Na+-dependent H+ efflux from proximal tubule: evidence for reversible Na+-H+ exchange

Removal of Na+ or addition of ouabain inhibits HCO3(-) and Na+ absorption in rabbit proximal tubule, a finding suggestive of Na+-H+ coupling. However, inhibition of Na+ transport might decrease H+ secretion by reducing energy metabolism rather than by inhibiting Na+-H+ exchange directly. H+ disappearance from the luminal fluid, which depends on direction and magnitude of Na+ gradient, should be a function of cell Na+, independent of cellular metabolism. If cell Na+ is increased by ouabain, H+ disappearance should increase; when cell Na+ is reduced by Na+ removal, less H+ should level the lumen for a given pH difference. Superficial early proximal convoluted tubules were dissected from rabbit kidney and perfused rapidly in vitro with CO2(-) and HCO3(-)-free solutions (pH 6.45). The bath resembled perfusate except that the pH was 7.4 and contained 6 g/dl of albumin. H+ efflux was calculated from the difference in pH between perfused and collected fluid, flow rate, and buffer capacity of the perfusate. When 145 mM Na+ was present in perfusate and bath, H+ efflux was 5.3 +/- 0.4 pmol . cm-1 . s-1 and increased by 39 +/- 16% when ouabain was added to the bath. Replacement of Na+ by choline or Li+ caused a 44 +/- 7% decrease in H+ efflux. Removal of luminal organic solutes markedly reduced H+ efflux; however, it was still enhanced by addition of ouabain to the bath. Even in the absence of Na+ or luminal organic solutes, a substantial apparent H+ leak permeability exists. Addition of 10(-4) M amiloride to a 10 mM Na+ medium caused a 34 +/- 6% reduction in H+ efflux. The results indicate that H+ transport in the proximal tubule is mediated, at least in part, by a reversible Na+-H+ exchanger driven by the difference between H+ and Na+ gradients.