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

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