Transport Mechanism of Choline in Rat Renal Brush-Border Membrane

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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 tetraehylammonium 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 K_m 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.