Proton transport mechanism in the cell membrane of Xenopus laevis oocytes

Mechanisms of H+ transport across the plasma cell membrane of prophase-arrested oocytes of Xenopus laevis were investigated by testing the effect of ion substitutions and inhibitors on cytoplasmic pH (pHi), membrane potential (Vm) and membrane resistance (Rm). During superfusion with control solution of pH=7.4, pHi was 7.49±0.12 (n=15), Vm was −61.9±7.8 mV (n=34) (cytoplasm negative), and Rm was 2.9±1.5 MΩ (n=19). These data confirm that H+ ions are not distributed at electrochemical equilibrium. By following pHi during recovery of the oocytes from an acid load (20 mmol/l NH4Cl) in the presence and absence of extracellular Na+ or amiloride (1 mmol/l), a Na/H exchanger was identified. On the basis of the known Na+ gradient across the cell membrane, this transporter could suffice to generate the observed H+ disequilibrium distribution. Utilizing blockers or ion-concentration-step experiments no evidence was obtained for an ATP-driven H+ pump or for passive acid/base transporters such as H+ conductances or Na+ (HCO 3 − )3 cotransport. The membrane depolarization observed in response to extracellular acidification appeared to result from a pH-dependent, Ba2+-inhibitable K+ conductance.