The Effect of Acetylcholine Release on Choline Fluxes in Isolated Synaptic Terminals

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Abstract
As in intact tissues, choline influx into guinea pig synaptosomes is enhanced after a period of depolarization-induced release of acetylcholine. The activation of uptake is dependent on the presence of Ca2+ and inhibited by high Mg2+ concentrations in the medium during depolarization. Choline transport in erythrocytes was not activated by prior treatment with K+. The permeability constant of the synaptosome membrane to choline was 2.7 .times. 10-8 cm/s and to acetylcholine 1.8 .times. 10-8 cm/s. Choline influx was studied after pre-loading synaptosomes with choline. Different radiolabels were used to measure efflux of preloaded choline and influx simultaneously. Isotopic dilution in flux studies was estimated and corrected for. Influx was stimulated by high internal concentrations of choline and efflux similarly stimulated by high outside concentrations of choline. The maximal influx and efflux at saturating opposite concentrations of choline were equal with a value of about 500 pmol/min per mg synaptosomal protein. A reciprocating carrier would explain the equality of the maximal influx and efflux. Acetylcholine competes with choline for binding to the carrier but is itself hardly transported. Increased acetylcholine concentrations inhibited both choline influx and efflux from the trans position. Raising intrasynaptosomal acetylcholine concentrations. High concentrations of acetylcholine apparently immobilize the carrier on the inside of the synaptic membrane. The stimulation of choline influx consequent upon depolarization is caused by release of ACh which results in relief of this immobilization. The enhanced supply of choline achieved by this mechanism is likely to be important in maintaining stores of the acetylcholine in vivo.