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Regulation of K + Excretion
Overview of K + Distribution and Excretion–Internal and External Balance
As the most abundant cation in intracellular fluid, K + plays an important role in a variety of cell functions. High K + concentration in cells and low K + concentration in extracellular fluid is essential for many cellular processes, including determining the electrical properties of cell membranes in both excitable (nerve, muscle) and nonexcitable (transporting epithelia) tissues. Cell K + also contributes importantly to the effective osmolality of intracellular fluid, and thus to the regulation of cell volume. Changes in cell K + modify intracellular acidity, and thereby indirectly influence a variety of metabolic processes. These important functions depend on the coordinated action of a variety of regulatory mechanisms that serve to maintain total K + content (50–55 mmol/kg body weight) and distribution.
Internal K + Balance
Figure 49.1 schematically shows several features of the distribution of K + in the body. More than 98% of body K + resides within cells, principally in skeletal muscle, whereas only 2% of total body K + is located in the extracellular fluid space. Maintenance of K + homeostasis is challenging, because the daily dietary intake of K + in the adult (~70 mEq) typically approaches the total K + content normally present within the extracellular fluid space (~70 mEq in 17 L of extracellular fluid, with a K + concentration averaging ~4 mEq/l). To maintain zero balance in the adult, dietary intake of K + must be matched by its elimination, a task performed primarily by the kidney.
Ingested potassium enters the extracellular fluid by reabsorption from the small intestine, a process not subject to specific regulation. K + that enters the extracellular fluid must temporarily and rapidly be translocated into cells to prevent dangerous increases in plasma K + levels. The buffering capacity of the combined cellular storage reservoirs, which includes muscle, liver, and red blood cells, is vast compared with the extracellular pool and is capable of sequestering large amounts of K + . The biochemical and hormonal factors that influence the internal balance of K + , typically by altering Na-K-ATPase activity, are listed in Figure 49.2 . Racial differences in K + distribution have also been reported.
The steep K + concentration gradient across the cell membrane depends on the regulated interplay between active uptake by Na + ,K + -ATPase and passive backleak through K + channels and carrier-mediated transport processes ( Figure 49.1b ). When K + enters the extracellular fluid, active Na + ,K + -ATPase-mediated K + uptake into cells occurs rapidly, and buffers against fluctuations of K + in the extracellular fluid. This process is efficient, and plasma K + concentration is kept remarkably constant in the range from 3.5 to 5.0 mM. Variations in K + intake are matched within hours by parallel adjustments in K + excretion, most of which is mediated by the kidney. However, during exercise and ischemia, extracellular K + may quickly rise significantly. To minimize cell ATP decrease and loss of K + production of AMP-activated protein kinase (AMPK), under these conditions, is stimulated by an increase in intracellular AMP-to-ATP ratio favoring translocation of K + into cells in states of exercise and ischemia. Although overshadowed by the kidney, the colon also excretes K + and responds to stimuli calling for a change in excretion rate.
External K + Balance: The Role of the Kidney
To accomplish excretion of the variable quantity of K + ingested daily, the kidney must first extract K + from blood in which K + circulates at a rather low concentration. Indeed, dietary K + may approximate that of sodium (80–120 mM/day), but the concentration of K + in plasma, and therefore the rate at which it is filtered by glomeruli, is only one-thirtieth of that of sodium. Nevertheless, the glomerular filtration rate (GFR) is normally high enough so that K + could be excreted by filtration alone. However, if the GFR is reduced to 10 to 15% of normal, as occurs with chronic renal failure, filtration alone would not be able to keep up with the normal dietary intake. Even if the GFR were reduced only by half, because not all filtered K + can escape reabsorption, it is likely that renal excretion would be inadequate and K + would be retained. Furthermore, even when GFR is normal, an excretion mechanism relying solely on filtration would have a limited capacity for adaptive increase, and could not achieve the 20-fold increase in K + excretion that has been observed in animals exposed to increased intake of K + by diets high in K + or parenteral infusions containing K + . Indeed, net urinary K + excretion reflects not only glomerular filtration, but also tubular reabsorption and secretion. Clearance studies from as early as the 1940s revealed that the kidney is capable of secretion enabling the transfer of K + from plasma to tubule fluid. Table 49.1 summarizes the main features of renal K + transport based on these studies.
Table 49.1
Main Features of Potassium Transport, Based on Clearance Experiments
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A simplified but generally adequate view of renal K + handling is that proximal nephron segments between the glomerulus and the distal convoluted tubule (DCT) reabsorb a rather fixed fraction (80–90%) of the filtered K + , whereas distal tubules and collecting ducts either secrete a variable quantity of K + into tubule fluid or effectively reabsorb K + . By varying the rate and even the direction of K + transport, the distal nephron is able to respond homeostatically to changes in dietary K + intake or to changes in extracellular K + caused by other gains (parenteral administration, release from cellular pools) or losses (from the GI tract or skin). In people ingesting ≈70 mmol/day K + , the usual rate of K + excretion in the postabsorptive period between meals is approximately 10–15% of the rate of K + filtration. In the hours following ingestion of K + -rich meals, the rate of urinary excretion of K + can increase greatly to approach or even exceed the rate at which it is filtered. Such increments in K + excretion can be attributed to an increase in the quantity secreted by distal nephron segments. Increased secretion may not entirely account for increases in K + excretion in all cases, however, and variations in reabsorption by tubule segments, either proximal or distal to the main secretory sites, may become important in some circumstances.
If K + intake is reduced or eliminated (or if the body is depleted of K + by prior renal or nonrenal losses), urinary K + excretion declines rapidly as the deficit in total body K + increases. Within a few days K + excretion can be reduced to very low levels, but K + conservation is less complete than that of sodium.
General Aspects of K + Transport Along the Nephron
The generalization that filtered K + is largely reabsorbed by proximal nephron segments and that excreted K + is secreted by distal segments provides a useful framework for integrating information about renal handling of K + , but a closer look at the cytologically distinct subdivisions of the nephron and the way each handles K + reveals a more complicated picture. K + is not continuously reabsorbed from all tubule segments proximal to the secretory sites in the distal tubules and collecting ducts. The rather constant fractional delivery of K + (10–20% of the filtered quantity) that has been found by collecting and analyzing samples of fluid from the earliest portion of the superficial DCT accessible to micropuncture is achieved by a sequence of reabsorption, secretion, and reabsorption as the glomerular filtrate travels through the proximal tubule and the loop of Henle. Secretion of K + into loops of Henle has been documented only for the juxtamedullary nephron population ; however, it seems likely that the more superficial nephrons behave similarly.
Figures 49.3 and 49.4 represent schematic representations of the renal elements responsible for K + excretion. The location and naming of the subdivisions generally follow the scheme outlined by Kriz and Bankir. Some features that distinguish superficial and deep nephrons are pictured. K + is filtered and reabsorbed from proximal convoluted tubules (PCT) of both superficial and deep nephrons. As the proximal straight tubule (PST) enters the outer medulla, the direction of K + transport reverses, and K + is secreted into the third proximal segment (S3) and the thin descending limb of the loop of Henle. Higher K + concentrations are attained in loops of deeper nephrons that penetrate further into the inner medulla.
K + can be reabsorbed by thick ascending limbs (TALs), and net reabsorption probably occurs in both medullary and cortical TALs of both deep and superficial nephrons. The portion of the distal tubule beyond the macula densa where each ascending limb contacts its parent glomerulus comprises several segments that are cytologically distinguishable. The DCT, extending beyond the macula densa, is functionally distinct from the TAL segment, and probably contributes modestly to K + excretion.
The next segment of the distal tubule – the connecting tubule (CNT), and the segment following it, the cortical collecting duct (CCD) – are major sites of K + secretion. In superficial nephrons, epithelium characteristic of the CCD appears some distance proximal to the first confluence of two distal tubules. This region has also been referred to as the late distal tubule or the initial collecting tubule (ICT). In deeper nephrons, the CNTs frequently join to form arcades before flowing into the collecting duct. The separate segments of the distal tubule are discussed subsequently in more detail. K + is secreted into the collecting duct throughout the cortex, and probably in the outer stripe of the outer medulla as well. However, in the inner stripe of the outer medulla, K + reabsorption appears once again and contributes to K + accumulation in the medullary interstitium. Both secretion and reabsorption of K + have been described along the terminal portions of the inner medullary collecting duct.
K + Transport by Individual Nephron Segments
Glomerulus
K + ions that are free in plasma water pass across the glomerular capillary membrane with little hindrance. Nonfilterable proteins in plasma may bind a small fraction of K + and restrict filtration. The net negative charge on these plasma proteins tends to reduce K + concentration in glomerular filtrate relative to plasma water (Donnan equilibrium), but the concentration of K + in plasma is approximately 6% lower than in plasma water. These factors tend to cancel each other out, and the concentrations of K + in glomerular filtrate and in plasma or serum, are approximately equal.
Although variations in GFR do cause proportional variations in the rate of K + filtration, they do not usually result in large changes in K + excretion, because mechanisms promoting glomerulotubular balance tend to stabilize the rate of K + delivery out of the proximal tubule and the loop of Henle. In the 1950s, when renal clearance methods were first applied systematically to the study of K + regulation by the kidney, it was observed that rates of glomerular filtration and final K + excretion could be varied independently of one another. However, if GFR is reduced enough to decrease sodium and water excretion, the rate of K + excretion will also decrease.
Proximal Convoluted Tubule: Direction, Magnitude, and Mechanism of Transport
Transepithelial K + Transport
Information about the direction and magnitude of K + transport processes along the nephron was obtained in the 1960s by in vivo micropuncture techniques. The main features of K + transport in various nephron segments are summarized in Figure 49.4 . Collections of tubular fluid samples showed that about 50% of filtered K + reaches the last accessible surface segment of the PCT. Collections from sites close to the glomerulus showed that reabsorption of K + , which occurs over most of the accessible proximal tubule, may be preceded by a small K + leak into the lumen. The downstream reabsorption of K + along the proximal tubule, like that of sodium, generally proceeds without developing a large concentration difference across the proximal tubule as a roughly similar fraction of water is also absorbed. Although K + concentration has been reported to increase slightly or to remain unchanged along the proximal tubule, a few studies have provided evidence that proximal reabsorption of K + can proceed against an electrochemical gradient, and that tubular fluid K + concentration may decline (by about 10%) between early and late proximal segments in rat kidneys.
Three mechanisms participate in K + reabsorption by the proximal tubule: solvent drag; diffusion; and apparent active transport. First, the consistently observed association between K + transport and fluid transport suggests that a fraction of proximal K + absorption depends on the simultaneous rate of fluid absorption. This dependence of K + transport on net fluid transport, and the finding of low reflection coefficients of K + , support the notion of direct coupling of K + and fluid through the same transport pathway: a solvent drag mechanism.
Second, diffusion of K + from luminal to peritubular fluid may also occur because fluid absorption may raise the K + concentration in the proximal tubule, thus creating a concentration difference favoring K + absorption. In vivo microperfusion experiments show that proximal K + transport is very sensitive to changes in luminal K + concentration and transepithelial voltage. The high K + permeability in the proximal tubule, and the dependence of K + transport on the transepithelial electrochemical potential difference is consistent with diffusive movement through a paracellular pathway. However, barium, a potent K + channel-blocker, has been shown to block a significant fraction of K + reabsorption, implying possible participation of a transcellular route for reabsorption of K + . However, little is known about this pathway.
Third, the direction of the electrochemical driving force for K + in at least part of the proximal tubule, and the special microenvironment of the paracellular compartment between proximal tubule cells, provides theoretical support for apparent active K + absorption. Such movement of K + ions against an electrochemical potential gradient in the early proximal tubule is implied by experimental findings demonstrating both a concentration of K + in the tubule lumen below that in arterial plasma, and a lumen-negative transepithelial potential. Moreover, dissociation between sodium-driven fluid movement and K + transport has also, albeit rarely, been observed. Micropuncture studies provide evidence that channel-dependent K + fluxes mediate a secretory component of K + movement into the early proximal tubule, but K + reabsorption further downstream exceeds such secretory K + fluxes. Direct measurements of the electrochemical driving force for K + across the apical cell membrane in amphibian proximal tubules also support active K + reabsorption. However, there is no evidence that K-H-ATPase activity identified in the apical membrane of mammalian proximal tubules contributes to K + reabsorption.
A cell model summarizing the complex mechanisms involved in proximal tubular K + transport is pictured in Figure 49.5 . It includes a sodium-K + exchange pump (Na + ,K + -ATPase) in the basolateral membrane, a K + conductance in the apical membrane, and a pathway for K + transport between cells. Two pathways for K + exit across the basolateral membrane are shown: a conductive channel and a K-Cl co-transporter. Note also that the transepithelial potential along the tubule changes from lumen-negative values in the early PCT to lumen-positive values in the late PCT.
Weinstein has suggested that the apparent reabsorptive movement of K + against an electrochemical driving force does not require an active, directly energy-driven reabsorptive maechanism for K + in the apical cell membrane. Given that Na + ,K + -ATPase-driven uptake of K + does occur in the cell membranes lining the paracellular compartment between cells, such transport could deplete this compartment of K + , particularly if the diffusion resistance to K + across the basolateral exit was low. A situation could then develop in which the luminal fluid equilibrates with the low-K + fluid in the interspace, effectively decreasing the K + concentration in the lumen below peritubular plasma levels. Exit of K + ions from the interspace into the peritubular fluid would be driven by bulk movement of fluid and K + along the hydrostatic pressure gradient that normally develops along the interspace from its luminal to basolateral end.
In the later part of the PCT, the transepithelial potential difference becomes lumen-positive, providing an additional driving force for net K + reabsorption. It is likely that K + movement driven by the transepithelial voltage occurs through the paracellular shunt pathway. In vivo microperfusion experiments using mannitol to vary tubular fluid osmolarity reveal that the direction of net K + transport is dependent on that of net fluid transport. Previous observations had already shown that inhibition of proximal sodium and water transport would also block K + reabsorption, an effect consistent with entrainment of K + ions by sodium-dependent fluid reabsorption.
Transepithelial electrochemical gradients of K + demonstrate the effect of changes in the electrical potential difference on tubule K + concentration. Marked differences were observed between K + -replete and K + -depleted animals: whereas a lumen-positive potential was recorded in replete animals, the transepithelial potential difference was reversed in K + -depleted animals, and the K + concentration ratio across the late proximal tubule was significantly elevated. These data support the view that diffusion along an electrochemical gradient can play a critical role in transport of K + across the proximal tubule.
Cell K + Transport
In proximal tubule cells, steady-state levels of cell K + depend on the balance between active uptake from interstitial fluid and passive leakage from the cytosol, either to the interstitium or to the tubule lumen. K + ions are actively taken up by the ATP-driven Na + -K + exchange pump located in the basolateral membranes. Microelectrode measurements of basolateral membrane voltage and of K + activities show that the electrical potential difference across cell membranes of both amphibian and mammalian tubule cells is too small to account for the measured intracellular K + activity by passive distribution. Also, inhibitor studies indicate that ATPase-driven accumulation of K + is responsible for high cell K + concentrations. Inhibition of Na + ,K + -ATPase activity reduces intracellular K + concentrations and content.
The basolateral sodium-K + pump operates in an electrogenic mode: the rate at which sodium ions are pumped out of the cell exceeds the rate at which K + ions are taken in. The contribution of such an electrogenic cation exchange to the steady-state voltage across the basolateral membrane is probably small; however, sudden activation of the pump, either by raising cell sodium, by increasing extracellular K + from low levels or by warming tubules previously cooled, leads to rapid hyperpolarization of the basolateral cell membrane to levels that can exceed the equilibrium potential that could be generated by passive diffusion of K + ions.
Figure 49.6 illustrates additional transporters in the basolateral membranes of proximal tubule cells. K + channels are present in both cell membranes, and serve several functions. They generate the cell-negative electrical potential which constitutes an important driving force for the entry of positively-charged solutes and the basolateral exit of negatively-charged solutes. Sodium-coupled electrogenic glucose and amino acid transport across the apical membrane of proximal tubule cells is facilitated by the cell-negative potential. Chloride diffusion, electrogenic Na + -HCO 3 cotransport and Ca 2+ /Na + exchange are also modulated by the magnitude of the K + -dependent basolateral membrane potential.
K + channels are also involved in volume regulation of proximal tubule cells. Both apical and basolateral K + channels are activated by cell swelling, either directly by stretching of the membrane or indirectly, by volume-dependent Ca 2+ entry through non-selective cation channels. Apical K + channels are also sensitive to changes in membrane voltage, with depolarization leading to increased activity. Apical K + channels in the proximal tubule are critical in stabilizing the cell-negative potential, especially during depolarizing Na + -coupled transport (i.e., with glucose or amino acids). The K + channel KCNQ1 and the accessory protein KCNE1 have been localized to the brush border of the mid-to-late proximal tubule. They have been proposed to play a role in net K + secretion in the early proximal tubule, and in polarizing the brush border membrane to maintain the electrical driving force for Na + -coupled transport. In support of this view, mice lacking KCNE1 exhibit increased renal excretion of Na + and glucose, and signs of volume-depletion.
Basolateral K + channels are inhibited by an increase in cell ATP, by a fall in pH, by cyclic AMP and taurine, and they have been implicated in renal cell damage by hypoxia.
Coupling Between Sodium Transport and Basolateral K + Channels
The constancy of intracellular K + in the presence of large changes in transepithelial net sodium transport depends on appropriate modulations of the basolateral K + conductance. Because a major pathway for sodium reabsorption is transcellular and involves the basolateral sodium-K + exchange pump, large changes in the rate of basolateral sodium extrusion necessarily cause large changes in K + uptake. However, by varying the magnitude of the basolateral-leak conductance in proportion to changes in pump rate, cells in renal and other transporting epithelia are able to maintain cytosolic K + activity, and cell volume, within narrow limits.
Pump-Leak Coupling
Transport-related changes in the coupling between active sodium extrusion across the basolateral membrane and apical and basolateral K + conductances are depicted in Figure 49.7a and 49.7b . Changes in cell volume and pH have a significant effect on K + channels, alkalosis increasing and acidosis decreasing the open probability. During substrate-induced stimulation of proximal sodium transport, cell pH rises with a time-course that matches the observed increase in basolateral K + conductance.
Changes in the concentration of Ca 2+ in tubule cells, especially those correlated with fluctuations in cell volume and nitric oxide, may also couple basolateral Na + ,K + -ATPase activity to K + permeability. Alterations in basolateral cell potential have also been implicated, since stimulation of electrogenic Na + ,K + pump activity hyperpolarizes tubule cells. Such membrane voltage changes are known to activate voltage-sensitive K + channels.
Renal K + channel activity, including that of channels in the basolateral membrane of proximal tubule cells, is also sensitive to alterations in cellular ATP. Small amounts of ATP are required for the activity of some K + channels in the renal tubule, but millimolar concentrations inhibit K + channel activity. This effect is reversed by ADP, and points to the involvement of the ATP/ADP ratio in the control of K + channel activity. It appears that transport-related changes in cell ATP levels modify cell K + conductance. Thus, stimulation of sodium transport in proximal tubules results in a significant fall in cell ATP, whereas inhibition of sodium transport increases cell ATP levels. This cross-talk mechanism, linking apical sodium transport to basolateral K + channel activity, is shown in Figure 49.7 . Thus, transport-related changes in the basolateral membrane potential of tubule cells may be involved in coupling the K + conductance to the pump activity. Measurements of the basolateral conductance of tubule cells, as well as patch-clamp studies, in which the open probability of basolateral K + channels was examined as a function of the membrane potential, show an increase in K + conductance with cell hyperpolarization. To the extent that stimulation of basolateral ATPase activity elevates the cell-negative potential, K + conductance would also be expected to increase interaction between basolateral and apical membrane transport.
Schultz has drawn attention to an additional relationship between transport events in the luminal and basolateral membrane of epithelial cells that involves changes in the pump-related K + conductance. In leaky epithelia, such as the proximal tubule, stimulation of co-transport of sodium ions with organic solutes such as glucose or amino acids augments sodium entry across the apical membrane. Since the co-transporter carries positive current into the cell, the cell-negative electrical potential is reduced. In tight epithelia, such as the CCD, mineralocorticoids increase apical sodium conductance, and thus bring about depolarization of the apical cell membrane. The increase in K + conductance that occurs with stimulation of sodium pumping across the basolateral membrane also provides an important transport-sustaining feedback loop. The rise in potassium conductance hyperpolarizes the basolateral membrane and renders net transport more effective. An essential feature of these effects is that maneuvers that increase sodium transport tend to curtail further sodium entry, because the depolarization of the apical cell membrane reduces the electrochemical driving force for further sodium transport into the cell.
Loop of Henle: K + Recycling, Direction, Magnitude, and Mechanism of Transport
In vivo micropuncture and microperfusion studies show net reabsorption of K + between the last accessible segment of surface proximal tubules and the first accessible segment of surface distal tubules. Thus, only a modest fraction, some 5–10% of the filtered K + , reaches the early distal tubule (see Figure 49.4 ). As shown in Figure 49.3 , this portion of the nephron comprises several morphologically and functionally distinct segments: the third segment of the proximal tubule (S3); the thin descending and thin ascending limbs; and the medullary and cortical thick ascending limbs of the loop of Henle.
The K + concentration in the loop of Henle fluid near the tip of the papilla can be as much as 10 times higher than the K + concentration in systemic plasma. de Rouffignac and Morel suggested that K + is added to tubule fluid along the descending limb of Henle’s loop after being absorbed from the ascending limb and collecting duct. The phenomenon constitutes K + recycling ( Figure 49.8 ). Jamison and co-workers further showed that K + delivery to the end of the descending limb of deep nephrons could equal the rate of K + filtration in normal rats, and exceeded it in rats either fed a high-K + diet or infused acutely with K + . The observation that isolated thin limbs of Henle lack a mechanism of active K + secretion supports the view that K + enters the descending limb passively. If K + is also secreted into the proximal straight tubule of superficial nephrons, and the delivery of K + to the DCT of deep nephrons is substantially less than the filtered quantity, then K + must be reabsorbed by ascending limbs in both populations of nephrons. Thus, K + is trapped in the medulla by countercurrent exchange between the ascending and descending limbs of the loop of Henle. Studies of isolated TAL perfused in vitro show that these segments are capable of absorbing K + .
As originally postulated by Jamison et al. and shown in Figure 49.8 , the main pathway by which K + can reach the renal medulla is absorption from the medullary collecting duct. Stokes has shown that the outer medullary collecting duct is adequately permeable to both sodium and K + , and therefore could permit K + reabsorption to occur passively. K + secretion by cells of the distal tubule and the cortical collecting duct into the tubular fluid provide the source of K + that accumulates in the renal medulla. Accordingly, more K + recycles with stimulation of secretion, whereas suppression of secretion attenuates the deposition of K + in the renal medulla. Thus, K + is secreted into tubule fluid in (at least) two sites along the nephron (see Figure 49.8 ), first into a proximal part of the nephron (the end-proximal tubule and descending limb), and second into a more distal region (the CT and CCD). When K + intake is suddenly increased after a period of K + deprivation, renal K + excretion, distal K + secretion, and K + recycling all increase. Stokes also proposed that rising medullary K + concentrations inhibit NaCl and water absorption by the loop of Henle, and thus increase the flow rate and sodium concentration of fluid entering the distal tubule, providing optimum conditions for the distal segments to secrete K + . Medullary recycling is thus responsible for maintaining high concentrations of K + in the renal medulla. Such trapping of K + is also expected to limit reabsorptive loss of K + from the terminal collecting ducts.
Thick Ascending Limb (TAL) Cell Transport Mechanisms
Studies of isolated mammalian TALs and amphibian ( Amphiuma ) early distal tubule have provided similar K + transport models ( Figure 49.9 ). The primary driving force for net K + reabsorption is again ATP-driven active Na + -K + exchange across the basolateral membrane, which generates the steep sodium gradient for the entry of Na + across the apical cell membrane.
Apical Membrane
Entry of Na + is primarily mediated by an Na + −2Cl − −K (or under certain conditions, Na,Cl) co-transport mechanism inhibited by the potent “loop” diuretics. Also present in the apical membrane is a second pathway for sodium entry into cells, the Na-H exchanger (not shown in Figure 49.9 ). Conductive pathways for K + are found in both the apical and basolateral membranes. The K + concentration gradient across the apical membrane favors K + secretion, which permits recycling of K + ions back to the tubule lumen. This back-diffusion provides a continuous luminal supply of K + for co-transport with sodium and chloride. The K + leak in the luminal membrane is also responsible for net K + secretion under those conditions in which the activity of the reabsorptive co-transport mechanism has been impaired.
Evidence supporting the operation of the Na + −2Cl − −K + co-transporter in the apical membrane includes the mutual dependence of these ions for transport. Net reabsorption of Na + , K + , and Cl − , and the lumen-positive potential are abolished in the luminal absence of any of the three ions. In addition, luminal application of furosemide also eliminates transport of all three ions, and collapses the transepithelial potential. Interference with the co-transport system, either by appropriate luminal ion substitution or by administration of furosemide, lowers cell chloride and sodium activity, thereby supporting the view that the co-transport system is the main apical pathway for entry of these ions into the cell. Additional evidence for the requirement of a steady supply of luminal K + to safeguard Na + −2Cl − −K + co-transport in the cortical TAL is the observation that sodium reabsorption decreases sharply following the depletion of K + from the lumen or following the administration of K + channel-blockers. These maneuvers also attenuate tubuloglomerular feedback.
Morphological and electrophysiological criteria suggest the presence of at least two cell types in TALs, each with different permeability properties of the apical and basolateral cell membranes. In the cortical TAL the K + permeability in the apical membrane exceeds that of the basolateral membrane, whereas the converse relationship was observed in the medullary TAL. These permeability differences may be associated with functional disparities, K + secretion being the prevalent transport mode in the cortical TAL, and reabsorption in the medullary TAL.
Patch-clamp techniques have identified at least two K + channels in the apical membrane that mediate K + recycling. The open probability of these apical K + channels is high, and they are inhibited by barium, low cell pH, millimolar concentration of ATP, and protein kinase C (PKC). Cell alkalinization and cyclic AMP, as well as NO and cyclic GMP, stimulate K + channels. Apical K + channels are also activated by furosemide, an effect mimicking the effects of aldosterone. Aldosterone in amphibian early distal tubules, enhances apical Na-H exchange and alkalinizes cell pH. When furosemide is given, apical sodium entry is blocked and the intracellular sodium concentration decreases. As a consequence, the driving force for apical Na-H exchange increases and alkalinizes the cytoplasm. Because apical K + channels are stimulated by cell alkalinization, their activity increases. This sequence of events explains why the full expression of the aldosterone effect on K + permeability depends on an intact Na-H exchange in the apical cell membrane.
A difference between the transport modes of cortical and medullary TAL segments has also been observed regarding the K + sensitivity of the apical co-transporter to K + . Figure 49.10 shows two cell models in which vasopressin switches the apical sodium chloride transporter from a K + -insensitive mode to one that depends critically on the presence of K + in the tubule lumen.
An interesting effect concerns the sequence of events following an increase in peritubular Ca 2+ , which has been shown to inhibit both apical K + channels and Na + −2Cl − −K + co-transport. Patch-clamp studies have demonstrated that the inhibitory effects of Ca 2+ are mediated through activation of P450 and PKC. It is likely that the extracellular Ca 2+ ion-sensing receptor (CaSR) present on the basolateral membrane of the TAL plays a role in this response. The CaSR is a G-protein-coupled surface receptor that is activated by increases in extracellular Ca 2+ ion concentrations, and has been shown to inhibit NaCl absorption and K + channels in the TAL by eicosanoids and P450 metabolites.
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