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 transpo rt. The stimulatory effect of an outward H+ gradient on choline uptake was weak compared with that on tetraethylammonium uptake. [C-14]Tetra ethylammonium uptake was cis-inhibited and trans-stimulated by choline , but the effects were less pronounced than those produced by unlabele d tetraethylammonium. [H-3]Choline uptake was trans-stimulated by unla beled choline, but not by tetraethylammonium. An interior-negative mem brane potential induced marked stimulation of choline uptake against i ts concentration gradient (overshoot phenomenon), and the uptake was s aturable with an apparent K(m) of 0.77 mM. Various compounds such as h emicholinium-3 inhibited the choline uptake by renal brush-border memb rane vesicles, but a sulfhydryl reagent did not. These findings sugges t that choline can be actively transported by a carrier-mediated syste m driven by cell interior-negative membrane potential in renal brush-b order membrane, and this system may play an important role in the tubu lar reabsorption of choline.