Evidence for Na+ dependent rheogenic HCO3- transport in fused cells of frog distal tubules.

The mechanism of HCO3- transport was studied applying microelectrodes in "giant" cells fused from single epithelial cells of the diluting segment of frog kidney. A sudden increase of extracellular HCO3- concentration from 10 to 20 mmol/l at constant pH hyperpolarized the cell membrane potential of the fused cell. This cell-voltage response was totally abolished by 10(-3) mol/l SITS and significantly reduced by 10(-4) mol/l acetazolamide or by omission of Na+ from the extracellular perfusate. Removal of Na+ from the perfusate caused a transient depolarization. Reapplication of Na+ induced a transient hyperpolarization. 10(-3) mol/l SITS abolished the cell-voltage response to removal and reapplication of Na+. In the intact diluting segment of the isolated perfused frog kidney peritubular perfusion of 10(-4) mol/l acetazolamide reduced the limiting transepithelial electrochemical gradient for H+ significantly from 30 +/- 4 mV to 14 +/- 3 mV. The results suggest: In the diluting segment of the frog kidney a Na+-dependent rheogenic HCO3- transport system exists across the peritubular cell membrane. This rheogenic peritubular Na+/HCO3- cotransporter cooperates with a Na+/H+ exchanger in the luminal membrane, thus driving HCO3- reabsorption. Reabsorption of HCO3- and secretion of H+ depend upon the presence of carbonic anhydrase.