The Mechanism of Bicarbonate Reabsorption in the Proximal and Distal Tubules of the Kidney*

Abstract

Considerable evidence (1-6) has been adduced to support the hypothesis that the reabsorption of filtered HC03 by the kidney is mediated by a single mechanism, operafive in both the proximal and distal portions of the nephron, which involves the secretion of cellular H hi exchange for luminal Na. The secreted H reacts with filtered HCO3 to form I-I CO3, which then decomposes to CO2 and H O. Difficulties arise, however, when the details of the process are examined. In the steady state, the rate at which the H2CO3 is removed from the luminal fluid must equal the rate at which H is secreted. Walser and Mudge (7) have estimated that for the uncatalyzed dehydration of H2C03 to account for the observed rates of HCO3 reabsorption, the steady-state concentration of H2C03 in luminal fluid must be at least tenfold greater than the concentration that would exist were H2C03 in equilibrium with the CO2 tension of luminal fluid and plasma. As a result of the excess H2CO3, the steady-state pH would be approximately 1 pH U lower than would be predicted from the luminal concentration of HCO3 and the CO2 tension of plasma, assuming complete equilibration of luminal H2C03 with plasma CO2. A marked disequilibrium pH1 would exist. Two lines of evidence have been advanced to support the presence of a disequilibrium pH in the proximal tubule. Rector and Carter (8) perfused single proximal tubules with NaHCO3 and estimated the steady-state intraluminal pH from the change in color of various acidbase indicators. The pH was found to be about 1.5 U lower than the predicted equilibrium pH. However, since the color change could result from loss of dyes by either reabsorption or binding to cell proteins, the validity of these results may be questioned. Bank and Aynedjian (9) measured intratubular pH by aspirating tubular fluid into quinhydrone microelectrodes and comparing the readings while the electrode was still in the tubular lumen with the values obtained when the fluid was removed from the tubule and permitted to reach equilibrium. With this technique, they found the intratubular pH to be 2.0 to 2.5 pH