Role of the Kidneys in the Regulation of Acid-Base Balance

The HCO ₃ ⁻ Buffer System

Bicarbonate (HCO ₃ ⁻ ) is an important buffer of the ECF. With a normal plasma [HCO ₃ ⁻ ] of 23 to 25 mEq/L and a volume of 14 L (for a 70-kg individual) the ECF can potentially buffer 350 mEq of H ⁺ . The HCO ₃ ⁻ buffer system differs from the other buffer systems of the body (e.g., phosphate) because it is regulated by both the lungs and kidneys. This is best appreciated by considering the following reaction:

As indicated the first reaction (hydration/dehydration of CO ₂ ) is the rate-limiting step. This normally slow reaction is greatly accelerated in the presence of carbonic anhydrase. ^a

a Carbonic anhydrase (CA) actually catalyzes the following reaction: ${\text{H}}_2\text{O}\stackrel{\text{CA}}{\to }{\text{H}}^{+}+{\text{OH}}^{-}+{\text{CO}}_2\to {\text{HCO}}_3^{-}+{\text{H}}^{+}\to {\text{H}}_{\text{2}}{\text{CO}}_{\text{3}}$

The second reaction, ionization of H ₂ CO ₃ to H ⁺ and HCO ₃ ⁻ , is virtually instantaneous.

The Henderson-Hasselbalch equation is used to quantitate how changes in CO ₂ and HCO ₃ ⁻ affect pH:

or

In these equations the amount of CO ₂ is determined from the partial pressure of CO ₂ (P co ₂ ) and its solubility (α) in solution. For plasma at 37°C, α has a value of 0.03. Also, pK′ is the negative logarithm of the overall dissociation constant for the reaction in Eq. 37.1 and has a value for plasma at 37°C of 6.1. Alternatively, the relationship between HCO ₃ ⁻ and CO ₂ on the [H ⁺ ] can be determined as follows:

Inspection of Eqs. 37.3 and 37.4 shows that pH and [H ⁺ ] vary when either [HCO ₃ ⁻ ] or P co ₂ is altered. Disturbances of acid-base balance that result from a change in [HCO ₃ ⁻ ] are termed metabolic acid-base disorders, whereas those resulting from a change in P co ₂ are termed respiratory acid-base disorders. These disorders are considered in more detail in a subsequent section. The kidneys are primarily responsible for regulating the [HCO ₃ ⁻ ] of ECF, whereas the lungs control the P co ₂ .

Overview of Acid-Base Balance

The diet of humans contains many constituents that are either acid or alkali. In addition, cellular metabolism produces acid and alkali. Finally, alkali is normally lost each day in feces. As described later, although diet dependent, the net effect of these processes is the addition of acid to body fluids. For acid-base balance to be maintained, acid must be excreted from the body at a rate equivalent to its addition. If acid addition exceeds excretion, acidosis results. Conversely, if acid excretion exceeds addition, alkalosis results.

As summarized in Fig. 37.1 , the major constituents of the diet are carbohydrates and fats. When tissue perfusion is adequate, O ₂ is available to tissues, and insulin is present at normal levels, carbohydrates and fats are metabolized to CO ₂ and H ₂ O. On a daily basis, 15 to 20 moles of CO ₂ are generated through this process. Normally this large quantity of CO ₂ is effectively eliminated from the body by the lungs. Therefore, this metabolically derived CO ₂ has no impact on acid-base balance. CO ₂ is usually termed volatile acid because it has the potential to generate H ⁺ after hydration with H ₂ O (see Eq. 37.1). Acid not derived directly from hydration of CO ₂ is termed nonvolatile acid (e.g., lactic acid).

The cellular metabolism of other dietary constituents also has an impact on acid-base balance. For example, cysteine and methionine, sulfur-containing amino acids, yield sulfuric acid when metabolized, whereas hydrochloric acid results from metabolism of lysine, arginine, and histidine. A portion of this nonvolatile acid load is offset by production of HCO ₃ ⁻ through metabolism of the amino acids aspartate and glutamate. On average the metabolism of dietary amino acids yields net nonvolatile acid production. Metabolism of certain organic anions (e.g., citrate) results in production of HCO ₃ ⁻ , which offsets nonvolatile acid production to some degree. Overall, in individuals ingesting a meat-containing diet, acid production exceeds HCO ₃ ⁻ production. In contrast, a vegetarian diet produces less nonvolatile acid. In addition to the metabolically derived acids and alkalis, the foods ingested contain acid and alkali. For example, the presence of phosphate (H ₂ PO ₄ ⁻ ) in ingested food increases the dietary acid load. Finally, during digestion, some HCO ₃ ⁻ is normally lost in feces. This loss is equivalent to the addition of nonvolatile acid to the body. In an individual ingesting a meat-containing diet, dietary intake, cellular metabolism, and fecal HCO ₃ ⁻ loss result in addition of approximately 0.7 to 1.0 mEq/kg body weight of nonvolatile acid to the body each day (50–100 mEq/day for most adults). This acid, referred to as net endogenous acid production (NEAP), results in an equivalent loss of HCO ₃ ⁻ from the body that must be replaced.

Nonvolatile acids do not circulate throughout the body but are immediately neutralized by the HCO ₃ ⁻ in ECF.

This neutralization process yields the Na ⁺ salts of the strong acids and removes HCO ₃ ⁻ from the ECF. Thus HCO ₃ ⁻ minimizes the effect of these strong acids on the pH of ECF. As noted previously, ECF contains approximately 350 mEq of HCO ₃ ⁻ . If this HCO ₃ ⁻ were not replenished, the daily production of nonvolatile acids (≈70 mEq/day) would deplete the ECF of HCO ₃ ⁻ within 5 days. To maintain acid-base balance the kidneys must replenish the HCO ₃ ⁻ lost by neutralization of the nonvolatile acids, a process termed renal net acid excretion (RNAE).

Net Acid Excretion by the Kidneys

Under steady-state conditions, NEAP must equal RNAE to maintain acid-base balance. Although NEAP varies from individual to individual and from day to day in anyone individual, it is not regulated. Instead, the kidneys regulate RNAE to match NEAP and in so doing replenish the HCO ₃ ⁻ (new HCO ₃ ⁻ ) lost by neutralization of nonvolatile acids. In addition, the kidneys must prevent the loss of HCO ₃ ⁻ in urine. This latter task is quantitatively more important because the filtered load of HCO ₃ ⁻ is approximately 4320 mEq/day (24 mEq/L × 180 L/day = 4320 mEq/day), compared with only 50 to 100 mEq/day needed to balance NEAP.

Both reabsorption of filtered HCO ₃ ⁻ and excretion of acid are accomplished via H ⁺ secretion by the nephrons. Thus, in a single day the nephrons must secrete approximately 4390 mEq of H ⁺ into the tubular fluid. Most of the secreted H ⁺ serves to reabsorb the filtered load of HCO ₃ ⁻ . Only 50 to 100 mEq of H ⁺ , an amount equivalent to NEAP, is excreted in urine. As a result of this acid excretion, urine is normally acidic.

The kidneys cannot excrete urine more acidic than pH 4.0 to 4.5. Even at a pH of 4.0, only 0.1 mEq/L of H ⁺ can be excreted. Therefore, to excrete sufficient acid, the kidneys excrete H ⁺ with urinary buffers such as phosphate (P _i ). ^b

b The titration reaction is: HPO ₄ ²⁻ + H ⁺ ↔ H ₂ PO ₄ ⁻ . This reaction has a pK of approximately 6.8.

Other constituents of urine can also serve as buffers (e.g., creatinine), although their role is less important than P _i . Collectively the various urinary buffers are termed titratable acid. This term is derived from the method by which these buffers are quantitated in the laboratory. Typically, alkali (OH ⁻ ) is added to a urine sample to titrate its pH to that of plasma (i.e., 7.4). The amount of alkali added is equal to the H ⁺ titrated by these urine buffers and is termed titratable acid.

Excretion of H ⁺ as a titratable acid is insufficient to balance NEAP. An additional and important mechanism by which the kidneys contribute to maintenance of acid-base balance is through synthesis and excretion of ammonium (NH ₄ ⁺ ) . The mechanisms involved in this process are discussed in more detail later in this chapter. With regard to renal regulation of acid-base balance, each NH ₄ ⁺ excreted in urine results in the return of one HCO ₃ ⁻ to the systemic circulation, which replenishes the HCO ₃ ⁻ lost during neutralization of the nonvolatile acids. Thus, production and excretion of NH ₄ ⁺ , like excretion of titratable acid, is equivalent to excretion of acid by the kidneys.

In brief the kidneys contribute to acid-base homeostasis by reabsorbing the filtered load of HCO ₃ ⁻ and excreting an amount of acid equivalent to NEAP. This process can be quantitated as follows:

where ( ${\text{U}}_{{\text{NH}}_4^{+}}+\stackrel{.}{\text{V}}$ ) and ( ${\text{U}}_{\text{TA}}+\stackrel{.}{\text{V}}$ ) are the rates of excretion (mEq/day) of NH ₄ ⁺ and titratable acid (TA), and ( ${\text{U}}_{{\text{HCO}}_3^{-}}+\stackrel{.}{\text{V}}$ ) is the amount of HCO ₃ ⁻ lost in urine (equivalent to adding H ⁺ to the body). ^c

c This equation ignores the small amount of free H ⁺ excreted in urine. As already noted, urine with a pH = 4.0 contains only 0.1 mEq/L of H ⁺ .

Again, maintenance of acid-base balance means that net acid excretion must equal nonvolatile acid production. Under most conditions, very little HCO ₃ ⁻ is excreted in urine. Thus net acid excretion essentially reflects titratable acid and NH ₄ ⁺ excretion. Quantitatively, titratable acid accounts for approximately one-third and NH ₄ ⁺ for two-thirds of RNAE.

HCO ₃ ⁻ Reabsorption Along the Nephron

As indicated by Eq. 37.7 , net acid excretion is maximized when little or no HCO ₃ ⁻ is excreted in urine. Indeed, under most circumstances, very little HCO ₃ ⁻ appears in urine. Because HCO ₃ ⁻ is freely filtered at the glomerulus, approximately 4320 mEq/day is delivered to the nephrons and is then reabsorbed. Fig. 37.2 summarizes the contribution of each nephron segment to reabsorption of filtered HCO ₃ ⁻ .

The proximal tubule reabsorbs the largest portion of the filtered load of HCO ₃ ⁻ . Fig. 37.3 summarizes the primary transport processes involved. H ⁺ secretion across the apical membrane of the cell occurs by both a Na ⁺ /H ⁺ antiporter and H ⁺ -ATPase (V-type). The Na ⁺ /H ⁺ antiporter (NHE3) is the predominant pathway for H ⁺ secretion (accounts for ≈ two-thirds of HCO ₃ ⁻ reabsorption) and uses the lumen-to-cell [Na ⁺ ] gradient to drive this process (i.e., secondary active secretion of H ⁺ ). Within the cell, H ⁺ and HCO ₃ ⁻ are produced in a reaction catalyzed by carbonic anhydrase (CA-II). The H ⁺ is secreted into the tubular fluid, whereas the HCO ₃ ⁻ exits the cell across the basolateral membrane and returns to the peritubular blood. HCO ₃ ⁻ movement out of the cell across the basolateral membrane is coupled to other ions. The majority of HCO ₃ ⁻ exits via a symporter that couples the efflux of Na ⁺ with HCO ₃ ⁻ (sodium bicarbonate symporter, NBC1). Some HCO ₃ ⁻ exits the cell by other transporters, but they are not as important as the Na ⁺ /HCO ₃ ⁻ symporter. As noted in Fig. 37.3 , carbonic anhydrase (CA-IV) is also present in the brush border and basolateral membrane of the cell. The brush border enzyme catalyzes dehydration of H ₂ CO ₃ in the luminal fluid, whereas the enzyme localized to basolateral membrane facilitates HCO ₃ ⁻ exit from the cell. The movement of CO ₂ into and out of the cell occurs via AQP1, which is localized to both the luminal and basolateral membranes.

The cellular mechanism for HCO ₃ ⁻ reabsorption by the thick ascending limb (TAL) of the loop of Henle is very similar to that in the proximal tubule. H ⁺ is secreted by a Na ⁺ /H ⁺ antiporter and H ⁺ -ATPase. Like in the proximal tubule, the Na ⁺ /H ⁺ antiporter (NHE3) is the predominant pathway for H ⁺ secretion. HCO ₃ ⁻ exit from the cell involves both a Na ⁺ /HCO ₃ ⁻ symporter (NBC1) and a Cl ⁻ /HCO ₃ ⁻ antiporter (anion exchanger, AE-2). Some HCO ₃ ⁻ may also exit the cell through Cl ⁻ channels present in the basolateral membrane.

The distal tubule ^d

d Here and in the remainder of the chapter we focus on the function of intercalated cells. The early portion of the distal tubule, which does not contain intercalated cells, also reabsorbs HCO ₃ ⁻ . The cellular mechanism appears to involve an apical membrane Na ⁺ /H ⁺ antiporter (NHE2) and a basolateral Cl ⁻ /HCO ₃ ⁻ antiporter (AE2).

and collecting duct reabsorb the small amount of HCO ₃ ⁻ that escapes reabsorption by the proximal tubule and loop of Henle. Fig. 37.4 shows the cellular mechanism of H ⁺ /HCO ₃ ⁻ transport by the intercalated cells located within these segments (see also Chapter 34 ).

One type of intercalated cell secretes H ⁺ (reabsorbs HCO ₃ ⁻ ) and is called the A- or α-intercalated cell. Within this cell, H ⁺ and HCO ₃ ⁻ are produced by hydration of CO ₂ ; this reaction is catalyzed by carbonic anhydrase (CA-II). H ⁺ is secreted into the tubular fluid via two mechanisms. The first involves an apical membrane H ⁺ -ATPase (V-type). The second couples secretion of H ⁺ with reabsorption of K ⁺ via an H ⁺ -K ⁺ -ATPase similar to those found in the stomach and colon (HKα1 and HKα2). HCO ₃ ⁻ exits the cell across the basolateral membrane in exchange for Cl ⁻ (via a Cl ⁻ /HCO ₃ ⁻ antiporter, AE-1) and enters the peritubular capillary blood.

A second population of intercalated cells secretes HCO ₃ ⁻ rather than H ⁺ into the tubular fluid (also called B- or β-intercalated cells ). ^e

e A third group of intercalated cells shares features of both H ⁺ -secreting and HCO ₃ ⁻ -secreting intercalated cells. The precise function of this cell type in acid-base transport is not fully understood.

In these cells the H ⁺ -ATPase (V-type) is located in the basolateral membrane and the Cl ⁻ /HCO ₃ ⁻ antiporter is located in the apical membrane (see Fig. 37.4 ). However, the apical membrane Cl ⁻ /HCO ₃ ⁻ antiporter is different from the one found in the basolateral membrane of the H ⁺ -secreting intercalated cells and has been identified as pendrin. The activity of the HCO ₃ ⁻ -secreting intercalated cell is increased during metabolic alkalosis, when the kidneys must excrete excess HCO ₃ ⁻ . However, under most conditions (e.g., ingestion of a meat-containing diet) H ⁺ secretion predominates in these segments. ^f

f Traditionally it was believed that intercalated cells were only involved in acid-base transport. There is now good evidence that NaCl reabsorption is also carried out by intercalated cells (B type). Reabsorption of NaCl occurs by the tandem operation of an apical membrane Cl ⁻ /HCO ₃ ⁻ antiporter (pendrin) and an apical membrane Na ⁺ /HCO ₃ ⁻ /2Cl ⁻ antiporter (NDCBE). This mechanism of NaCl reabsorption is inhibited by thiazide diuretics.

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 = 4.0–4.5) is produced there. In comparison the permeability of the proximal tubule to H ⁺ and HCO ₃ ⁻ is much higher, and the 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 the tubular fluid is critically important for excretion of urinary titratable acids and NH ₄ ⁺ .

Regulation of H ⁺ Secretion

A number of factors influence secretion of H ⁺ and thus reabsorption of filtered HCO ₃ ⁻ by the cells of the nephron. 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 RNAE, whereas RNAE is reduced during alkalosis.

AT THE CELLULAR LEVEL

Cells of the kidney express receptors that monitor acid-base status and therefore play a critical role in regulating H ⁺ and HCO ₃ ⁻ transporters along the nephron ( Fig. 37.5 ). For example, a G protein–coupled H ⁺ receptor (GPCR–GPR4) has been localized to the collecting duct. Activation of this receptor by an increase in ECF [H ⁺ ] stimulates H ⁺ secretion. Also in the collecting duct, HCO ₃ ⁻ -secreting intercalated cells (B- or β-ICs) express a basolateral insulin-related receptor (IRR) that is a tyrosine kinase. It is activated by an increase in ECF [HCO ₃ ⁻ ] and stimulates HCO ₃ ⁻ secretion by the cell. A soluble adenylyl cyclase (sAC) regulated by intracellular HCO ₃ ⁻ appears to also play a role in regulating collecting duct H ⁺ secretion. In the proximal tubule, basolateral membrane receptor tyrosine kinases (ErbB1 and ErbB2) sense changes in ECF P co ₂ . Activation of these receptors by an increase in P co ₂ results in generation of angiotensin II, which, acting from the lumen via AT-1A receptors, stimulates H ⁺ secretion/HCO ₃ ⁻ reabsorption. Also in the proximal tubule, the nonreceptor tyrosine kinase (Pyk2) senses intracellular [H ⁺ ]. When it is activated by an increase in intracellular [H ⁺ ], H ⁺ secretion/HCO ₃ ⁻ reabsorption is stimulated. Finally, the gating of several ion channels (e.g., the renal outer medullary K ⁺ channel [ROMK]) is affected by changes in either ECF or ICF pH. These too have the potential to serve as cellular acid-base sensors.

The response of the kidneys to changes in acid-base balance includes both immediate changes in the activity and/or number of transporters in the membrane and longer-term changes in the synthesis of transporters. For example, with metabolic acidosis, H ⁺ secretion is stimulated by multiple mechanisms, depending on the particular nephron segment. First, the decrease in intracellular pH that occurs with acidosis will create a more favorable cell-to-tubular fluid H ⁺ gradient and thereby make 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. Lastly, transporters may be shuttled to the membrane from intracellular vesicles. With long-term acidosis the abundance of transporters increases, either by increased transcription of appropriate transporter genes or by increased translation of transporter mRNA.

AT THE CELLULAR LEVEL

In the proximal tubule, metabolic acidosis increases the transport kinetics of the Na ⁺ /H ⁺ antiporter (NHE3) and increases apical membrane expression of the Na ⁺ /H ⁺ antiporter, H ⁺ -ATPase, and the basolateral Na ⁺ /3HCO ₃ ⁻ symporter (NBCe1). In the collecting duct, acidosis leads to exocytic insertion of H ⁺ -ATPase into the apical membrane of intercalated cells. With long-term acidosis the abundance of key acid-base transporters is increased in the proximal tubule (NHE3 and NBCe1) and in collecting duct intercalated cells (H ⁺ -ATPase and AE1). Lastly, acidosis decreases expression of the Cl ⁻ /HCO ₃ ⁻ antiporter pendrin in HCO ₃ ⁻ -secreting intercalated cells.

Although some of the effects just described may be attributable directly to acidosis, many of these changes in cellular H ⁺ transport are mediated by hormones or other factors. Three known mediators of the renal response to acidosis are endothelin, cortisol, and angiotensin II. Endothelin ( ET-1 ) is produced by endothelial and proximal tubule cells. With acidosis, ET-1 secretion is enhanced. In the proximal tubule, ET-1 stimulates phosphorylation and subsequent insertion of the Na ⁺ /H ⁺ antiporter into the apical membrane, and insertion of the Na ⁺ /3HCO ₃ ⁻ 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 increases the abundance of the Na ⁺ /H ⁺ antiporter and Na ⁺ /3HCO ₃ ⁻ symporter in the proximal tubule. Angiotensin II is produced in proximal tubule cells in response to acidosis. It is secreted into the tubular fluid, where it binds to the angiotensin I receptor and thereby stimulates H ⁺ secretion/HCO ₃ ⁻ reabsorption by the proximal tubule. Both cortisol and angiotensin II also stimulate production and secretion of NH ₄ ⁺ by the proximal tubule, which as described later is an important component of the kidney’s response to acidosis.

Acidosis also stimulates secretion of parathyroid hormone (PTH). PTH inhibits phosphate (P _i ) reabsorption by the proximal tubule (see Chapter 36 ). In so doing, more P _i is delivered to the distal nephron, where it serves as a urinary buffer and thus increases the capacity of the kidneys to excrete titratable acid.

The response of the kidneys to alkalosis is less well characterized. RNAE is decreased because of increased urinary HCO ₃ ⁻ excretion and because excretion of titratable acid and NH ₄ ⁺ is reduced. The factors that regulate this response are not well characterized.

Other factors not necessarily related to maintaining acid-base balance can influence secretion of H ⁺ by the cells of the nephron. Because a significant H ⁺ transporter in the nephron is the Na ⁺ /H ⁺ antiporter, factors that alter Na ⁺ reabsorption can secondarily affect H ⁺ secretion. For example, with volume contraction (negative Na ⁺ balance), Na ⁺ reabsorption by the nephron is increased (see Chapter 35 ), including reabsorption of Na ⁺ via the Na ⁺ /H ⁺ antiporter. As a result, H ⁺ secretion is enhanced. This occurs by several mechanisms. One mechanism involves the renin-angiotensin-aldosterone system, which is activated by volume contraction. As noted earlier, angiotensin II acts on the proximal tubule to stimulate the apical membrane Na ⁺ /H ⁺ antiporter as well as the basolateral Na ⁺ /3HCO ₃ ⁻ symporter. To a lesser degree, angiotensin II stimulates H ⁺ secretion in the TAL of Henle’s loop and 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 34 ). 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 secretion of H ⁺ by intercalated cells. In addition to this indirect effect, aldosterone (and angiotensin II) act directly on intercalated cells to stimulate H ⁺ secretion via H ⁺ -ATPase and H ⁺ ,K ⁺ -ATPase.

Another mechanism by which ECF volume (ECFV) contraction enhances H ⁺ secretion (HCO ₃ ⁻ reabsorption) is through changes in peritubular capillary Starling forces. As described in Chapters 34 and 35, ECFV 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 HCO ₃ ⁻ is reabsorbed.

Potassium 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. Hypokalemia acidifies the cells as intracellular K ⁺ is exchanged for H ⁺ , whereas hyperkalemia alkalinizing cells as intracellular H ⁺ is exchanged for K ⁺ . Hypokalemia also stimulates H ⁺ secretion by the collecting duct. This occurs as a result of increased expression of the H ⁺ ,K ⁺ -ATPase in intercalated cells.