Proximal tubular cell alkalinization induced by Na(+)-glucose cotransport

To study the ionic mechanisms of crosstalk between luminal and peritubular membranes induced by Na(+)-glucose cotransport in the proximal tubular cells, we applied the ion-selective microelectrode techniques and measured intracellular ion activities of Na+ (Nai) and H+ (pHi), as well as peritubular membrane potential (EM) in doubly-perfused bullfrog kidney. In control conditions with peritubular HCO3- (15 mM), luminal perfusion with 10 mM D-glucose induced a 12 mV depolarization of peritubular membrane (glucose-induced depolarization) with a transient increase in Nai by 3 mM and an elevation in pHi by 0.1. In the absence of peritubular HCO3- (HEPES-Ringer at constant pH), the luminal glucose produced a sustained increase in Nai and suppression of pHi elevation without affecting glucose-induced depolarization. Peritubular perfusion with DIDS (10(-4) M) abolished the change in pHi induced by luminal glucose. Furthermore, the luminal glucose increased the transport number for K+ (gK/gm) in the peritubular membrane by 20%. These results suggest that 1) glucose-induced depolarization suppresses peritubular HCO3- exit, followed by an elevation in pHi, 2) the luminal glucose increases K+ conductance of the peritubular membrane, probably by the elevation in pHi, and 3) this increased K+ conductance repolarizes the peritubular membrane, which can support the driving force for Na(+)-glucose entry across the luminal membrane. Thus, we conclude that the crosstalk between peritubular and luminal membranes by Na(+)-glucose cotransport is mainly modulated by changes in cell pH and membrane potential in the proximal tubule.