Total CO2 flux in isolated collecting tubules during carbonic anhydrase inhibition

These studies evaluated the effects of sodium transport inhibitors on total CO2 flux (JtCO2) in the rabbit cortical collecting tubule after inhibition of carbonic anhydrase. When ouabain was added to tubules during carbonic anhydrase inhibition, reabsorptive JtCO2 rose and potential difference (PD) decreased. Amiloride added to perfusate after carbonic anhydrase inhibition decreased PD and did not alter JtCO2. If ouabain was added to an ethoxyzolamide-treated tubule with amiloride present in perfusate, no effects were detectable. Amiloride added to the bath of ethoxyzolamide-treated tubules in high concentration (10-3 M), decreased potential and increased JtCO2. If amiloride was added to the bath of cortical collecting tubules from fasted rabbits, JtCO2 rose. This response was not seen in cortical tubules from fed animals or in medullary collecting tubules. These data demonstrate the existence of carbonic anhydrase-independent acidification in this segment in vitro. The data from studies with amiloride in the presence of intact carbonic anhydrase are consistent with action on a basolateral sodium-proton exchange mechanism. A cellular model that postulates a basolateral sodium-proton exchanger in an acidifying cell is offered.