Evidence for OH-/H+ permeation across the peritubular cell membrane of rat renal proximal tubule in HCO3(-)-free solutions.

Membrane potentials and intracellular pH were measured on rat renal proximal tubular cells in vivo to test whether sodium-bicarbonate cotransport across the peritubular cell membrane accepts OH- (or H+ in opposite direction) or whether it requires the CO2, HCO3-, CO3= buffer to operate. It was found that step changes of peritubular pH in nominally HCO3(-)-free and CO2-free solutions produced qualitatively similar initial potential responses and cell pH responses as changes in peritubular HCO3- concentrations. These responses, however, were considerably smaller and they were neither reduced in Na+-free solutions nor inhibited by the stilbene derivative SITS which is known to block Na+ (HCO3-)n cotransport completely. We conclude that the cotransporter requires the CO2, HCO3-, CO3= buffer for its physiological operation but that high rates of OH- or H+ can also be transferred across the peritubular cell membrane in HCO3(-)-free solutions, probably through a separate transport system.