Evidence for OH−/H+ permeation across the peritubular cell membrane of rat renal proximal tubule in HCO 3 − -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 qualitively 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.