Evidence for coupled sodium/hydrogen exchange in the rat superficial proximal convoluted tubule

Recent in vitro studies from the rat and rabbit have suggested a tightly coupled sodium/hydrogen ion exchanger on the luminal membrane of proximal tubules. The steep sodium gradient from the lumen to cell supplies indirect energy for hydrogen ions to be pumped from the cell to the lumen. However, a proton translocating pump has been demonstrated in other epithelia, which is independent of sodium transport and directly driven by ATP. To examine the role that sodium might play in the process of acidification, rat proximal convoluted tubules and their surrounding peritubular capillaries were perfused in vivo with artificial ultrafiltrate-like perfusion solutions. Total CO2 absorption was measured by microcalorimetry during alterations in sodium transport by replacement of the sodium with an impermeant cation, choline, or by inhibition of the (Na+ + K+)-ATPase by removing potassium from both perfusion solutions. Under control conditions the absolute rate of total CO2 absorption was 140 pmol/mm X min. In the choline substitution and potassium removal experiments, absolute total CO2 absorption fell to 23 and 28 pmol/mm X min, respectively. The data suggest that: 1) in the rat superficial proximal convoluted tubule approximately 80% of the bicarbonate absorption is tightly coupled to sodium transport; 2) this process is driven indirectly by the (Na+ + K+)-ATPase system; and 3) the residual 20% of acidification appears to be mediated by another mechanism or may be a consequence of technical limitations.