PARAMETERS NOT INFLUENCED BY VESAMICOL - MEMBRANE-POTENTIAL, CALCIUM-UPTAKE, AND INTERNAL CALCIUM-CONCENTRATION OF SYNAPTOSOMES

In our previous study vesamicol, an inhibitor of the acetylcholine transporter of the cholinergic vesicles, inhibited veratridine-evoked external Ca2+-dependent acetylcholine release from striatal slices but did not influence acetylcholine release observed in Ca2+-free medium (4). Here we examined if the effect of veratridine on membrane potential, Ca2+ uptake, and intracellular Ca2+ concentration of synaptosomes was altered by vesamicol in parallel with the inhibition of acetylcholine release. The depolarizing effect of 10-mu-M veratridine (from 67 +/- 2.3 mV resting membrane potential to 50.7+/-2.5 mV) was not significantly influenced by vesamicol (1-20-mu-M). Vesamicol (1-20-mu-M) had no effect on either the overall curve of the veratridine-evoked Ca-45(2+) uptake or the amount of Ca2+ taken up by synaptosomes. Veratridine caused a rise in intrasynaptosomal Ca2+ concentration as measured by Fura2 fluorescence, and the same increase both in characteristics and in magnitude was observed in the presence of vesamicol (20-mu-M). The K+-evoked (40 mM) increase of Ca2+ uptake and of intracellular calcium concentration were also unaltered by vesamicol. In high concentration (50-mu-M) vesamicol inhibited both the fall in membrane potential and the elevated Ca2+ uptake by veratridine, indicating a possible nonspecific effect on potential-dependent Na+ channels at this concentration. Vesamicol, in lower concentration (20-mu-M) when neither of the above parameters was changed, completely prevented veratridine-evoked increase of [C-14]acetylcholine release. This was observed only when vesamicol was present in the media throughout the experiment after loading the preparation with [C-14]Choline. The results suggest that vesamicol does not interfere with veratridine-induced changes in isolated nerve terminals other than with the release of acetylcholine, thus further supporting the involvement of a vesamicol-sensitive vesicular transmitter pool in Ca2+-dependent veratridine-elicited acetylcholine release.