Evaluation of acetate uptake and its relative conversion to acetylcholine in Torpedo electric organ synaptosomes under different ionic and metabolic conditions.
The uptake of acetate and its incorporation into acetylcholine were measured under various conditions in nerve terminals isolated from the electric organ in order to characterize acetate uptake and to study the relationship between acetate uptake and acetylcholine synthesis in a pure cholinergic preparation. It was found that increasing extracellular choline up to 10(?4) M had no effect on either acetate uptake or the conversion of acetate to ACh, while the addition of hemicholinium-3 to the incubation medium led to decreases in both parameters. Hence, it appears that endogenous levels of choline are sufficient to support ongoing acetylcholine synthesis in this preparation and that this synthesis depends to some extent on the uptake of extracellular choline. Nonetheless, in the absence of choline uptake, both the uptake of acetate and the conversion of acetate to acetylcholine remained substantial, indicating that internal sources of choline as well can be used for acetylcholine synthesis. Acetate uptake displayed a marked requirement for external Na(+) and was decreased following depolarization of the synaptosomes by an elevated K(+) concentration. The conversion of acetate to acetylcholine followed a similar pattern, except that a small reduction in acetylcholine synthesis was observed in the absence of external Ca(2+), while acetate uptake was unaffected. The addition of ATP, AMP-PNP or phosphate to the incubation medium caused an increase in both the uptake and incorporation of acetate, but adenosine had no effect on either of these functions. Choline uptake, meanwhile, was unchanged in the presence of ATP, phosphate or adenosine. Acetate uptake appears to be more closely linked to its intracellular metabolism than to the transmembrane movement of choline itself. The mechanism by which acetate crosses the nerve terminal membrane has not been established, but the possibility that acetate is a substrate for a monocarboxylate transport system such as has been described in other systems can be ruled out as inhibitors of anion permeability do not block acetate uptake in this preparation.
O'Regan
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Département de Neurochimie, Laboratoire de Neurobiologie Cellulaire, C.N.R.S., 91190 Gif-sur-Yvette, France.