Transport mechanism of choline in rat renal brush-border membrane

The transport mechanism of choline was examined using rat renal brush-border membrane vesicles in comparison with tetraethylammonium transport. The stimulatory effect of an outward H+ gradient on choline uptake was weak compared with that on tetraethylammonium uptake. [14C]Tetraethylammonium uptake was cis-inhibited and trans-stimulated by choline, but the effects were less pronounced than those produced by unlabeled tetraethylammonium. [3H]Choline uptake was trans-stimulated by unlabeled choline, but not by tetraethylammonium. An interior-negative membrane potential induced marked stimulation of choline uptake against its concentration gradient (overshoot phenomenon), and the uptake was saturable with an apparent Km of 0.77 mM. Various compounds such as hemicholinium-3 inhibited the choline uptake by renal brush-border membrane vesicles, but a sulfhydryl reagent did not. These findings suggest that choline can be actively transported by a carrier-mediated system driven by cell interior-negative membrane potential in renal brush-border membrane, and this system may play an important role in the tubular reabsorption of choline.