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.
Rajendran
V M
VM
Department of Internal Medicine, Yale University, New Haven, Connecticut 06510.
Binder
H J
HJ
D000818:Animals; D001639:Bicarbonates / Q000378:metabolism*; D002352:Carrier Proteins / Q000378:metabolism*; D002462:Cell Membrane / Q000378:metabolism; D003106:Colon / Q000378:metabolism*; D006859:Hydrogen / Q000378:metabolism*; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D011522:Protons; D051381:Rats; D017207:Rats, Sprague-Dawley; D012964:Sodium / Q000378:metabolism*; D027982:Sodium-Bicarbonate Symporters; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism
D001639:Bicarbonates; D002352:Carrier Proteins; D011522:Protons; D027982:Sodium-Bicarbonate Symporters; D017923:Sodium-Hydrogen Exchangers; D006859:Hydrogen; D012964:Sodium