Figure 10. Effect of one-unit pH-gradient with varying proton concentrations on Na-HCO₃ cotransport activities in basolateral membrane vesicles from proximal colon.

Proton/HCO₃ gradient-driven ²²Na uptake (0.1 mM) was measured as described in the Method section with one unit acid outward pH gradient at different extravesicular and intravesicular pH in presence and absence of inhibitors (1 mM amiloride or 1 µM HOE694). One-unit outward acid pH-gradients with both low (pHo/pHi: 8.0/7.0) and high (pHo/pHi: 7.0/6.0) proton concentrations stimulated amiloride-insensitive Na-HCO₃ cotransport, while amiloride-sensitive Na-HCO₃ cotransport was substantially stimulated only at high proton concentrations. Uptake obtained in presence of amiloride represented amiloride-insensitive proton/HCO₃ gradient-driven uptake. Amiloride-sensitive uptake (which also includes HOE694 sensitive Na-H exchange) was calculated by subtracting uptake obtained in presence of amiloride from that of uptake in the absence of inhibitors. HOE694-sensitive uptake was calculated by subtracting uptake obtained in presence of HOE694 from that of uptake in the absence of inhibitors. Amiloride-sensitive proton/HCO₃ gradient-driven uptake was calculated by subtracting HOE694-sensitive uptake from that of amiloride-sensitive uptake. *p<0.001 - compared to 8.0/7.0 (pHo/pHi) in amiloride-sensitive; ns: not significant - compared to 8.0/7.0 (pHo/pHi) in amiloride-insensitive.