Inhibition of trichocyst exocytosis and calcium influx in Paramecium by amiloride and divalent cations.

Regulated exocytosis of defensive secretory organelles, the trichocysts, as well as a transient Ca(2+)-influx can be induced in Paramecium by aminoethyldextran (Kerboeuf and Cohen, J Cell Biol (1990) 111, 2527). Knoll et al (Febs Lett (1992) 304, 265) reported that veratridine was also a secretagogue for Paramecium. Here we show that, like aminoethyldextran, veratridine induces a transient Ca(2+)-influx. Both aminoethyldextran- and veratridine-induced exocytosis and associated Ca(2+)-influx were: i) blocked in the ndl2 thermosensitive mutant at the non-permissive temperature; and ii) inhibited by amiloride and four divalent cations, Ba2+, Mg2+, Sr2+ and Co2+. This suggests that, although of different chemical nature, aminoethyldextran and veratridine act through the same physiological pathway. In addition, the inhibitory doses are comparable to the ones found to inhibit a hyperpolarization-sensitive Ca(2+)-current described in Paramecium (Preston et al (1992) J Gen Physiol 100, 233). The possibility that the activation of this Ca(2+)-current by the secretagogue represents an early step in the regulation of trichocyst exocytosis is discussed.