Significance of Carbonic Anhydrase for HCO ⁻₃ Absorption and H⁺ Secretion in Renal Tubules

Abstract

In the present paper I shall discuss some problems dealing with the role of carbonic anhydrase in renal bicarbonate absorption I shall consider only the first part of the nephron, the so-called proximal tubule, because this is the site where most of the renal HCO ⁻₃ absorption occurs. As observed in micropuncture experiments the proximal tubule absorbs 70%–80% of the filtered Na⁺ ions and water, ~ 75% of the filtered Cl⁻ions and ~ 95% of the filtered HCO ⁻₃ , as well as practically all of the filtered glucose and amino acid molecules. Microscopically the wall of the proximal tubule consists of a typical single-layered epithelium of uniform, flat cells which are held together at their luminal end by quite leaky terminal bars. All substances which are reabsorbed by the cells enter the cells via the luminal cell membrane (brushborder) and leave the cells via the highly folded peritubular cell membrane. A great number of the transport mechanisms which are located in these cell membranes have been identified in recent years. Briefly we can say that the primum moyens of almost all fluxes is a Na⁺/K⁺ pump located in the peritubular cell membrane which consumes energy by splitting ATP. This pump effects many different things. Firstly it brings about active transtubular Na⁺ ion transport by exporting the Na⁺ ions, which leak from the lumen into the cell, across the peritubular cell membrane into the interstitial space. Secondly it provides the energy for the absorption of glucose and amino acids — these substances are driven from lumen to cell by cotransport with Na⁺ ions, accumulate in the cytoplasm and diffuse then into the interstitium along their concentration gradient — and thirdly it builds up the passive driving forces for the absorption of Cl⁻ ions and water. Other transport mechanisms, however, are less well defined.