EFFECT OF MEMBRANE-SURFACE POTENTIAL ON THE UPTAKE AND THE INHIBITIONOF CATIONIC COMPOUNDS IN RAT INTESTINAL BRUSH-BORDER MEMBRANE-VESICLES AND LIPOSOMES

The effect of membrane surface potential on the uptake of tryptamine, an organic cation, by rat intestinal brush-border membrane vesicles wa s investigated. In the presence of an inside-negative K+-diffusion pot ential, the manner of initial uptake of tryptamine appeared to be pi-I -dependent and the uptake in the acidic medium was lower than that in the neutral medium. Changes in surface potential of brush-border membr ane vesicles were monitored using 8-anilino-1-naphthalenesulfonic acid (ANS) and the results suggested that the membrane surface potential ( negative charge on the membrane surface) decreased in the acidic mediu m. A good correlation was observed between the K+-diffusion potential- dependent uptake of tryptamine and membrane surface potential monitore d by ANS at various pH levels. The uptake of tryptamine by liposomes ( large unilamellar vesicles), which contained various amounts of dipalm itoylphosphatidylserine (DPPS), was also examined. The uptake of trypt amine decreased with a decrease of DPPS content in the liposomes, and was correlated with the membrane surface potential monitored by ANS. M oreover, the effect of organic cations on the uptake of tryptamine by intestinal brush-border membrane vesicles was examined. The uptake of tryptamine was inhibited by tetracaine and imipramine. The inhibitory effect of these cations was well correlated with changes in the membra ne surface potential in the presence of tetracaine or imipramine. Thes e results suggest that the Kf-diffusion potential-dependent uptake of tryptamine by intestinal brush-border membrane vesicles is affected by membrane surface potential, and the inhibition of tryptamine uptake o riginates in changes in the membrane surface potential caused by the o rganic cations.