The transport mechanisms of organic cations and their zwitterionic derivatives across rat intestinal brush-border membrane. II. Comparison of the membrane potential effect on the uptake by membrane vesicles.

Further investigation of organic cation transport mechanisms were continued using rat intestinal brush-border membranes following our previous report. The net uptake of organic cations was superior to that of their zwitterionic derivatives. This result agreed with the absorption behaviour of these compounds from rat intestinal loop. The uptake of tyramine and 5-benzyloxytryptamine was significantly stimulated by the valinomycin-generated K(+)-diffusion potential (inside-negative). On the other hand, the uptake of zwitterionic derivatives was not affected by the valinomycin-induced K(+)-diffusion potential. The voltage-clamped brush-border membrane vesicles exhibited a complete disappearance of the overshoot-uptake of organic cations. Therefore, this permeation mechanism across the intestinal brush-border membrane seems to be different from the well-known H(+)-antiport system of organic cation found in other organs such as kidney and liver, and depends upon an inside-negative H(+)- or K(+)-diffusion potential.