Voltage dependence of H+ buffering mediated by sodium bicarbonate cotransport expressed in Xenopus oocytes.

The electrogenic sodium bicarbonate cotransporter (NBCe1) is expressed in many epithelial cells and, in the brain, in glial cells. Little is known about the physiological significance of the NBCe1 for proton homeostasis and for other acid/base-coupled transporters in these cells. We have measured the voltage-dependent transport activity of an NBC from human kidney, type hkNBCe1, expressed in oocytes of the frog Xenopus laevis, by recording membrane current and the changes in intracellular pH and sodium at different membrane potentials between -20 and -100 mV. The apparent intracellular buffer capacity was increased and became dependent upon membrane voltage when the NBCe1 was expressed; the measured buffer capacity increased by up to 7 mm/10 mV of membrane depolarization. Lactate transport by the electroneutral monocarboxylate transporter became enhanced and dependent upon membrane potential, when the monocarboxylate transporter (isoform 1) was co-expressed with NBCe1 in oocytes. Our results indicate that the electrogenic NBCe1 renders the cell membrane potential an effective regulator of intracellular H(+) buffering and acid/base-coupled metabolite transport.