Ionic requirements of proximal tubular sodium transport. III. Selective luminal anion substitution.

The effect of substitution of luminal anions on sodium and fluid absorption in rat renal proximal convoluted tubules was studied with continuous luminal microperfusion methods. Substitution of bicarbonate in the control Ringer perfusion fluid by 25 mM acetate reduced net sodium reabsorption by 40%; substitution by chloride reduced it by 25%; and substitution by cyclamate reduced it by 70%. Infusion of acetazolamide reduced net sodium and fluid transport in all cases except chloride-Ringer perfusion. Cyanide added to the perfusion fluid inhibited fluid and sodium movement completely when there was no imposed chloride concentration gradient, but only reduced fluid and solute movement by 68% when a nominal 36 mM transepithelial chloride concentration gradient existed. We conclude from these observations that passive forces for sodium reabsorption can account for a moderate amount of sodium transport, that the effects of acetazolamide in low concentrations are dependent on the presence of bicarbonate in the lumen, and that some anions alter net sodium transport either by reducing the availability of permeant anion for co-transport with sodium or by a direct effect on the sodium and/or chloride transport systems.