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 basolat eral membrane. Previous studies demonstrated that anion antiport activ ity is positively affected by Na, but evidence was given that this cat ion is not translocated by the carrier protein. Basolaterial membranes isolated from rat jejunum were used to give more insight on Na effect . Uptake studies, performed together with vesicle sidedness determinat ions, indicated that the greatest stimulation of Cl-dependent HCO3 upt ake occurs when Na is present at both vesicle surfaces. The kinetic de pendence of Cl/HCO3, exchange on equal intra- and extravesicular Na co ncentration showed a hyperbolic relationship, and the calculated kinet ic 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 part ially substituted only by Li and not by other monovalent cations. Resu lts of this study suggest that Na could act as a nonessential activato r of the Cl/HCO3 exchanger. A possible role of the Na-sensitive modifi er site in the physiology of jejunal enterocyte is suggested.