Rat jejunal basolateral membrane Cl/HCO3 exchanger is modulated by a Na-sensitive modifier site

A Cl/HCO₃ exchanger mediates HCO₃ extrusion across rat jejunal basolateral membrane. Previous studies demonstrated that anion antiport activity is positively affected by Na, but evidence was given that this cation is not translocated by the carrier protein. Basolateral membranes isolated from rat jejunum were used to give more insight on Na effect. Uptake studies, performed together with vesicle sidedness determinations, indicated that the greatest stimulation of Cl-dependent HCO₃ uptake occurs when Na is present at both vesicle surfaces. The kinetic dependence of Cl/HCO₃ exchange on equal intra- and extravesicular Na concentration showed a hyperbolic relationship, and the calculated kinetic parameters were V _max=0.153 ± 0.006 nmol mg protein^-1 sec^-1, K _m =23.0 Mm. Ion replacement studies indicated that Na can be partially substituted only by Li and not by other monovalent cations. Results of this study suggest that Na could act as a nonessential activator of the Cl/HCO₃ exchanger. A possible role of the Na-sensitive modifier site in the physiology of jejunal enterocyte is suggested.