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 Ca(2+)-dependent acetylcholine release from striatal slices but did not influence acetylcholine release observed in Ca(2+)-free medium (4). Here we examined if the effect of veratridine on membrane potential, Ca(2+)-uptake, and intracellular Ca2+ concentration of synaptosomes was altered by vesamicol in parallel with the inhibition of acetylcholine release. The depolarizing effect of 10 microM veratridine (from 67 +/- 2.3 mV resting membrane potential to 50.7 +/- 2.5 mV) was not significantly influenced by vesamicol (1-20 microM). Vesamicol (1-20 microM) had no effect on either the overall curve of the veratridine-evoked 45Ca2+ 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 microM). The K(+)-evoked (40 mM) increase of Ca2+ uptake and of intracellular calcium concentration were also unaltered by vesamicol. In high concentration (50 microM) 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 microM) when neither of the above parameters was changed, completely prevented veratridine-evoked increase of [14C]acetylcholine release. This was observed only when vesamicol was present in the media throughout the experiment after loading the preparation with [14C]choline.(ABSTRACT TRUNCATED AT 250 WORDS)