HCO₃⁻ Reabsorption along the Nephron

As indicated by Equation 36-7, net acid excretion is maximized when little or no HCO₃⁻ is excreted in urine. Indeed, under most circumstances, very little HCO₃⁻ appears in urine. Because HCO₃⁻ is freely filtered at the glomerulus, approximately 4320 mEq/day is delivered to the nephrons and then reabsorbed. Figure 36-1 summarizes the contribution of each nephron segment to reabsorption of the filtered HCO₃⁻.

Figure 36-1 Segmental reabsorption of HCO₃⁻. The fraction of the filtered load of HCO₃⁻ reabsorbed by the various segments of the nephron is shown. Normally, the entire filtered load of HCO₃⁻ is reabsorbed and little or no HCO₃⁻ appears in urine. CCD, cortical collecting duct; DT, distal tubule; IMCD, inner medullary collecting duct; PT, proximal tubule; TAL, thick ascending limb.

The proximal tubule reabsorbs the largest portion of the filtered load of HCO₃⁻. Figure 36-2 summarizes the primary transport processes involved. H⁺ secretion across the apical membrane of the cell occurs by both an Na⁺-H⁺ antiporter and H⁺-ATPase. The Na⁺-H⁺ antiporter (NHE3) is the predominant pathway for H⁺ secretion and uses the lumen-to-cell [Na⁺] gradient to drive this process (i.e., secondary active secretion of H⁺). Within the cell, H⁺ and HCO₃⁻ are produced in a reaction catalyzed by carbonic anhydrase. The H⁺ is secreted into tubular fluid, whereas HCO₃⁻ exits the cell across the basolateral membrane and returns to the peritubular blood. Movement of HCO₃⁻ out of the cell across the basolateral membrane is coupled to other ions. The majority of HCO₃⁻ exits via a symporter that couples the efflux of 1Na⁺ with 3HCO₃⁻ (sodium bicarbonate cotransporter: NBC1). In addition, some of the HCO₃⁻ may exit in exchange for Cl⁻ (via Na⁺-independent and/or Na⁺-dependent Cl⁻-HCO₃⁻ antiporters). As noted in Figure 36-2, carbonic anhydrase is also present in the brush border of the proximal tubule cells. This enzyme catalyzes the dehydration of H₂CO₃ in luminal fluid and thereby facilitates reabsorption of HCO₃⁻.

Figure 36-2 Cellular mechanism for the reabsorption of filtered HCO₃⁻ by cells of the proximal tubule. Only the primary H⁺ and HCO₃⁻ transporters are shown. CA, carbonic anhydrase.

The cellular mechanism for reabsorption of HCO₃⁻ by the thick ascending limb of the loop of Henle is very similar to that in the proximal tubule. H⁺ is secreted by an Na⁺-H⁺ antiporter and H⁺-ATPase. As in the proximal tubule, the Na⁺-H⁺ antiporter is the predominant pathway for secretion of H⁺. Exit of HCO₃⁻ from the cell involves both a 1Na⁺-3HCO₃⁻ symporter (although the isoform is different from that in the proximal tubule), and a Cl⁻-HCO₃⁻ antiporter (anion exchanger: AE-2). A K⁺-HCO₃⁻ symporter in the basolateral membrane may also contribute to exit of HCO₃⁻ from the cell.

The distal tubule^* and collecting duct reabsorb the small amount of HCO₃⁻ that escapes reabsorption by the proximal tubule and loop of Henle. Figure 36-3 shows the cellular mechanism of H⁺/HCO₃⁻ transport by intercalated cells located within these segments (see Chapter 32).

Figure 36-3 Cellular mechanisms for the reabsorption and secretion of HCO₃⁻ by intercalated cells of the collecting duct. Only the primary H⁺ and HCO₃⁻ transporters are shown. CA, carbonic anhydrase.

One type of intercalated cell secretes H⁺ (reabsorbs HCO₃⁻) and is called the A- or α-intercalated cell. Within this cell, H⁺ and HCO₃⁻ are produced by the hydration of CO₂; this reaction is catalyzed by carbonic anhydrase. H⁺ is secreted into tubular fluid via two mechanisms. The first involves an apical membrane H⁺-ATPase. The second couples the secretion of H⁺ with the reabsorption of K⁺ via an H⁺,K⁺-ATPase similar to that found in the stomach. The HCO₃⁻ exits the cell across the basolateral membrane in exchange for Cl⁻ (via a Cl⁻-HCO₃⁻ antiporter: AE-1) and enters the peritubular capillary blood. Other HCO₃⁻ transporters have been localized to this cell. However, their role in H⁺ secretion (HCO₃⁻ reabsorption) has not been completely defined.

A second population of intercalated cells secrete HCO₃⁻ rather than H⁺ into the tubular fluid (also called B- or β-intercalated cells).^† In these cells, the H⁺-ATPase is located in the basolateral membrane, and the Cl⁻-HCO₃⁻ antiporter is located in the apical membrane (Fig. 36-3). However, the apical membrane Cl⁻-HCO₃⁻ antiporter is different from the one found in the basolateral membrane of the H⁺-secreting intercalated cells and has been identified as pendrin. Other HCO₃⁻ transporters have been localized to the HCO₃⁻-secreting intercalated cell, but their precise role in the function of the cell has not been defined. The activity of the HCO₃⁻-secreting intercalated cell is increased during metabolic alkalosis, when the kidneys must excrete excess HCO₃⁻. However, under most conditions (i.e., ingestion of a meat-containing diet), H⁺ secretion predominates in these segments.

The apical membrane of collecting duct cells is not very permeable to H⁺, and thus the pH of tubular fluid can become quite acidic. Indeed, the most acidic tubular fluid along the nephron (pH of 4.0 to 4.5) is produced there. In comparison, the permeability of the proximal tubule to H⁺ and HCO₃⁻ is much higher, and tubular fluid pH falls to only 6.5 in this segment. As explained later, the ability of the collecting duct to lower the pH of tubular fluid is critically important for the excretion of urinary titratable acids and NH₄⁺.

Regulation of H⁺ Secretion

A number of factors regulate secretion of H⁺ and thus reabsorption of HCO₃⁻ by cells of the nephron (Table 36-1). From a physiological perspective, the primary factor that regulates H⁺ secretion by the nephron is a change in systemic acid-base balance. Thus, acidosis stimulates H⁺ secretion, whereas H⁺ secretion is reduced during alkalosis. The response of the kidneys to changes in acid-base balance includes both immediate changes in the activity or number of transporters in the membrane (or both) and longer-term changes in the synthesis of transporters. For example, with metabolic acidosis, whether produced by a decrease in ECF [HCO₃⁻] or by an increase in the partial pressure of carbon dioxide (PCO₂), the pH of cells of the nephron decreases. This will stimulate H⁺ secretion by multiple mechanisms, depending on the particular nephron segment. First, the decrease in intracellular pH will create a more favorable cell-to–tubular fluid [H⁺] gradient and thereby make the secretion of H⁺ across the apical membrane more energetically favorable. Second, the decrease in pH may lead to allosteric changes in transport proteins, thereby altering their kinetics. This has been reported for the Na⁺-H⁺ antiporter (NHE3) in the proximal tubule. Finally, transporters may be shuttled to the membrane from intracellular vesicles. This mechanism occurs in both the intercalated cells of the collecting duct, where acidosis stimulates the exocytotic insertion of H⁺-ATPase into the apical membrane, and in the proximal tubule, where insertion of the Na⁺-H⁺ antiporter and H⁺-ATPase into the apical membrane occurs. With long-term acidosis, the abundance of transporters increases, either by increased transcription of appropriate transporter genes or by increased translation of transporter mRNA. Examples include the Na⁺-H⁺ antiporter and the 1Na⁺-3HCO₃⁻ symporter in the proximal tubule and H⁺-ATPase in the intercalated cell.

Table 36-1 Factors Regulating H⁺ Secretion (HCO₃⁻ Reabsorption) by the Nephron

* Effect on the proximal tubule is established. It may also regulate H⁺ secretion in other nephron segments.

Although some of the effects just described may be directly attributable to the decrease in intracellular pH, most of these changes in cellular H⁺ transport are mediated by hormones or other factors. Two important mediators of the renal response to acidosis are endothelin and cortisol. Endothelin-1 (ET-1) is produced by endothelial and proximal tubule cells, and thus it exerts its effects via autocrine and paracrine mechanisms. With acidosis, secretion of ET-1 is enhanced. In the proximal tubule, ET-1 stimulates the phosphorylation and subsequent insertion of the Na⁺-H⁺ antiporter into the apical membrane and insertion of the 1Na⁺-3HCO₃⁻ symporter into the basolateral membrane. ET-1 may mediate the response to acidosis in other nephron segments as well. Acidosis also stimulates secretion of the glucocorticoid hormone cortisol by the adrenal cortex. Cortisol in turn acts on the kidneys to increase transcription of the Na⁺-H⁺ antiporter and 1Na⁺-3HCO₃⁻ symporter genes in the proximal tubule, as well as increase translation of the mRNA of these transporters.

Alkalosis, caused by an increase in ECF [HCO₃⁻] or a decrease in PCO₂, inhibits secretion of H⁺ secondary to an increase in the intracellular pH of nephron cells. Thus, the responses just described for the renal adaptation to acidosis are reversed.

Table 36-1 also lists other factors that influence secretion of H⁺ by cells of the nephron. However, these factors are not directly related to the maintenance of acid-base balance. Because H⁺ secretion in the proximal tubule and thick ascending limb of the loop of Henle is linked to the reabsorption of Na⁺ (via the Na⁺-H⁺ antiporter), factors that alter Na⁺ reabsorption secondarily affect H⁺ secretion. For example, the process of glomerulotubular balance ensures that the reabsorption rate of the proximal tubule is matched to the glomerular filtration rate (GFR) (see Chapter 33). Thus, when the GFR is increased, the filtered load to the proximal tubule is increased, and more fluid (including HCO₃⁻) is reabsorbed. Conversely, a decrease in the filtered load results in decreased reabsorption of fluid and thus HCO₃⁻.

Alterations in Na⁺ balance, through changes in ECF volume, also have an impact on H⁺ secretion. With volume contraction (negative Na⁺ balance), secretion of H⁺ is enhanced. This occurs via several mechanisms. One mechanism involves the renin-angiotensinaldosterone system, which is activated by volume contraction and leads to enhanced reabsorption of Na⁺ by the nephron (see Chapter 34). Angiotensin II acts on the proximal tubule to stimulate the apical membrane Na⁺-H⁺ antiporter, as well as the basolateral 1Na⁺-3HCO₃⁻ symporter. This stimulatory effect includes increased activity of the transporters and exocytotic insertion of transporters into the membrane. To a lesser degree, angiotensin II stimulates H⁺ secretion in the early portion of the distal tubule, a process also mediated by the Na⁺-H⁺ antiporter. Aldosterone’s primary action on the distal tubule and collecting duct is to stimulate Na⁺ reabsorption by principal cells (see Chapter 33). However, it also stimulates intercalated cells in these segments to secrete H⁺. This effect is both indirect and direct. By stimulating Na⁺ reabsorption by principal cells, aldosterone hyperpolarizes the transepithelial voltage (i.e., the lumen becomes more electrically negative). This change in transepithelial voltage then facilitates the secretion of H⁺ by the intercalated cells. In addition to this indirect effect, aldosterone acts directly on intercalated cells to stimulate H⁺ secretion. The precise mechanism or mechanisms for this stimulatory effect are not fully understood.

Another mechanism by which ECF volume contraction enhances H⁺ secretion (HCO₃⁻ reabsorption) is via changes in peritubular capillary Starling forces. As described in Chapters 33 and 34, ECF volume contraction alters the peritubular capillary Starling forces such that overall proximal tubule reabsorption is enhanced. With this enhanced reabsorption, more of the filtered load of HCO₃⁻ is reabsorbed.

With volume expansion (positive Na⁺ balance), secretion of H⁺ is reduced because of low levels of angiotensin II and aldosterone, as well as alterations in peritubular Starling forces that reduce overall proximal tubule reabsorption.

Parathyroid hormone (PTH) has both inhibitory and stimulatory effect on renal H⁺ secretion. Acutely, PTH inhibits H⁺ secretion by the proximal tubule by inhibiting the activity of the Na⁺-H⁺ antiporter and by also causing the antiporter to be endocytosed from the apical membrane. Long-term, PTH stimulates renal acid excretion by acting on the thick ascending limb of Henle’s loop and the distal tubule. Because secretion of PTH is increased during acidosis, this long-term stimulatory effect on renal acid excretion is a component of the renal response to acidosis. The stimulatory effect of PTH on acid excretion is due in part to the delivery of increased amounts of P_i to more distal nephron sites, where it is then titrated and excreted as titratable acid.^*

Finally, K⁺ balance influences secretion of H⁺ by the proximal tubule. Hypokalemia stimulates and hyperkalemia inhibits H⁺ secretion. It is thought that K⁺-induced changes in intracellular pH are responsible, at least in part, for this effect, with hypokalemia acidifying and hyperkalemia alkalinizing the cells. Hypokalemia also stimulates H⁺ secretion by the collecting duct. This occurs as a result of increased expression of H⁺-K⁺-ATPase in the intercalated cells.