene (autosomal recessive or autosomal dominant NDI). More than 90% of patients with congenital NDI suffer from X-linked NDI. In both cases, patients cannot concentrate their urine despite normal or elevated plasma concentrations of vasopressin, leading to a massive loss of water through the kidney. So far, NDI is managed by salt restriction combined with hydrochlorothiazide diuretics to reduce urine output.⁶⁶ However, no cure is available so far.

Abnormal water handling of central origin includes SIADH in which AVP levels are abnormally elevated and not suppressed when plasma osmolality concentration falls below the osmotic threshold for physiologic AVP secretion.⁶⁷ SIADH is characterized by impaired water excretion in the absence of renal insufficiency, adrenal insufficiency, or any recognized stimulus for AVP secretion. Renal water retention and extracellular fluid expansion are compensated for by increased urinary Na⁺ excretion leading to life-threatening hyponatremia. The most common causes of the syndrome of inappropriate AVP secretion are neoplasia, neurologic disorders, congestive heart failure, liver cirrhosis, preeclampsia, and drugs such as thiazide diuretics or selective serotonin reuptake inhibitor antidepressants.⁶⁷ Four patterns of AVP dysregulation in patients with SIADH have been observed.⁶⁸ The most common pattern (in ˜40% of patients with SIADH) is the excessive and unregulated release of AVP, which is unrelated to plasma osmolality. In the second most common pattern (˜30% of patients), referred to as “reset osmostat,” AVP release continues to regulate water excretion at a lower plasma osmolality set-point. Although most tumors (e.g., lung carcinoma) manifest the first type of SIADH, some also present with the second type, thus the pattern of abnormal AVP secretion cannot be utilized to predict the cause of SIADH. A third rare pattern is characterized by an inability to stop AVP secretion at low plasma osmolalities, but the osmoregulation of AVP is otherwise normal. This pattern may be due to dysfunction of inhibitory hypothalamic neurons, leading to persistent low-grade basal AVP secretion. In the fourth pattern, a rare clinical picture of SIADH, the normal osmoregulation of AVP secretion is not altered (˜10% of patients) but AVP levels are low or undetectable. It is thought that a nephrogenic SIADH
(NSIADH) may be responsible for this picture. In fact, in some children, it appears to be due to an activating mutation of V₂R. In other patients, it may be due to abnormal control of aquaporin-2 water channels in renal collecting tubules or production of an antidiuretic principle other than AVP.⁶⁹ In a study with SIADH patients, treatment with V₂R antagonists, commonly referred to as vaptans, increased serum Na⁺ concentration and decreased its excretion. Tolvaptan has been recently approved in the United States and Europe for the treatment of hyponatremia associated with SIADH, as well as cirrhosis and congestive heart failure. Recently, a dual vasopressin V₁R and V₂R antagonist, conivaptan, improves hyponatremia in rats with SIADH, suggesting a therapeutic potential for conivaptan in the treatment of SIADH.⁷⁰

AVP has also been shown to modify vascular tone in renal microvessels. Short-term infusion of AVP does not alter either renal blood flow or the GFR.⁷¹ Alternatively, chronic AVP administration increases the intraglomerular capillary pressure and GFR through tubuloglomerular feedback.⁷² In addition, a sustained increase in water intake and the consequent AVP suppression reduce proteinuria and the severity of glomerular and tubular damage in 5/6 nephrectomized rats.⁷³,⁷⁴ Thus, it seems likely that a chronic AVP-induced hyperfiltration may alter the glomerular barrier and start a series of events leading to enhanced protein loss and glomerulosclerosis. An increase in urinary albumin excretion represents an early predictor of glomerular damage in diabetes mellitus and a risk factor for cardiovascular complications in hypertension. Studies show that the Brattleboro rat, a model of central diabetes insipidus with complete lack of AVP, is protected from hyperfiltration, albuminuria, and renal hypertrophy after streptozotocin-induced diabetes mellitus.⁷⁵ This suggests that AVP plays a role in hyperfiltration and glomerular damage induced by diabetes. These observations are also of relevance to humans. A marked increase in AVP plasma levels is well documented in diabetes mellitus.⁷⁶ AVP through V₁R induces contraction of cortical efferent, but not afferent, arterioles. Administration of a V₁R selective antagonist to noninsulin-dependent diabetic patients modestly reduces albuminuria partly by decreasing intraglomerular capillary pressure.⁷⁷ Thus, although V₁Rs (but not V₂Rs) are downregulated in diabetes mellitus, they could mediate part of the increase in albumin excretion. Interestingly, administering desmopressin, a selective V₂R agonist, to healthy humans and patients with central diabetes insipidus significantly increases urinary albumin excretion, but this effect is absent in those with hereditary nephrogenic diabetes insipidus secondary to V₂R mutations.⁷⁸ These findings suggest that the AVP-induced rise in albuminuria depends on V₂Rs. This is further confirmed by the observation that plasma copeptin levels correlate with microalbuminuria
in the PREVEND study.⁷⁹ However, V₂ receptors have not been found in glomeruli or proximal tubules, suggesting indirect effects. Recent evidence suggests a strong interaction between AVP and the renin-angiotensin system (RAS).⁸⁰,⁸¹ In fact, chronic RAS blockade by angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers (ARBs) prevents the desmopressin-induced albuminuria, indicating that RAS mediates the effects of AVP on glomerular hemodynamics.⁸² Recent studies using V₁R knockout mice show that AVP regulates body fluid homeostasis and the GFR by activating RAS through V₁Rs in MD cells, and subsequently the V₂R-aquaporin 2 system.⁸³ Simultaneous AVP and RAS blockade may represent a good therapeutic approach for delaying renal disease progression.

Another interesting observation is that mesangial cells express V₁Rs, which mediate AVP-induced cell contraction.⁸⁴ In addition, prolonged exposure to AVP promotes mitogenesis and proliferation of these cells, which ultimately leads to an increased accumulation of extracellular matrix, a pathologic feature found in various glomerular diseases.⁸⁵,⁸⁶ In fact, addition of AVP to cultured mesangial cells increases in a dose-dependent manner the synthesis and release of matrix proteins, such as type I and IV collagen, fibronectin, and transforming growth factor β.⁸⁷ In addition, AVP inhibited the synthesis of matrix metalloproteinase (MMP)-2, which degrades matrix proteins including type IV collagen, and stimulated endothelin (ET)-1 secretion from mesangial cells, another mitogenic factor.⁸⁸

In the remnant kidney model in rat, a model of progressive CKD in humans, V₁R antagonists (but not V₂R) prevent proteinuria and glomerulosclerosis in the initial phases of disease, but have limited effectiveness in established renal damage.⁸⁹,⁹⁰ Similar observations were reported in 5/6 nephrectomized, salt-loaded spontaneously hypertensive rats (SHRs), in which the increase in urinary protein excretion and the progression of nephrosclerosis were attenuated with a V₁R antagonist but not with a V₂R antagonist.⁹¹

AVP has also been linked to autosomal dominant polycystic disease (ADPKD), an inherited disorder characterized by the development within renal tubules of innumerable cysts that progressively expand to cause renal insufficiency.⁹² It has been shown that 3′-5′-cyclic adenosine monophosphate (cAMP) stimulates tubule cell proliferation and transepithelial fluid secretion, both of which contribute to enlarge renal cysts.⁹³ AVP operates continuously in ADPKD patients to promote cAMP production in the distal nephron and collecting ducts via V₂Rs, thereby contributing to cyst enlargement and renal dysfunction.⁹⁴ Studies in animal models of ADPKD provide compelling evidence that blocking AVP’s actions dramatically improves disease progression.⁹⁵,⁹⁶ This prompted the initiation of an international clinical trial⁹⁷ testing the efficacy of tolvaptan, a V₂R inhibitor, in the treatment of ADPKD.⁹⁸ Alternatively, a recent review discusses that the impact of simply increasing the amount of solute-free water drunk evenly throughout the day by patients with ADPKD on decreasing plasma AVP concentrations and mitigating the actions of cAMP on the renal cysts.⁹⁹

In conclusion, in recent years, AVP has been implicated in the initiation and progression of many kidney diseases, playing a role beyond water metabolism. Further studies are required to determine whether AVP antagonists and/or AVP suppression by a high water intake can be useful for the treatment of nephropathies, from ADPKD to diabetic and nondiabetic CKD.

The discovery of renin goes back to 1898, when the Finnish physiologist Robert Tigerstedt and his student Per Gunnar Bergman found that extracts of rabbit renal cortex had a slowly developing and sustained pressor effect. Based on its origin, they named this substance renin.¹⁰⁰ This effect was not observed with extracts of renal medulla and persisted despite removal of sympathetic activation. Subsequently, efforts to verify these experiments were unsuccessful until the 1930s when Harry Goldblatt and his colleagues demonstrated that clamping the renal artery in dogs produced chronic hypertension.¹⁰¹ This work converged with other subsequent experiments performed by leading scientists such as Juan Fasciolo and Bernardo Houssay to suggest the presence of a vasoactive substance produced by the kidney, other than renin.¹⁰²,¹⁰³ This substance was later isolated from the blood and was named hypertensin. Based on their physiologic properties, renin and hypertensin were clearly two different compounds, but the relationship between them was not established. Renin was later identified by an Argentine group as a proteolytic enzyme that acts on a plasma constituent to produce hypertensin as the final product of the enzymatic reaction.¹⁰⁴ Subsequent research led to the characterization of the components of the renin-angiotensin system and Braun-Menéndez and Page gave the final nomenclature of the whole enzymatic system in 1958: the renin substrate was named angiotensinogen, hypertensin renamed angiotensin, and the enzymes that metabolize angiotensin were named angiotensinases.¹⁰⁵,¹⁰⁶

Ang II in the systemic circulation is produced primarily from circulating angiotensinogen through the proteolytic action of renin. Most of the circulating renin is derived from the kidney, although other organs can secrete prorenin in the circulation. Angiotensinogen is mostly formed and secreted in the circulation by liver cells, allowing for systemic formation of angiotensin peptides, principally Ang II. The circulating concentrations of angiotensinogen are more than 1,000 times greater than those of Ang I and Ang II, therefore changes in plasma renin activity is the major determinant of Ang I formation (Fig. 8.3). Renin secretion is stimulated by various dynamic parameters such as renal perfusion pressure, tubular fluid sodium chloride concentration at the MD, sympathetic discharge to the kidney, and endocrine and paracrine hormones and growth factors.¹⁰⁷

Stimulation of renin release by juxtaglomerular cells is mediated by increased intracellular cAMP, whereas a rise in cytosolic free calcium is inhibitory.¹⁹⁴ Renin release responds inversely to changes in renal perfusion pressure.¹¹⁴ Elevation of intrarenal arterial pressure inhibits renin release and induces a “pressure” natriuresis. At least two mechanisms have been postulated. Increased afferent arteriolar wall tension secondary to increased renal perfusion elevates intracellular calcium in juxtaglomerular cells and inhibits renin secretion.¹¹⁴,¹⁹⁴ Increased perfusion pressure also stimulates NO production and release by endothelial cells. NO, in turn, suppresses renin secretion.¹⁹⁵,¹⁹⁶ Conversely, decreased renal perfusion results in increased production of prostacyclin (prostaglandin I₂), which enhances renin release.¹⁹⁷ Mechanical strain leads to upregulation of the AT1 receptor and increased Ang II production in conditionally immortalized podocytes. The resulting activation of a local tissue angiotensin system leads to an increase in podocyte apoptosis, mainly in an AT1 receptor-mediated fashion.¹⁹⁸

Decreased NaCl delivery to MD cells stimulates renin secretion, whereas increased urinary NaCl exerts an opposite effect.¹⁸⁰ Schlatter and coworkers¹⁹⁹ demonstrate that changes in luminal Cl^– concentration alter the rate of Na⁺-K⁺-2Cl^– transport in MD cells.¹⁹⁹ The precise mechanism by which variation in the activity of this transporter translates to a signal that regulates renin release by adjacent juxtaglomerular granular cells is not entirely clear. Postulated mediators include adenosine, which inhibits renin secretion via activation of A₁ receptors on juxtaglomerular cells, and alterations in interstitial osmolality, which may affect renin secretion directly.⁹⁶ Experimental evidence also suggests that NO produced by MD cells and endothelial cells regulates renin secretion.¹⁹⁵,²⁰⁰

The importance of renal sympathetic innervation in controlling renin secretion is well recognized.²⁰¹ Stimulation of postjunctional β-adrenergic receptors increases renin release. The role of α-adrenergic receptors, on the other hand, is controversial.²⁰¹ Ample evidence suggests that dopamine stimulates renin secretion by direct activation of dopamine A1 (DA₁) receptors on juxtaglomerular cells.²⁰¹,²⁰²

Several endocrine and paracrine hormones regulate renin secretion by the kidney. ANP has been shown to inhibit renin release from isolated juxtaglomerular cells.²⁰³ Other inhibitory hormones include AVP, endothelin, and adenosine (A₁-receptor agonists).¹¹⁴,¹⁹⁵ Regulation of renin secretion by Ang II is probably the most physiologically relevant.¹⁶⁹ Ang II inhibits renin secretion and renin gene expression in a negative feedback loop. Treatment of transgenic mice bearing the human renin gene with an ACE inhibitor increases renin expression in the kidney by five- to tenfold.²⁰⁴ Similarly, ACE inhibition in rats augments renal renin mRNA expression, an effect that is reversed by infusion of Ang II.²⁰⁵ The effects of Ang II are believed to be direct and not dependent on changes in renal hemodynamics or tubular transport. Arachidonic acid metabolites produced in the kidney also play an important role in renin secretion.¹⁹⁵ Intrarenal infusion of arachidonic acid increases (and indomethacin decreases) plasma renin activity in rabbits.²⁰⁶ Several studies have since confirmed that prostaglandins of the I series are potent stimulators of renin secretion.¹⁹⁵,¹⁹⁷ On the other hand, lipoxygenase products of arachidonic acid metabolism (12-HPETE, 15-HPETE, and 12-HETE) and cytochrome P450-mediated epoxides (14,15-epoxyeicosatrienoic acid) have been shown to inhibit renin release in renal cortical slices.²⁰⁷,²⁰⁸

The renin-angiotensin system has been characterized as an endocrine, paracrine, and autocrine system. Contribution of systemically formed mediators to local control of dynamics within tissues is difficult to delineate. Recent evidence suggests that local formation is a major determinant of Ang levels in organs and tissues. In the brain, for example, Ang peptide levels are regulated in an autonomous manner.²⁰⁹ Local renin-angiotensin systems have been identified in several organs, including the kidney, heart, vasculature, brain, and adrenals.²¹⁰ Although most organs have elements of the renin-angiotensin system, the adult kidney is unique in expressing all the components of the system.¹⁵⁶,¹⁶⁹

Compared with plasma levels, the renal Ang II contents are much higher despite suppression of renin secretion and release.¹²⁶ Renin is principally produced by juxtaglomerular cells of the distal afferent arteriole, but has been shown to be expressed in the proximal tubule cells,¹²⁶ whereas its substrate, angiotensinogen, is expressed by proximal tubule cells. ACE activity in the kidney has also been localized to the proximal tubule, with the highest concentration present on the brush border. Several studies have provided evidence for production of Ang II by the kidney, mainly concentrated in the proximal tubules,¹²⁶ suggesting that the intrarenal renin-angiotensin
system is, indeed, functional.¹⁵⁶ Some investigators have proposed that this local system plays a role in proximal tubule NaCl and HCO₃ absorption, pathogenesis of essential hypertension, and expression of the phenotype of autosomal dominant polycystic kidney disease.¹⁵⁶ Independent regulation of renal Ang II production has not been definitively demonstrated. Circulating Ang II stimulates renal angiotensinogen mRNA production and intact urine angiotensinogen suggests its presence along the whole nephron and that renin and ACE activity are available all through the nephron.¹²⁶

Endogenous Ang II in both peritubular blood and luminal fluid is important for maximal expression of the stimulatory influence of this peptide on proximal tubule fluid uptake.²¹¹ Intraluminal conversion of Ang I to Ang II can occur in the cortical collecting duct, resulting in enhanced apical sodium entry.¹⁶⁵

Renal degradation of Ang II is constitutively high, unaffected by chronic levels of arterial blood pressure, and is independent of long-term changes in levels.²¹²

Low-density lipoproteins (LDLs) increase Ang II production by mesangial cells, which, in turn, results in increased O₂ production, cell proliferation, and hypertrophy—these effects of Ang II are mediated by the AT₁ receptor.²¹³

The RAAS has been implicated in the pathophysiology of several diseases of the cardiovascular system and the kidney, mostly hypertension and renal injury. The local renal renin-angiotensin system is activated in the renovascular type of hypertension in the stenotic kidney and accounts for most of the Ang II concentration in the renal tissue.²¹⁴ In addition, it plays a crucial role in other forms of hypertension. Administration of a renin inhibitor to normal subjects and hypertensive patients showed sustained local renal vascular responses but time-dependent decreased drug activity at the systemic level.²¹⁵ Several other studies demonstrated the importance of the intrarenal renin-angiotensin system in the pathophysiology of hypertension.²¹⁶,²¹⁷

Ang II promotes fibrogenesis and oxidative stress in the kidney by stimulating fibrogenic mediators, altering renal hemodynamics especially facilitating glomerular hypertension, inducing tubulointerstitial hypoxia, and enhancing free radical formation. Studies have shown that the degree of mesangial hypercellularity and expansion in IgA nephropathy correlated closely with glomerular expression of mRNAs for renin, ACE, chymase, and AT1 and AT2 receptors.²¹⁷,²¹⁸ The role of the intrarenal renin angiotensin system has been also established in the pathogenesis of membranous nephropathy.²¹⁹ In diabetic nephropathy, the renin-angiotensin system plays a crucial role in disease progression: the intrarenal generation of Ang II is increased, despite suppression of the systemic RAAS. Details of the pathophysiologic mechanisms implicated in diabetic nephropathy are beyond the scope of this chapter.

The role of the RAAS in end-stage renal disease (ESRD) is even more complicated and implicates many parameters related to residual renal function, presence of renal replacement therapy and modality, and drug treatment of associated conditions. In addition, renal transplantation adds a new dimension to the complexity by introducing an immune component through graft rejection and immunosuppressive drugs. Despite the substantial decrease in renal blood flow and function in patients with ESRD on dialysis, studies have shown that those with remaining kidneys have an activated intrarenal renin-angiotensin system.¹¹⁷ On the other hand, renal transplant diseases, especially rejection and cyclosporine-induced nephropathy, were associated with increased activity of systemic and local RAAS in several studies.²²⁰,²²¹

In conclusion, the RAAS is one of the most powerful pressure and volume regulatory systems in the body, involved in physiologic responses and pathophysiologic processes at both the systemic and local levels. Autonomous and independent control of local renin-angiotensin systems in various organs and tissues may exist and contribute to disease progression. New areas of interest in the RAAS are continuously emerging, facilitating the understanding of mechanisms of diseases and development of specific drug targets.

The cDNA for human ANP was isolated in 1984 and, shortly afterward, the gene was localized to the short arm of chromosome 1.²²²,²²³ The chromosomal gene consists of three exons and two introns encoding for a mature mRNA transcript approximately 900 bases long.²²³

Translation of human ANP mRNA results in a 151-amino acid preprohormone.²²⁴ Pro-ANP, a 126-residue molecule, is formed after cleavage of the signal peptide sequence of prepro-ANP and represents the major storage form of the hormone in atrial granules.²²⁵ The circulating, biologically active form of ANP, often referred to as AN_99-126 or ANP_1-28, is a peptide comprising the 28 carboxy-terminal amino acids of the parent molecule.²²⁴ The amino acid sequence of ANP_99-126 is highly conserved among mammalian species.²²⁴ A disulfide bond between cysteine residues 105 and 121 gives ANP_99-126 its ring structure, which is essential for biologic activity.²²⁶

Pro-ANP (1-30), (31-67), and (68-98) are secreted from the heart and circulate in the plasma. Pro-ANP (1-30) and (31-67) increase sodium and water excretion and binding sites have been found in the proximal tubules and collecting ducts. Pro-ANP (31-67) inhibits the Na⁺-K⁺ pump at the medullary collecting duct through a prostaglandindependent mechanism and no effect on cGMP production. Pro-ANP (1-30) infusion in rats clearly increases urine output, sodium, and potassium excretion, the mechanism of which still needs to be elucidated.²²⁷

Cardiac atria contain the highest concentrations of ANP and serve as the major source of circulating hormone.²²⁴ ANP_99-126 secretion from cardiomyocytes occurs largely in response to atrial stretch resulting from increased atrial transmural pressure.²²⁸ Physiologic stimuli for the release of ANP_99-126, include acute salt and volume loading, supine posture (head-down tilt), and head-out water immersion.²²⁸,²²⁹,²³⁰ An increased rate of atrial contraction has also been shown to stimulate ANP_99-126 secretion.²³¹,²³² Ang II, vasopressin, epinephrine, and phenylephrine stimulate ANP_99-126 secretion from the heart largely because of their systemic vasopressor effects.²³³ On the other hand, glucocorticoids and endothelin raise ANP_99-126 levels possibly by acting directly on atrial myocytes.²³⁴ Leptin decreases ANP secretion via an NO-mediated mechanism.²³⁵ Physiologic and pathologic conditions in which elevated plasma levels of ANP_99-126 have been detected are summarized in Table 8.3.

The local synthesis of natriuretic peptides is increased in the kidney and in the vasculature in obstructive uropathy.²³⁶ The activation of the renin-angiotensin system during low sodium intake antagonizes the biologic effect of ANP by interfering in the intracellular metabolism of cGMP.²³⁷

Three subtypes of ANP receptors (NPRs) have been identified.²³⁸,²³⁹,²⁴⁰ NPR-A and -B have intrinsic guanylate cyclase activity that catalyzes production of cGMP after ligand binding. cGMP then serves as an intracellular second messenger that mediates the biologic activities of ANP_99-126.²⁴¹ NPR-B appears to have 50-fold higher affinity for a related natriuretic factor originally purified from porcine brain, known as C-type natriuretic peptide (CNP).²⁴² NPR-C, previously known as the (clearance) receptor, is devoid of guanylate cyclase activity and, therefore, does not confer biologic activity and is thought to mediate clearance of circulating ANP along with metalloendoprotease (E.C.3.4.24.11)²⁴² and of other related hormones, such as brain natriuretic peptide (BNP).²⁴³ Selective downregulation of NPR-C in the kidney in response to dietary salt supplementation may contribute to local elevation in ANP levels and may be functionally significant in attenuating the development of salt-sensitive hypertension.²⁴⁴ NPR density is decreased in diabetic rats with significant increase in plasma ANP levels.²⁴⁵ ANP also antagonizes AVP-mediated increases in water permeability in IMCD cells.²⁴⁶

The major sites of action of ANP_99-126 are the kidneys, adrenal glands, and vascular smooth muscle.²⁴⁷ Short-term administration of ANP_99-126 in laboratory animals and in humans induces pronounced natriuresis, diuresis, alteration in renal hemodynamics and tubular function, suppression of renin release, inhibition of aldosterone secretion by the adrenal glands, and decreased vasomotor tone, resulting in transient drop in systemic blood pressure. From these actions it has been postulated that ANP plays an important physiologic role in protecting against extracellular volume overload.²⁴⁸

ANP-induced increase in the GFR is well established.²⁴⁹,²⁵⁰,²⁵¹ ANP_99-126 increases efferent arteriolar resistance, resulting in increased P_GC and filtration fraction.²⁴⁹ In addition, ANP_99-126 relaxes mesangial cells in vitro, suggesting that it can increase filtration area by cGMP generation in podocytes²⁵² and K_f in vivo.²⁵³ Indeed, when baseline K_f is low, as in water-deprived animals, ANP_99-126 enhances GFR mainly by increasing K_f.²⁵³ If preexisting vascular constriction is present in isolated perfused kidney, ANP tends to vasodilate renal vessels and increase renal blood flow (RBF).²⁵⁴ In whole animals, however, ANP_99-126 infusion causes either a decline or no change in RBF.²⁵⁴ The effects of ANP_99-126 on RBF are influenced by its systemic actions on blood pressure and the renin-angiotensin system. ANP seems to increase NO production at both renal and cardiac levels, further explaining its natriuretic and diuretic effects.²⁵⁵ Finally, ANP has been reported to induce redistribution of blood flow from the cortex to the medulla and to increase vasa recta flow, leading to dissipation of the medullary solute gradient.²⁵⁶,²⁵⁷

ANP_99-126 has both direct and indirect effects on tubular transport of Na, chloride, and water.²⁵⁸ In the proximal tubule, ANP_99-126 antagonizes Ang II-induced Na reabsorption²⁵⁹ and, along with endothelin-3, inhibits the sodium-glucose transporter²⁶⁰ and, like urodilatin, inhibits Na⁺-ATPase activity.²⁶¹ In the IMCD, it directly inhibits Na transport by binding to ANP-R1 receptors and influencing amiloride-sensitive Na channels and the activity of apical Na-K-2Cl cotransporters.²⁶² Other mechanisms by which ANP_99-126 induces natriuresis and diuresis include suppression of renin and aldosterone release,²⁶³ inhibition of the tubular actions of AVP,²⁴⁶ and dissipation of the medullary solute gradient by impairing the increase in intracellular Ca²⁺ concentration. ANP blocks both the stimulatory and inhibitory effects of AVP on Na⁺-dependent pHi recovery.²⁶⁴

Transgenic mice over-express pro-ANP in hepatocytes. These transgenic animals have a hypotensive phenotype (20 to 30 mm Hg lower than control littermates) without compensatory tachycardia. GFR remained normal despite hypotension. Moreover, significant diuresis or natriuresis during steady state was not detected. Contrary to observations made after short-term infusion of ANP_99-126, plasma renin activity also did not change while aldosterone levels were elevated.²⁶⁵,²⁶⁶,²⁶⁷

Disruption of the gene that encodes pro-ANP results in knockout mice that lack expression of ANP. Homozygous ANP knockout mice fed a standard diet have both mildly elevated blood pressure (average increase of 8 mm Hg) and cardiac hypertrophy compared to wild type mice, suggesting that ANP has a physiologic role in maintaining the normotensive state. This hypertension appears to be sensitive to dietary salt intake as feeding the homozygous ANP knockout mice a diet with an intermediate salt content (2%) would further increase blood pressure by an average of 20 mm Hg compared to wild type mice.²⁶⁸ Knockout mice lacking ANP-R1 activity (known as GC-A null mice) have both elevated blood pressure and cardiac hypertrophy, similar to pro-ANP knockout mice, but the GC-A null mice have a salt-insensitive form of hypertension.²⁶⁷ It is unclear why there should be a difference in the phenotype of these two types of knockout mice. It is possible that other guanylyl cyclase receptors, such as guanylyl cyclase C, can help regulate blood pressure in the face of changes in dietary salt intake and compensate for the lack of GC-A receptors.²⁶⁸ ANP (-/-) mice have a blunted pressure-natriuresis response and suppressed expression of local renal renin-angiotensin system.²⁶⁹

BNP is a 32-amino acid peptide with structural homology to ANP_99-126. Although originally isolated from porcine brain,²⁷⁰ it is also secreted by cardiac ventricles and, to a lesser extent, from atria. The biologic effects of BNP infusion are similar to those of ANP_99-126. Unlike ANP, BNP secretion seems to be constitutive and unrelated to myocyte stretch. The kidney-specific degradation of ANP provides a mechanism for preferential regulation of kidney function by BNP, independent of peripheral ANP concentration.²⁷⁰ In 1990, another homologous peptide, CNP, was isolated from porcine brain.²⁷¹ CNP is produced in the brain, where it achieves concentrations much higher than those of ANP and BNP. In contrast, circulating levels of CNP are lower. CNP lacks natriuretic, diuretic, and hypotensive effects and probably acts in a paracrine fashion in the CNS. The physiologic significance of BNP and CNP remains unclear.

Measurement of circulating ANP and ANP analogs is now a well established marker in assessment of volume overload and left ventricular dysfunction and for predicting mortality, in the presence or absence of renal dysfunction.²⁷²,²⁷³,²⁷⁴,²⁷⁵,²⁷⁶ A number of studies, reviewed recently,²⁷⁷ provide support for the use of ANP/ANP analogs in protection against and treatment of acute kidney injury (AKI).²⁷⁸,²⁷⁹,²⁸⁰,²⁸¹ In these studies, low (but not high)-dose ANP infusions reduced the need for renal replacement therapy and hospital and intensive care unit length of stay, but did not appear to improve mortality in the management of cardiac surgery-associated AKI. Hyporesponsiveness to ANP in congestive heart failure may be due to reduced NPR-A expression.²⁸² In dogs, maximizing cGMP action with type V phosphodiesterase inhibitors augments the natriuretic responses to exogenous ANP.²⁸³ Animal studies have shown the usefulness of encapsulated ANP gene transfected cells as a new tool for ANP gene delivery with possible implication for future therapy.²⁸⁴

The effects of aldosterone on its target cells have long been considered to be mediated exclusively through the genomic pathway and were characterized by a 45-minute lag period; however, evidence has been provided for rapid effects of the hormone that may involve nongenomic mechanisms.⁵,⁶,⁷,¹⁹,³² On the other hand, in a series of in vitro studies, aldosterone was shown to have a half maximal effect on both rapid (15 minutes) and delayed (120 minutes) Na flux.⁴,³² The Na/H exchanger has been identified as a target for nongenomic regulation. Aldosterone rapidly increases Na/H exchanger activity in a variety of cells, including distal colon and renal epithelial cell lines,³ but rapidly inhibits apical NHE₃ and HCO₃^– reabsorption in medullary thick ascending limb (MTAL) and has a dose-dependent biphasic effect on the Na/H exchanger (low doses stimulate and high doses inhibit it).³¹²

Aldosterone acts through nongenomic pathways to regulate many different ion transport proteins and signaling pathways in a variety of renal epithelial cells, such as proximal tubule cells derived from human renal cortex, MDCK-C11 cells (a cell line that exhibits properties of collecting duct intercalated cells), and principal cells isolated from rabbit cortical collecting duct and both M-1 and RCCD2 cortical collecting duct cell lines.⁶ Studies using isolated perfused tubules also demonstrate that aldosterone, via nongenomic mechanisms, regulates the transepithelial transport function of different nephron segments, such as proximal S3 segment,²¹ renal MTAL, type A intercalated cells of outer medullary collecting ducts, and principal cells in the connecting tubule and inner medullary collecting ducts.⁶,³¹³ In renal epithelial cells, an elevation in cytosolic Ca₂ serves as a second messenger for the nongenomic Na^–/H exchanger activation initiated by aldosterone.² The existence of a nongenomic action of aldosterone is in general attributed to conditions where this action is observed over a short period of time (minutes) against the much longer time (hours, days) needed for a genomic action.³¹³

Rapid nongenomic aldosterone effects are characterized by their rapid onset of action (within minutes) and an insensitivity to inhibitors of transcription (e.g., actinomycin D) and of protein synthesis. Aldosterone also acts via rapid nongenomic effects in vivo in humans at the renal vasculature. Antagonizing the endothelial NO synthase unmasks these effects. Therefore, rapid nongenomic aldosterone effects increase renal vascular resistance and thereby mediate arterial hypertension if endothelial dysfunction is present.³¹⁴ Evidence suggests that there is a nonclassical membrane-bound aldosterone receptor.³¹⁵ These data come from kinetic studies that demonstrate saturable, radiolabeled binding of aldosterone to cell surface membranes that have kinetics compatible with physiologic activity.³¹⁶,³¹⁷ In some cell lines, aldosterone can have very rapid physiologic effects that are not blocked by inhibitors of cell transcription and translation. For instance, in human mononuclear leukocytes, aldosterone can stimulate release of IP₃ or calcium within 30 seconds of exposure.³¹⁸,³¹⁹ These membrane-bound receptors may explain some of the effects of aldosterone that occur prior to gene transcription, such as early stimulation of
sodium reabsorption³²⁰ or early stimulation of salt intake,³²¹ possibly via phospholipase C/PKC signaling pathways.

Glucocorticoids appear to be important for the normal maintenance of GFR.³⁴⁷ In both adrenalectomized animals and in humans with adrenal insufficiency, GFR is reduced compared to controls. Adrenalectomized rats given physiologic doses of glucocorticoids regain a normal GFR.³⁴⁸ Furthermore, short-term administration of pharmacologic doses of glucocorticoids has been reported to increase inulin clearance in both normal animals and humans.³⁴⁹,³⁵⁰ Micropuncture studies in normal rats indicate that glucocorticoids enhance GFR by increasing glomerular plasma flow.³⁴⁷,³⁵¹ The latter results from selective vasodilation of both afferent and efferent renal arterioles.³⁵¹ The mechanisms by which glucocorticoids alter the glomerular microcirculation remain obscure. Because amino acid infusion causes similar glomerular hemodynamic changes, Baylis and colleagues suggest that glucocorticoids may increase GFR through their effects on catabolism of proteins to free amino acids.³⁴⁷

Glucocorticoid actions on the proximal tubule include enhancement of gluconeogenesis, ammoniagenesis, and Na reabsorption.³⁵² Lag in ammonium excretion resulting in acid retention is well described in subjects in a glucocorticoid-depleted state.³⁵³ In adrenalectomized animals, glucocorticoid replacement restores proximal tubule ammoniagenesis and the ability of the kidney to respond to the chronic phase of acidosis.³⁵⁴ Furthermore, in whole animals, glucocorticoid excess accelerates renal base generation, resulting in metabolic alkalosis.³⁵⁵ Glucocorticoids regulate ammoniagenesis possibly through altering glutamine uptake and metabolism by proximal tubule cells.³⁵⁵ Glucocorticoids increase proximal tubule Na reabsorption by at least two mechanisms: enhanced Na-K-ATPase activity and Na-H exchange.³⁵⁶,³⁵⁷ Several experiments suggest that glucocorticoids inhibit Na-dependent phosphate and sulfate reabsorption in the proximal tubule.³⁵² These observations are supported by clinical reports of phosphaturia and lower serum phosphate levels in patients with Cushing disease and subjects given high doses of glucocorticoids.³⁵⁸

Both patients with Addison disease and adrenalectomized animals have decreased urinary concentrating ability,³⁵⁹ due in part to reduction in RBF, GFR, and hydroosmotic
permeability of the collecting tubule.³⁶⁰ In addition, adrenal corticosteroids contribute to urinary concentration by stimulating Na, K, and HCO₃ transport in the thick ascending limb of Henle’s loop.³⁶¹ It is not entirely clear whether glucocorticoids, mineralocorticoids, or both mediate the effects of corticosteroids on renal-concentrating mechanisms.

Systemic excess of glucocorticoids is known to cause hypertension. The effect of glucocorticoids may be in part mediated by the suppression of endothelial nitric oxide synthase (eNOS).² Acute administration of glucocorticoids, however, may have beneficial effects on the cardiovascular system in part through nontranscriptional activation of eNOS.³⁰⁸

Norepinephrine (noradrenaline), epinephrine (adrenaline), and dopamine are collectively called catecholamines. These endogenous amines are derived from the amino acid tyrosine and the enzymes involved in their biosynthesis have been identified, cloned, and characterized³⁶² and include: tyrosine hydroxylase, dopa decarboxylase, dopamine β-hydroxylase, and phenylethanolamine-n-methyltransferase. The first step in catecholamine synthesis involves the hydroxylation of tyrosine by tyrosine hydroxylase, which is regarded as a rate-limiting step. This enzyme is activated by stimulation of sympathetic nerves or the adrenal medulla and is subject to feedback inhibition by catechol compounds. Catecholamines are synthesized in nerve terminals of the postsynaptic neurons of the sympathetic nervous system and in the adrenal medulla. Most of the steps occur in the cytoplasm whereas some take place in storage vesicles.³⁶³ Storage of catecholamines in vesicles ensures their regulated release, protects them from enzymatic degradation, and prevents their leakage outside the cell. Termination of action of norepinephrine and epinephrine includes reuptake by nerve terminals, reuptake by nonneuronal cells, and metabolic transformation. Major enzymes involved in catecholamine metabolism include monoamine oxidase (MAO) and catechol-O-methyl transferase (COMT). The main source of epinephrine in the body derives from the adrenal medulla whereas noepinephrine originates mostly from nerve terminals.³⁶³ At the renal level, catecholamines derive from renal efferent nerves (norepinephrine and, to a lesser extent, dopamine), from the circulation (epinephrine and norepinephrine), from the adrenal medulla (epinephrine), and from renal proximal tubule cells (dopamine) via α₁, α₂, β₁, and β₂ receptors for epinephrine and norepinephrine and D₁– and D₂-like receptors for dopamine.³⁶⁴,³⁶⁵,³⁶⁶,³⁶⁷

Catecholamines play an important role in the regulation of RBF, GFR, renin secretion, and tubular transport. Their effects depend on site of action and receptor type. The kidney is one of the major long-term regulators of blood pressure (BP). This is achieved by the interplay of several hormonal, neural, and humoral factors that would regulate principally two parameters: sodium homeostasis and peripheral vascular resistance.³⁶⁸ In the following section, we discuss the renal functions of catecholamines separately on their receptors.

The KKS is a complex multienzyme system that can be divided into two types: (1) a circulating KKS that belongs to the coagulation system and (2) a tissue KKS that acts in a paracrine or autocrine fashion. The KKS leads to the production of kinins, namely bradykinin and lys-bradykinin (kallidin), from kininogens through the action of kininogenase, namely kallikrein (Fig. 8.5).³⁹⁹ Two types of kallikreins have been identified—a plasma and a tissue kallikrein—both of which are serine proteases that are encoded by different genes and differ in their distribution and regulation.³⁹⁹ Mice lacking tissue kallikrein show dramatically reduced levels of renal kinin, suggesting that tissue kallikrein is the main system involved in the kidney,⁴⁰⁰ although little is known regarding the putative role of plasma kallikrein under pathologic conditions. Renal tissue kallikrein is synthesized in large amounts by connecting tubule cells and is mainly secreted into the urinary fluid and to a lesser extent to the peritubular interstitium.³⁹⁹

In humans, two types of kininogens have so far been described: a high molecular weight (HK) form present in blood and a low molecular weight (LK) form present in various tissues. It is generally accepted that tissue kallikrein prefers LK but is capable of cleaving HK, whereas plasma kallikrein cleaves HK exclusively.³⁹⁹ Kininogens are synthesized in the liver and circulate in blood plasma at concentrations of 45 to 120 µg per mL for LK and of 65 to 115 µg per mL for HK, but are also found in other body fluids and organs such as kidney.⁴⁰¹

Novel functions of kininogens have been recently discovered. Derivatives of HK have been shown to be involved in the regulation of endothelial cell proliferation, angiogenesis, and apoptosis.⁴⁰² In addition, some seem to possess potent and broad-spectrum microbicidal properties against both gram-positive and gram-negative bacteria, and thus may represent an alternative to conventional antibiotic therapy.⁴⁰²

Tissue kallikrein is secreted by many cells throughout the body but some tissues produce particularly large quantities such as the kidney, lung, intestine, brain, and glandular tissues (salivary and sweat glands and pancreatic exocrine gland). This enzyme is activated intracellularly from a precursor, prokallikrein, to produce tissue kallikrein.⁴⁰³ The enzyme responsible for this conversion has not yet been identified.

Regulatory mechanisms of tissue kallikrein remain partly unknown. Aprotinin, a polypeptide purified from the lung, and kallistatin have been shown to inhibit the activity of renal and other tissue kallikreins.⁴⁰⁴ In addition, it has been shown that salt restriction stimulates the synthesis of renal kallikrein through an unclear mechanism, possibly implicating aldosterone.⁴⁰⁵ Interestingly, it was also recently shown that potassium intake triggers an increase in renal kallikrein secretion through an aldosterone-independent mechanism
involving membrane depolarization of kallikrein-secreting cells in the renal connecting tubules, followed by enhanced calcium influx.⁴⁰⁶,⁴⁰⁷ In addition to dietary sodium and potassium intake, hereditary factors may determine tissue kallikrein activity. In fact, a loss of function polymorphism in the human tissue kallikrein gene (R53H) has been identified with a frequency of 0.03 in Caucasians.⁴⁰⁸ Interestingly, these partially tissue kallikrein-deficient subjects develop a form of arterial dysfunction characterized by remodeling of the brachial artery, which is not adapted to a chronic increase in wall shear stress.⁴⁰⁸

Although of little relevance for the kidney, it is noteworthy that plasma kallikrein is regulated through a different, more complex, mechanism. Plasma kallikrein is synthesized and secreted by the liver as an inactive zymogen. It is activated by the intrinsic coagulation cascade whereby contact of plasma with negatively charged macromolecular surfaces initiates a proteolytic cascade that ultimately converts prekallikrein to plasma kallikrein.⁴⁰⁹

The half-life of bradykinin in plasma is short (˜30 seconds), suggesting that its actions are regulated locally through its production and degradation within tissues. In fact, both bradykinin and kallidin can be metabolized through two pathways.⁴¹⁰ Kininase I (also called carboxypeptidase-N), as well as carboxypeptidase-M, remove the C-terminal arginine from the kinins to generate their des-Arg derivatives, which are agonists of B1R (see later). Kininase II (also known as ACE), neprilysin (endopeptidase 24.11), and endothelin-converting enzyme cleave off the two C-terminal amino acids (Phe and Arg) of the kinins, thereby inactivating them.⁴¹¹,⁴¹² It is noteworthy that kallidin can be converted into bradykinin by a plasma aminopeptidase.

Kinins exert their biologic effects by acting on two types of G protein-coupled receptors called bradykinin B1 receptor (B1R) and B2 receptor (B2R) (Fig. 8.6). Although B2R is ubiquitous and expressed throughout the kidney, B1R is mainly expressed after induction by endotoxin, cytokines, ischemia, and other noxious stimuli.⁴¹¹ Interestingly, treatment with an inflammatory signal (lipopolysaccharide) induces the expression of B1R mRNA in all renal segments except the outer medullary collecting ducts. B1R, once induced by inflammatory mediators and tissue damage, assumes some of the physiologic roles of B2Rs.⁴¹³ Bradykinin and kallidin are equipotent and both have higher affinity for B2R. Interestingly, metabolites (des-Arg9-BK [DABK] and Lys-DABK) of bradykinin and kallidin, which result from the action of carboxypeptidases, have higher affinity for B1R.

Both receptors activate similar intracellular signaling cascades involving phospholipase C activation and intracellular calcium mobilization. In addition, through calciumdependent and -independent mechanisms, KKS increases NO and prostaglandin synthesis, both of which seem to mediate some of the effects of kinins.⁴¹⁰ Alternative mediators of kinins also include endothelium-derived hyperpolarizing factor (EDHF), norepinephrine, substance P, cytokines, and tissue plasminogen activator.⁴¹⁰

Experimental evidence suggests that kinins regulate renal blood flow and renal excretion of sodium and water.⁴¹⁴,⁴¹⁵ Studies on the role of the renal KKS, using congenitally kininogen-deficient Brown-Norway Katholiek rats and B2R knockout mice, amongst other models, revealed that this system starts to induce natriuresis and diuresis when sodium accumulates in the body as a result of excess sodium intake or aldosterone release, for example, by angiotensin II. Thus, it is hypothesized that the system works as a safety valve for sodium accumulation. In fact, mice lacking B2R and/or B1R develop salt-sensitive hypertension.⁴¹⁶ Interestingly, humans with essential hypertension have low levels of urinary
kallikrein.⁴⁰⁵

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