Renal Epithelial Transport and Nephron Function

The normal human glomerular filtration rate (GFR) is approximately 180 L/day. Assuming a normal plasma Na⁺ concentration of 140 mmol/L, that means that 25,200 mmol of Na⁺ are filtered each day. To maintain Na⁺ balance, the kidney must reabsorb more than 99% (24,950 mmol) of the filtered load of Na⁺. This staggering amount of solute and water reabsorption is achieved by the actions of the million nephrons in each human kidney. Renal epithelial cells transport solute and water from the apical cell membrane to the basolateral cell membrane. The polarization of structures that differentiate the apical membrane from the basolateral membrane allows the vectorial transport of solute and water (Fig. 21-2). The basolateral cell membrane expresses the ubiquitous Na⁺/K⁺-adenosine triphosphatase (ATPase) (the Na⁺ pump) that exchanges 3 Na⁺ ions for 2 K⁺ ions, resulting in decreased intracellular Na⁺ providing a chemical gradient and an electronegative cell interior for Na⁺ entry from the lumen through the apical membrane (a potential difference of approximately 60 mV), which also attracts Na⁺. The concentration gradient then favors passive efflux of the K⁺ that entered the cell to the intercellular space. Because the ECF concentration of K⁺ is low relative to Na⁺, a recycling of K⁺ between the cell and interstitial fluid is necessary to maintain the Na⁺ pump.

FIGURE 21–2 Polarized renal epithelial cells. Distinct transporters are present in apical cell and basolateral cell membranes to mediate the net transepithelial transport of Na⁺ and water. Operation of the basolateral cell membrane Na⁺/K⁺-ATPase (1) initiates the movement of Na⁺ and water by decreasing the intracellular Na⁺ concentration (2) and maintaining a negative interior potential in the cell. Na⁺ enters the cell from the lumen (3) down an electrochemical gradient via three types of transport mechanisms: channel (A), symport (B) in which X may be glucose, amino acids, or PO₄, and antiport (C).

The transport of Na⁺ across the apical cell membrane adjacent to the tubular fluid is achieved by passive diffusion via proteins that form a pore or channel (see Fig. 21-2, A), and two types of carrier-mediated transport, a cotransport (symport) pathway that transports Na⁺ and another solute species (such as Cl⁻ or amino acids) in the same direction (see Fig. 21-2, B), and a countertransport (antiport) pathway that transports Na⁺ and another solute species (H⁺) in the opposite direction (see Fig. 21-2, C). In each case the low intracellular Na⁺ concentration as a consequence of the action of the Na⁺/K⁺-ATPase pump provides the electrochemical gradient for Na⁺ entry.

This transepithelial transport causes the osmolality of the lateral intercellular spaces to increase as a result of the accumulation of solute, producing an osmotic gradient that permits water to flow by two routes (see Fig. 21-2):

In the primary pathway of water flow, water moves from lumen to cell to interstitial fluid to capillary, as a direct result of the transepithelial osmotic gradient.

Tubular Reabsorption: Transport by the Proximal Tubule

The GFR of healthy adults ranges from 1.7 to 1.8 mL/min/kg, and approximately two thirds of the water and NaCl filtered at the glomerulus is reabsorbed by the proximal tubule. , glucose, amino acids, and other organic solutes are also reabsorbed. When GFR increases, salt and water excretion increases, but fractional reabsorption in the proximal tubule does not change. This is termed glomerulotubular balance. It moderates but does not entirely eliminate the effects of alterations in the GFR on salt and water excretion.

The transport of Na⁺, , and Cl⁻ is important for the actions of several diuretics on the proximal tubule. Na⁺ is reabsorbed primarily with in the early proximal tubule, whereas Na⁺ is reabsorbed primarily with Cl⁻ in the late proximal tubule (Fig. 21-3). At the apical cell membrane of the early proximal tubule, Na⁺ entry is coupled with H⁺ efflux via the Na⁺/H⁺ antiporter (NHE3), which is a protein containing 10 to 12 transmembrane spanning domains and a hydrophilic C-terminal domain and is subject to regulation by a variety of factors, including angiotensin II, which increases its activity. H⁺ extruded from the cell combines with to form H₂CO₃, which rapidly forms CO₂ and water in the presence of the enzyme carbonic anhydrase. CO₂ rapidly enters the cell via simple diffusion and is rehydrated to form carbonic acid. Because the concentration of cellular H⁺ is low, the reaction proceeds as follows: CO₂ + H₂O → H₂CO₃ → H⁺ + . Thus a constant supply of H⁺ is furnished for countertransport with Na⁺. that accumulates is cotransported with Na⁺ across the basolateral cell membrane into the interstitial fluid and, subsequently, into the blood. Although the cytoplasmic hydration reaction occurs spontaneously, the rate is inadequate to allow reabsorption of the load filtered (approximately 4000 mEq/day). However, little or none of the filtered is excreted because of the presence of carbonic anhydrase. The net effect of coupling of the Na⁺/H⁺ antiporter to the carbonic anhydrase-mediated hydration and rehydration of CO₂ is preservation of . The final step is its transfer from the interstitial fluid into peritubular capillaries.

FIGURE 21–3 Ion transport in early and late proximal tubule cells. CA, Carbonic anhydrase; NHE3, the Na⁺/H⁺ exchange protein.

In the late proximal tubule (see Fig. 21-3), Na⁺ is reabsorbed primarily with Cl⁻. Reabsorption is secondary to activation of both the NHE3, which couples inward Na⁺ transport with outward H⁺ transport, and a Cl⁻ base (formate) exchanger that transports Cl⁻ from lumen to cell in exchange for a base. The parallel operation of both exchangers results in net Na⁺ and Cl⁻ absorption by the late proximal tubule. Passive transport of Na⁺ and Cl⁻ also occurs between cells through the paracellular pathway.

Reabsorptive transport systems of the proximal tubule deliver large amounts of fluid and solutes to the interstitial space, which raises pressure in the interstitium. For reabsorption to continue, pressure must be decreased. The permeable peritubular capillary can easily carry away reabsorbed fluids and solutes. Pushed by interstitial pressure and pulled by the oncotic pressure of intracapillary proteins (higher in postglomerular than in preglomerular capillaries), filtered fluid and solutes return to the blood.

In summary, the proximal tubule reabsorbs approximately 70% of filtered water, Na⁺, and Cl⁻, 85% of filtered , and 50% of filtered K⁺ (Table 21-2). These percentages are relatively constant, even when filtered quantities increase or decrease. As a result, minor fluctuations in GFR do not influence fluid and electrolyte excretion very much. The driving force for reabsorption of water and electrolytes is the Na⁺/K⁺-ATPase. Passive movements of other ions and water are initiated and sustained by active transport of Na⁺ across basolateral cell membranes. Osmotic equilibrium with plasma is maintained to the end of the proximal tubule. Most of the filtered is not actually reabsorbed directly from the lumen; rather, it is converted to CO₂ and water in the vicinity of the brush border membranes, within which large concentrations of carbonic anhydrase are located. The direction of this reaction is H₂CO₃ → CO₂ + H₂O (established by the high concentration of carbonic acid in luminal fluid resulting from the secretion of H⁺). Carbonic anhydrase in the cytoplasm catalyzes formation of carbonic acid. Cellular H⁺ is then exchanged for luminal Na⁺, and is reabsorbed across the basolateral cell membranes. In this indirect way, filtered is reabsorbed.

TABLE 21–2 Summary of Reabsorption in the Proximal Tubule^*

Tubular Reabsorption: Transport by the Loop of Henle

Diuretics have no discernible actions in the descending limb of the loop of Henle. These cells permit water to diffuse from the lumen to the medullary interstitium, where higher osmotic pressures are encountered, but they do not contain specialized transport systems and are relatively impermeable to Na⁺ and Cl⁻.

In contrast, the thick ascending limb and its transport functions are an important site of action of the loop (also called high-ceiling) diuretics (Fig. 21-4). Approximately 25% to 35% of filtered Na⁺ and Cl⁻ is reabsorbed by the loop of Henle. The Na⁺/K⁺-ATPase in the basolateral membrane provides the gradient for Na⁺ and Cl⁻ absorption. Na⁺ entry across the apical membrane is mediated by an electroneutral transport protein that binds one Na⁺, one K⁺, and two Cl⁻ ions and is referred to as the Na⁺/K⁺/2Cl⁻ (NKCC2) cotransporter. Although the ascending limb is highly permeable to Na⁺, K⁺, and Cl⁻, it is impermeable to water. Thus the continuous reabsorption of these ions without reabsorption of water dilutes the luminal fluid, thus the name diluting segment. Na⁺ entry down an electrochemical gradient drives the uphill transport of K⁺ and Cl⁻. This system depends on the simultaneous presence of these three ions in the luminal fluid. Once inside the cell, K⁺ passively reenters the lumen (K⁺ recycling) via conductive K⁺ channels in the apical membrane. Cl⁻, on the other hand, exits the cell via conductive Cl⁻ channels in the basolateral membrane. Depolarization of the basolateral membrane occurs as a consequence of Cl⁻ efflux, creating a lumen-positive (relative to the interstitial fluid) transcellular potential difference of approximately 10 mV. This drives paracellular cation transport, including Na⁺, Ca⁺⁺, and Mg⁺⁺. Inhibition of the NKCC2 cotransporter not only results in excretion of Na⁺ and Cl⁻ but also in excretion of divalent cations, such as Ca⁺⁺ and Mg⁺⁺.

FIGURE 21–4 Ion transport by thick ascending limb cells. This segment, also referred to as the diluting segment, is impermeable to water, and thus the tubular lumen concentration of ions decreases.

Based on its molecular structure, the NKCC2 cotransporter belongs to a family referred to as electroneutral Na⁺/Cl⁻ cotransporters, which also includes the Na⁺/Cl⁻ cotransporter (NCC) sensitive to thiazide diuretics. These proteins have a structure similar to NHE3 and appear to be up regulated by reduction of intracellular Cl⁻ activity and cell shrinkage. Bartter’s syndrome (a renal tubular disorder) type I kindreds apparently have mutations in the gene encoding the NKCC2 transporter; Types II and III of this syndrome result from mutations in channels. The well-recognized countercurrent mechanism in the renal medulla depends on the activity of this cotransport system, and drugs that inhibit this pathway diminish the ability of the kidney to excrete urine that is either more concentrated or more dilute than plasma.

In summary, fluid is reabsorbed from the lumen of the descending limb of the loop as it progresses deeper into the medullary areas of higher osmotic pressure. Electrolyte concentrations increase to a maximum at the bend and gradually decrease as the NKCC2 cotransport mechanism and Na⁺ pump, working in tandem, achieve reabsorption of Na⁺, K⁺, and Cl⁻. The thick ascending limb reabsorbs 25% of filtered NaCl and 40% of filtered K⁺, but not water, whereas the entire loop reabsorbs 15% of the fluid.

Tubular Reabsorption: Transport by the Distal Convoluted Tubule

In contrast to the proximal tubule and loop of Henle, there is less reabsorption of water and electrolytes in the distal convoluted tubule. It reabsorbs approximately 10% of the filtered load of NaCl. Similar to the thick ascending limb, this segment is impermeable to water, and the continuous reabsorption of NaCl further dilutes tubular fluid. NaCl entry across the apical membrane is mediated by the electroneutral NCC cotransporter sensitive to thiazide diuretics (Fig. 21-5). Unlike the NKCC2 cotransporter of the thick ascending limb, this cotransporter does not require participation of K⁺. As in other segments, the basolateral Na⁺/K⁺-ATPase provides the low intracellular Na⁺ concentration that facilitates downhill transport of Na⁺. The distal tubule does not have a pathway for K⁺ recycling, and therefore the transepithelial voltage is near zero. Therefore the reabsorption of Ca⁺⁺ and Mg⁺⁺ is not driven by electrochemical forces. Instead, Ca⁺⁺ crosses the apical membrane via a Ca⁺⁺ channel and exits the basolateral membrane via the NCC exchanger. Thus, by inhibiting the NCC cotransporter, thiazide diuretics indirectly affect Ca⁺⁺ transport through changes in intracellular Na⁺. Another mechanism of increased Ca⁺⁺ reabsorption with thiazide diuretics is an increase in the intracellular concentrations of Ca⁺⁺-binding proteins.

FIGURE 21–5 Ion transport by the distal convoluted tubule cell. As in the case for the thick ascending limb, this segment is relatively impermeable to water.

Recently, inactivating mutations have been found in the human gene encoding the NCC transporter in patients with Gitelman’s syndrome, characterized by hypotension, hypokalemia, hypomagnesemia, and hypocalciuria, similar to the effects of thiazides. Pseudohypoaldosteronism type II is an autosomal dominant disease characterized by hypertension, hyperkalemia, and sensitivity to thiazide diuretics. It has been suggested that an activating mutation of the NCC cotransporter is responsible. Recently, two protein kinases have also been linked to the pathogenesis of this syndrome. They are found in the distal nephron and are thought to control the activity of the cotransporters.

Tubular Reabsorption: Transport by the Collecting Tubule

The collecting tubule is the final site of Na⁺ reabsorption, and approximately 3% of filtered Na⁺ is reabsorbed by this segment. Although the collecting tubule reabsorbs only a small percentage of the filtered load, two characteristics are important for diuretic action. First, this segment is the site of action of aldosterone, a hormone controlling Na⁺ reabsorption and K⁺ secretion (see Chapter 39). Second, virtually all K⁺ excreted results from its secretion by the collecting tubule. Thus the collecting tubule contributes to the hypokalemia induced by diuretics.

The collecting tubule is composed of two cell types with separate functions. Principal cells are responsible for the transport of Na⁺, K⁺, and water, whereas intercalated cells are primarily responsible for the secretion of H⁺ or . Intercalated cells are of two types, A and B, the former responsible for secretion of H⁺ via an H⁺-ATPase (primary active ion pump) in the apical cell membrane, and the latter responsible for secretion of via a Cl⁻/ exchanger in the apical membrane. In contrast to more proximal cells, the apical membrane of principal cells does not express cotransport or countertransport systems; rather it expresses separate channels that permit selective conductive transport of Na⁺ and K⁺ (Fig. 21-6). Na⁺ is reabsorbed through a conductive Na⁺ channel. The low intracellular Na⁺ as a result of the basolateral Na⁺/K⁺-ATPase generates a favorable electrochemical gradient for Na⁺ entry through epithelial Na⁺ channels (ENaCs). Because Na⁺ channels are present only in the apical cell membrane of principal cells, Na⁺

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