A rapid calcium influx during exocytosis in Paramecium cells is followed by a rise in cyclic GMP within 1 s.

The synchrony of trichocyst exocytosis in Paramecium allows temporal correlation of associated events. Using quenched flow we observed a Ca2+ influx concurrent with exocytosis within 80 ms after stimulation with the secretagogue aminoethyldextran. Cyclic AMP did not change in depency of stimulation. Cyclic GMP transiently increased after 500 ms, culminating at 2 s, and thus considerably lags behind exocytosis induction and influx of Ca2+. Both Ca2+ influx and rise in cGMP are known to be induceable also by Ba2+ or veratridine, allegedly via the opening of ciliary Ca2+ channels. However, only veratridine stimulated exocytosis. We conclude that both aminoethyldextran and veratridine induce an exocytosis-associated Ca2+ influx, which is responsible for the rise in cGMP, through an as yet unknown pathway.