Abstract
Synaptic plasma membrane vesicles isolated from the highly cholinergic nervous tissue of insects were used to study the translocation of choline across the membrane via a high-affinity carrier-mediated mechanism energized by ion gradients as the sole driving force. The uphill movement of choline, energized mainly by the Na+ gradient, attained levels of choline severalfold the final equilibrium value at the peak of the overshoot. Efflux of choline required the presence of internal sodium ions and was promoted by external choline if Na+ was present. External choline inhibited choline efflux in the absence of sodium. It is concluded that the efflux of choline is in many aspects symmetrical with its uptake.