Release of [3H]-acetylcholine from the isolated retina of the rat by potassium depolarization: dependence on high affinity choline uptake.

1. The effect of potassium depolarization on the release of [3H]-acetylcholine ([3H]-ACh) from the isolated retina of the rat was studied. 2. Exposure of retinae to medium containing KCl (50 mM) evoked a large increase in the efflux of [3H]-ACh with only a small concurrent increase in the efflux of [3H]-choline. The KCl-evoked release of [3H]-ACh was almost abolished in calcium-free medium. 3. Incubation of retinae with [3H]-choline in sodium-free medium, or medium containing hemicholinum-3 (HC-3), procedures that are believed to inhibit selectively the high affinity choline transport system, reduced the retinal uptake of [3H]-choline by approximately 50% and the synthesis of [3H]-ACh by about 97%. 4. The potassium-evoked release of [3H]-ACh was almost abolished in retinae that had been loaded with [3H]-choline in sodium-free medium or medium containing HC-3, and subsequently superfused in normal medium. 5. It is suggested that as in other areas of the nervous system, a sodium-dependent, high affinity uptake system for choline is important in retinal cholinergic nerve terminals.