Differential modulation of Na-HCO3 cotransport and Na-H exchange by pH in basolateral membrane vesicles of rat distal colon.

This study was designed to investigate the function of intravesicular proton on two pH gradient-dependent transport processes, a novel Na-HCO3 cotransport and a Na-H exchange, that are present in basolateral membrane vesicles of rat distal colon. Increasing intravesicular proton concentration saturated 22Na uptake via both Na-H exchange and Na-HCO3 cotransport; reduced the apparent Km for sodium for Na-H exchange from 94.9 to 35.6 mM, without alteration in the Vmax; but enhanced the Vmax for Na-HCO3 cotransport from 4.3 to 11.7 nmol/mg protein.6 s, while not changing the Km for sodium. The effect of a 10-fold proton concentration gradient at two different absolute proton concentrations on both systems was also determined. 22Na uptake via Na-HCO3 cotransport, but not via Na-H exchange, was enhanced at the higher proton concentration, indicating that the magnitude of the proton concentration gradient is primarily responsible for proton stimulation of Na-H exchange, whereas the absolute proton concentration is critical for proton enhancement of Na-HCO3 cotransport. These studies also demonstrate saturation of both transport systems as a function of intravesicular proton concentration without an exponential component. These results indicate that proton stimulated Na-H exchange and Na-HCO3 cotransport are regulated by distinct and separate mechanisms that may reflect their different cellular functions.