Glomerular Filtration Rate and Proximal Tubular Reabsorption
The rates of glomerular filtration and proximal tubular reabsorption are important primarily in determining the rate of sodium and water delivery to the more distal portions of the nephron, where the renal concentrating and diluting mechanisms are operative. Fluid reabsorption in the proximal tubule is isosmotic; therefore, tubular fluid is neither concentrated nor diluted in the proximal portion of the nephron. Rather, after approximately 70% of glomerular filtrate is reabsorbed in the proximal tubules, the remaining 30% of fluid entering the loop of Henle is still isotonic to plasma. The reabsorption of sodium chloride is primarily driven by the Na/H 3 transporter whereas the isotonic removal of water is facilitated by the robust expression of the water channel aquaporin 1 (AQP1
), depicted in Figure 1-2
. A decrease in glomerular filtration rate (GFR) or an increase in proximal tubular reabsorption, or both, may diminish the amount of fluid delivered to the distal nephron and thus limit the renal capacity to excrete water. Similarly, a diminished GFR and increased proximal tubular reabsorption may limit the delivery of sodium chloride to the ascending limb, where the tubular transport of these ions without water initiates the formation of the hypertonic medullary interstitium. With diminished delivery of sodium chloride to the ascending limb, the resultant lowering of medullary hypertonicity impairs maximal renal concentrating capacity.
Figure 1-2 Schematic representation of key elements of the kidney tubule that mediate water reabsorption. Direct mediators are those involved in sodium (depicted as red circles), urea (green circles), and water (blue circles) transport. PCT, proximal convoluted tubule; CTAL, cortical thick ascending limb; MTAL, medullary thick ascending limb; DCT, distal convoluted tubule; CCD, cortical collecting duct; OMCD, outer medullary collecting duct; IMCD, inner medullary collecting duct; AQP, aquaporin; NHE, Na+/H+ exchanger; Na/Glucose, sodium-glucose cotransporter; NBC, sodium bicarbonate cotransporter; NaK, Na+/K+ ATPase; NKCC2, Na+/2CI–/K+ cotransporter; ENaC, epithelial sodium channel; UT-A, urea transporter A isoform. (From Hasler U, Leroy V, Martin PY, et al. Aquaporin-2 abundance in the renal collecting duct: new insights from cultured cell models. Am J Physiol Renal Physiol. 2009;(297:1), with permission.)
Descending and Ascending Limbs of the Loops of Henle, Distal Tubule, and Collecting Ducts
Because the urine that emerges from the proximal tubule is isosmotic, the first nephron segment actually involved in urinary concentration is the descending limb of Henle’s loop. There are two types of descending limbs. The short loops originate in superficial and mid cortical glomeruli and turn in the outer medulla. The long loops originate in deep cortical and juxtamedullary glomeruli and penetrate variable distances into the inner medulla. Short and long descending limbs are anatomically distinct; the long limbs in particular display considerable interspecies variability (6
). Interestingly, no correlation is apparent between a species’ maximal concentrating ability and the ratio of short and long loops. In fact, in rodents with highest urinary concentrations, the number of short loops is considerably greater than the number of long loops. Approximately 15% of nephrons possess long loops in the human kidney; the other 85% of nephrons have short loops. The descending thin limb is very water permeable as it also has, in its first portion, abundant expression of AQP1
). Thus, tubular fluid is concentrated as it descends primarily, but probably not exclusively, by the extraction of water.
Somewhat proximal to the hairpin turn, there is a transition from the descending limb to the ascending thin limb of Henle’s loop. This segment, as well as the remainder of ascending limb, is water impermeable, As will be further discussed, the nature and particular site at which the movement of solutes (urea and NaCl) occur has not been fully defined. Active sodium transport has not been demonstrated convincingly, and this segment’s morphologic appearance with few mitochondria does not suggest active metabolic work.
The thick ascending limb of Henle’s loop appears both structurally and functionally distinct from its thin counterpart. The epithelium is remarkably uniform among species with tall, heavily interdigitating cells with large mitochondria. The observation that fluid emerges into the early distal tubule hypotonic (about 100 mOsm/kg H2
O) supports the view that active sodium chloride transport out of this water-impermeable segment provides the single effect required for the operation of the countercurrent multiplier. The primary
mechanism of chloride absorption in the thick ascending limb is mediated by an electroneutral sodium, potassium, and chloride (Na+
) cotransport (Fig. 1-2
The distal convoluted tubule is the segment between the macula densa and collecting ducts. This is a morphologically heterogenous segment (6
) that is also water impermeable and unresponsive to vasopressin. The collecting ducts are formed in the cortex by the confluence of several distal tubules. They descend through the cortex and outer medulla individually, but successively fuse together on entering the inner medulla. In humans, a terminal inner medullary collecting duct draws from as many as 7,800 nephrons. The collecting ducts possess vasopressin-sensitive adenylate cyclase in all species studied; they are virtually impermeable to water in the absence of the hormone. The vasopressin-sensitive water channel AQP2
mediates water reabsorption in this segment of the nephron in concert with AQP3
). The collecting duct in its cortical and medullary segments is also impermeable to urea, but in response to the vasopressin-sensitive urea transporter UT 1, the inner medullary collecting duct is rendered urea permeable (9
Kokko and Rector, as reviewed by Sands (10
), have proposed a model of urinary concentration that is in concert with the anatomic features and the permeability characteristics of the various segments of the system, while limiting the active transport of solute to the thick portion of the ascending limb of Henle in the outer medulla. The components of the mechanism, as depicted in Figure 1-3A
, are as follows:
The water-impermeable thick ascending limb of Henle’s loop actively cotransports sodium, chloride, and potassium, thereby increasing the tonicity of the surrounding interstitium and delivering hypotonic fluid to the distal tubule. Urea is poorly reabsorbed and therefore retained in the tubule.
Under the influence of vasopressin in the cortical and outer medullary collecting ducts, tubular fluid equilibrates with the isotonic and hypertonic interstitium, respectively. Low urea permeability in this portion of the nephron allows its concentration to further increase.
In the presence of vasopressin, the inner medullary collecting duct is rendered more permeable to urea. Therefore, in this segment of nephron, in addition to water reabsorption, urea is reabsorbed as it diffuses passively along its concentration gradient into the interstitium, where it constitutes a significant component of the medullary interstitial tonicity.
The resulting increase in interstitial tonicity creates the osmotic gradient that abstracts water from a highly water permeable and solute impermeable descending limb of Henle’s loop. This process elevates the concentration of sodium chloride in the tubular fluid. When tubular fluid arrives at the bend of the loop, its tonicity is the same as that of the surrounding interstitium. However, the sodium chloride concentration of the tubular fluid is higher and the urea concentration lower than that of the interstitium.
Tubular fluid then enters the thin ascending limb, which is more permeable to sodium than urea. The sodium gradient provides for passive removal of sodium chloride from this segment into the interstitium.
To prevent urea removal from the inner medulla to the cortex, the ascending and descending vasa recta act as a countercurrent exchanger and “trap” urea in the inner medulla. The ascending vasa recta also may deposit urea into adjacent descending thin limbs of a short loop of Henle, thereby recycling it to the inner medullary collecting tubule. The descending limbs of short loops do not enter the inner medulla; thus, the addition of urea to these loops does not interfere with the removal of water from the descending thin limb in the inner medulla, a step that is so crucial to the concentrating process.
This passive model of urinary concentration has a number of attractive features, and many of its aspects have been experimentally supported (10
). However the requisite difference in sodium and urea permeabilities that is needed for this model to operate passively (point 5 above) have not been met, as mathematical models that employ available permeabilities fail to generate the desired osmotic gradients. Thus, alternatives have been tested. Among these, a three-dimensional reconstruction of the components of the rat inner medulla coupled with the development of mathematical models has emerged with a variation of the previous model, designated as the solute mixing passive model, depicted in Figure 1-3B
). This model recognizes that the water permeability of the descending limb does not extend into the inner medulla as AQP1
is absent in the lower half of this limb of Henle’s loop. The high urea permeability and the passive exit of sodium from tubular fluid occur before the bend of the loop, equally in the descending as well as ascending thin loops rather than exclusively in the latter. Although this model predicts the generation of concentrated urine, it does not produce one that is maximally concentrated (4
What remains widely agreed upon is that the single effect in the ascending limb of Henle, so critical to the operation of the countercurrent system and urinary concentration, also serves to dilute the urine. In the absence of vasopressin, and thus with water impermeability of the collecting ducts, the continued reabsorption of solute in the remainder of the distal nephron results in a maximally dilute urine (50 mOsm/kg). Thus, it
should be apparent that impairment of sodium, chloride, and potassium cotransport in the ascending limb of the loop of Henle will limit the renal capacity both to concentrate and to dilute the urine.
Figure 1-3 (A) Schematic representation of the passive urinary concentrating mechanism. Both the thin ascending limb in the inner medulla and the thick ascending limb in the outer medulla, as well as the first part of the distal tubule, are impermeable to water, as indicated by the thickened lining. In the thick ascending limb, active sodium, chloride, and potassium cotransport renders the tubule fluid dilute and the outer medullary interstitium hyperosmotic (1). In the last part of the distal tubule and the collecting tubule in the cortex and outer medulla, water is reabsorbed down its osmotic gradient (2), increasing the concentration of urea that remains behind. In the inner medulla, both water and urea are reabsorbed from the collecting duct (3). Some urea reenters the loop of Henle (data not shown). This medullary recycling of urea, in addition to trapping of urea by countercurrent exchange in the vasa recta (data not shown), causes urea to accumulate in large quantities in the medullary interstitium (indicated by UREA), where it osmotically extracts water from the descending limb (4) and thereby concentrates sodium chloride in descending limb fluid. When the fluid rich in sodium chloride enters the sodium chloride-permeable (but water-impermeable) thin ascending limb, sodium chloride moves passively down its concentration gradient (5), rendering the tubule fluid relatively hyposmotic to the surrounding interstitium. (From Jamison RL, Maffly RH. The urinary concentrating mechanism. N Engl J Med. 1976;295(19):1059, Copyright © 2017 Massachusetts Medical Society. Reprinted with permission from Massachusetts Medical Society.) (B) Illustrating the unique permeabilities and solute fluxes of current solute separation, solute mixing passive model for concentrating urine. Thick tubule border indicates AQP1-null, water-impermeable segment of DTL as well as water-impermeable ATL and TAL. The AQP1-null segment of the DTL is essentially impermeable to inorganic solutes and water. In this model, both the ATLs and the DTLs (including the AQP1-null segment) are highly permeable to urea. In contrast to the original passive model, passive NaCl reabsorption without water begins with the prebend segment and is most significant around the loop bend. Also, in contrast to previous models, urea moves passively into the entire DTL and early ATL, but as this urea-rich fluid further ascends in the ATL, it reaches regions of lower interstitial urea concentration and diffuses out of the ATL again. Thus, the loops act as countercurrent exchangers for urea. (Republished with permission of American Society of Nephrology, from Dantzler W. et al Urine-concentrating mechanism in the inner medulla: function of the thin limbs of the loops of Henle. Clin J Am Soc Nephrol. 2014;9(10):1781-1789; permission conveyed through Copyright Clearance Center, Inc.)
Medullary Blood Flow
Medullary blood flow, whose rate can be regulated independently of whole kidney blood flow, may also affect the renal capacity both to concentrate and to dilute the urine, because the preservation of the medullary hypertonicity in the interstitium is dependent on the countercurrent exchange mechanism in the vasa recta. Although medullary blood flow constitutes only 5% to 10% of total renal blood flow, this flow is still several times more rapid than the tubular flow. The vasa recta possess AQP1 and serve as a countercurrent exchanger that permits the preservation of interstitial tonicity. Blood that enters the descending vasa recta becomes increasingly concentrated as water diffuses out of and solutes diffuse into this portion of the nephron. The hairpin configuration of the vasa recta, however, does not allow the solute-rich blood to leave the medulla. In the ascending portion of the vasa recta, water diffuses into the vasa recta and solute moves out, thus maintaining interstitial hypertonicity. Even with an intact countercurrent exchange system in the vasa recta, circumstances that increase medullary blood flow may “wash out” the medullary concentration gradient and thereby diminish renal concentrating capacity. Moreover, even in the absence of vasopressin, the collecting duct is not completely water impermeable; therefore, a further decrease in the hypertonic medullary interstitium during an increase in medullary blood flow may decrease the vasopressin-independent osmotic water movement from the collecting duct and thereby increase water excretion.
The renal concentrating and diluting processes are ultimately, and most importantly, dependent on the presence or absence respectively of arginine vasopressin (AVP) to modulate the water permeability of the collecting duct. AVP, a cyclic hexapeptide (mol wt 1,099) with a tail of three amino acids, is the antidiuretic hormone (ADH) in humans (Fig. 1-5
). The presence of a basic amino acid (arginine or lysine) in the middle of the intact hormone at position 8 is crucial for antidiuresis, as is the asparagine at position 5. AVP is synthesized in the supraoptic and paraventricular magnocellular nuclei in the hypothalamus. In these nuclei, a biologically inactive macromolecule is cleaved into the smaller, biologically active AVP. Both oxytocin and AVP are encoded in human chromosome 20 in close proximity to each other, depicted in Figure 1-6
. The preprohormone gene is approximately 2,000 base pairs in length and comprises three exons (Fig. 1-6
). AVP is encoded in the first exon following a signal peptide. Although spanning all three exons, the binding protein neurophysin is encoded primarily in exon B and the terminal glycoprotein in exon C. The promoter has cis-acting elements, including a glucocorticoid response element, a cyclic adenosine monophosphate (cAMP) response element, and four AP-2 binding sites (11
). The precursor prohormone, called propressophysin, is cleaved by removal
of the signal peptide after translation. Vasopressin, with its binding protein neurophysin II, and the glycoprotein are transported in neurosecretory granules down the axons and stored in nerve terminals in the pars nervosa. There is no known physiologic role of the neurophysins, but they neutralize the negative charge of vasopressin. The release of stored peptide hormone and its neurophysin into the systemic or hypophyseal portal circulation occurs by an exocytosis. With increased plasma osmolality, electrical impulses travel along the axons and depolarize the membrane of the terminal axonal bulbs. The membrane of the secretory granules fuses with the plasma membrane of the axonal bulbs, and the peptide contents are then extruded into the adjacent capillaries. The Brattleboro rat, a strain with an autosomal recessive defect that causes AVP deficiency, is afflicted by a single base deletion in exon B. This leads to a shift in the reading frame, with loss of the translational stop code. Although transcribed and translated in the hypothalamus, the translational product is neither transported nor processed in these mutant rats.
Figure 1-5 Structure of the human antidiuretic hormone, arginine vasopressin. (From Schrier RW, Miller PD. Water metabolism in diabetes insipidus and the syndrome of inappropriate antidiuretic hormone secretion. In: Kurtzman NA, Martinez Maldonado M, eds. Pathophysiology of the Kidney. Springfield, IL: Charles C Thomas; 1977.)
Figure 1-6 The arginine vasopressin (AVP) gene and its protein products. The three exons encode a 145-amino acid prohormone with an NH-2-terminal signal peptide. The prohormone is packaged into neurosecretory granules of magnocellular neurons. During axonal transport of the granules from the hypothalamus to the posterior pituitary, enzymatic cleavage of the prohormone generates the final products: AVP, neurophysin, and a COOH-terminal glycoprotein. When afferent stimulation depolarizes the AVP-containing neurons, the three products are released into capillaries of the posterior pituitary. (From Brenner BM, ed. The Kidney. 8th ed. Philadelphia: Saunders Elsevier; 2008, with permission.)
The regulation of AVP release from the posterior pituitary is dependent primarily on two mechanisms: osmotic and nonosmotic pathways (Fig. 1-7
Osmotic Release of Vasopressin
The osmotic regulation of AVP is dependent on “osmoreceptor” cells in the anterior hypothalamus in proximity but separate from supraoptic nuclei. These cells, most likely by altering their cell volume, recognize changes in ECF osmolality. Cell volume is decreased most readily by substances that are restricted to the ECF, such as hypertonic saline or hypertonic mannitol, and thus enhance osmotic water movement from cells; these substances are very effective in stimulating AVP release. Since the effects of saline and mannitol are comparable, this supports the view that the response is due to changes in effective osmolality rather than to sodium per se. In contrast, urea moves readily into cells and therefore does not alter cell volume; hypertonic urea does not effectively stimulate AVP release. The effects of increased osmolality on vasopressin release are associated with measurable (twofold to fivefold) increases in vasopressin precursor messenger RNA (mRNA) in the hypothalamus. The osmoreceptor cells are very sensitive to changes in ECF osmolality. An increase of ECF osmolality by 1% stimulates AVP release, whereas water ingestion causing a 1% decrease in ECF osmolality suppresses AVP release (Fig. 1-8
). A role for members of the transient receptor potential vallinoid family (TRPV 1 and 4) in osmoregulation has been suggested as knockouts of these proteins in mice become somewhat hypernatremic and display blunted vasopressin secretion in response to hypertonic stimuli as summarized by Cohen (12
A close correlation between AVP and plasma osmolality has been demonstrated in subjects with various states of hydration, although there are considerable genetically determined individual variations in both the
threshold and sensitivity (Fig 1-8
). In humans, the osmotic threshold for vasopressin release is between 280 and 290 mOsm/kg. The system is so efficient that plasma osmolality usually does not vary more than 1% to 2 %, despite great variations in water intake. There is also a close correlation between AVP and urinary osmolality, allowing for the maintenance of tonicity of body fluids.
Figure 1-7 Osmotic and nonosmotic stimulation of arginine vasopressin release. (From Robertson GL, Berl T. Pathophysiology of water metabolism. In: Brenner BM, ed. The Kidney. 6th ed. Philadelphia: WB Saunders; 2000:875, with permission.)
Figure 1-8 Antidiuretic hormone levels, urinary osmolality, and thirst as functions of serum osmolality. (From Narins RG, Krishna GC. Disorders of water balance. In: Stein JH, ed. Internal Medicine. Boston: Little, Brown; 1987:794, with permission.)
Nonosmotic Release of Vasopressin
Vasopressin release can occur in the absence of changes in plasma osmolality (5
). Although a number of such nonosmotic stimuli exist, physical pain, emotional stress, and a decrement in blood pressure or volume are the most prominent ones. A 7% to 10% decrement in either blood pressure or blood volume causes the prompt release of vasopressin (Fig. 1-7
). Because the integrity of the circulatory volume takes precedence over mechanisms that maintain tonicity, activation of these nonosmotic pathways overrides any decline in the osmotic stimulus that otherwise would suppress the hormone’s release. This process accounts for the pathogenesis of hyponatremia in various pathophysiologic states, including cirrhosis, heart failure, and several endocrine disorders.
There is considerable evidence for the existence of baroreceptor sensors in the low-pressure (venous) areas of the circulation, particularly in the atria. Atrial distention causes a decrease in plasma AVP levels, and a water dieresis; this reflex is mediated by the vagus nerve. Alternatively, arterial baroreceptors in the aorta and carotid sensors send impulses through the vagus and glossopharyngeal nerves to the nucleus tractus solitarii of the medulla. Unloading of these arterial baroreceptors decreases tonic inhibition and leads to the nonosmotic release of vasopressin. Denervation of these arterial baroreceptors has been shown to abolish the nonosmotic release of AVP.
It is possible that angiotensin II is a mediator of AVP release in these states because many of the pathophysiologic states associated with nonosmotic AVP release are characterized by enhanced plasma renin activity and therefore increased angiotensin II levels. The experimental results in this regard are however conflicting. Activation of the sympathetic nervous system seemed be involved in the nonosmotic stimulation of AVP. In this regard, the supraoptic nuclei are heavily innervated by noradrenergic neurons. Other pathways that could stimulate the nonosmotic secretion of AVP have been proposed; for example, the antidiuresis associated with nausea and pain has been ascribed to an emetic and to a cerebral pain center, respectively. A role for baroreceptor pathways however has not been convincingly excluded even in these settings. Other biogenic amines, polypeptides, and even cytokines have been implicated as modulators of AVP release in addition to catecholamines.
Cellular Action of Vasopressin
Once released from the posterior pituitary, vasopressin exerts its biologic action on water excretion by binding to V2
receptors in the basolateral membrane of the collecting duct (Fig. 1-9
). The receptors to which vasopressin binds have been cloned. The V1
receptor on blood vessels and elsewhere is a 394 amino acid protein with seven transmembrane domains (14
). The 370 amino acid V2
receptor, which is present only in the kidney and has a similar configuration, has been cloned for both rats (15
) and humans (16
). Although the V1
receptor messenger is plentiful in the glomerulus, it is also detected in the collecting duct, where the V2
message is predominant.
Binding of AVP to its V2
receptor increases adenylate cyclase activity resulting in the generation of cyclic adenosine 3′,5′-monophosphate (cAMP) from adenosine triphosphate. The V2
receptor is coupled to the catalytic unit of adenylate cyclase by the stimulatory guanine nucleotide binding regulatory protein, Gs. This is a heterotrimeric protein whose α
subunit binds and hydrolyzes guanosine triphosphate. The heightened cAMP formation activates protein kinase A (PKA), which in turn phosphorylates serines and threonines. The activation of PKA brings about the phosphorylation of the water channel AQP2
at serine 256 in intracellular vesicles, and thereby increases the trafficking of the water channel to the luminal membrane (17
). This sequence of events results in a marked increase in the water permeability of the luminal membrane and thereby the collecting tubule. AQP2
is a member of an increasingly large family of water channels whose archetypal member, AQP1
, cloned by Agre and coworkers (18
) is, as mentioned previously, abundant in the proximal tubule and the proximal half of the descending limb of Henle. In contrast, AQP2
is limited to the vasopressin-sensitive principal cell of the collecting duct, and particularly to the cytoplasm and luminal membrane. Vasopressin also is involved in the long-term regulation of the expression of AQP2
are widely distributed, including the collecting duct principal cell, where they are localized at the basolateral membrane. In this basolateral membrane, they serve as conduits for water exit from the cell. Other AQP6
also are expressed in the kidney. AQP6
is present in intercalated cells, AQP7
in the S3
segment of the proximal tubules, AQP8
in proximal tubules and collecting ducts, and AQP11
in the proximal tubule (19
). The physiologic roles of these water channels in the kidney are not clear. The cytoskeleton also plays an important role in the trafficking of the AQP2
water channel to the luminal membrane, a process involving both exocytic insertion associated with AVP stimulation and endocytic retrieval associated with suppression of AVP action.
Figure 1-9 Schematic representation of the cellular action of vasopressin. The binding of vasopressin to the V2 receptor in the basolateral membrane initiates a cascade of events resulting in AQP2 insertion in the luminal membrane, see text for details. (From Bichet D. Nephrogenic and central diabetes insipidus. In: Schrier RW, ed. Diseases of the Kidney and Urinary Tract. Vol 3. 7th ed. New York: Lippincott Williams & Wilkins; 2012:2553, with permission from Wolters Kluwer Health.)