Evidence for Na+ dependent rheogenic HCO 3 ? transport in fused cells of frog distal tubules

The mechanism of HCO ₃ ^– transport was studied applying microelectrodes in giant cells fused from single epithelial cells of the diluting segment of frog kidney. A sudeen increase of extracellular HCO ₃ ^– 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: (i) In the diluting segment of the frog kidney a Na⁺-dependent rheogenic HCO ₃ ^– transport system exists across the peritubular cell membrane. (ii) This rheogenic peritubular Na⁺/HCO ₃ ^– cotransporter cooperates with a Na⁺/H⁺ exchanger in the luminal membrane, thus driving HCO ₃ ^– reabsorption. (iii) Reabsorption of HCO ₃ ^– and secretion of H⁺ depend upon the presence of carbonic anhydrase.