The kinetics of acetylcholine turnover in a resting cholinergic nerve terminal and the magnitude of the cytoplasmic compartment.

The magnitude of the cytosolic pool of acetylcholine in the cholinergic electromotor nerve terminals of Torpedo marmorata has been calculated to be 22 +/- 3% of the total by comparing the isotopic ratio of acetylcholine with that of choline when slices of electric organ were incubated with 10 microM deuterated choline. The calculation is based on a two-compartment model that assumes the presence, in unstimulated tissue, of a vesicular pool of acetylcholine that does not exchange, under resting conditions, with a second cytosolic pool; the latter, by contrast, is subject to 'futile recycling' and comes into isotopic equilibrium with the tissue choline pool.