Relationship between sodium and bicarbonate transport in the rat proximal convoluted tubule

Abstract

The relationship between net Na and HCO3- transport was studied by microperfusion of proximal convoluted tubules and peritubular capillaries. HCO3- absorption was unchanged as long as the Na concentration remained above 40 meq/l, despite reduction of Na transport to 10% of its control value. At a Na concentration of 5 meq/l, fluid absorption was completely abolished but HCO3- transport was reduced to 39% of its control value. Even at reduction of luminal and peritubular Na concentrations to nominally zero, HCO3- transport continued at 23% of its control value. Amiloride, at a Na concentration of 5 meq/l, inhibited HCO3- absorption in a dose-dependent manner. Elevating peritubular pH to 8.4 drastically reduced net HCO3- transport; fluid absorption was only slightly inhibited. These results are consistent with a dual mechanism of acidification: a Na-H exchange that saturates at low extracellular Na concentrations and an additional Na-independent mechanism of H+ secretion.