Veratridine-mediated Ca2+ dynamics and exocytosis in Paramecium cells

We analyzed [Ca2+](i) transients in Paramecium cells in response to veratridine for which we had previously established an agonist effect for trichocyst exocytosis (Erxleben & Plattner, 1994. J. Cell Biol. 127:935-945; Plattner et al., 1994. J. Membrane Biol. 158:197-208). Wild-type cells (7S), nondischarge strain nd9-28 degrees C and trichocyst-free strain "trichless"' (t1), respectively, displayed similar, though somewhat diverging time course and plateau values of [Ca2+](i) transients with moderate [Ca2+](o) in the culture/assay fluid (50 mu M or 1 mM). In 7S cells which are representative for a normal reaction, at [Ca2+](o) = 30 nM (c.f [Ca2+](i)(rest) = similar to 50 to 100 nM), veratridine produced only a small cortical [Ca2+](i) transient. This increased in size and spatial distribution at [Ca2+](o) = 50 mu M of 1 mM. Interestingly with unusually high yet nontoxic [Ca2+](o) = 10 mM, [Ca2+](i) transients were much delayed and also reduced, as is trichocyst exocytosis. We interpret our results as follows. (i) With [Ca2+](o) = 30 nM, the restricted residual response observed is due to Ca2+ mobilization from subplasmalemmal stores. (ii) With moderate [Ca2+](o) = 50 mu M to 1 mM, the established membrane labilizing effect of veratridine may activate not only subplasmalemmal stores but also Ca-o(2+) influx from the medium via so far unidentified (anteriorly enriched) channels. Visibility of these phenomena is best in t1 cells, where free docking sites allow for rapid Ca2+ spread, and least in 7S cells, whose perfectly assembled docking sites may "consume" a large part of the [Ca2+](i) increase. (iii) With unusually high [Ca2+](o), mobilization of cortical stores and/or Ca-o(2+) influx may be impeded by the known membrane stabilizing effect of Ca-o(2+) counteracting the labilizing/channel activating effect of veratridine. (iv) We show these effects to be reversible, and, hence, not to be toxic side-effects, as confirmed by retention of injected calcein. (v) Finally, Mn2+ entry during veratridine stimulation, documented by Fura-2 fluorescence quenching, may indicate activation of unspecific Me2+ channels by veratridine. Our data have some bearing on analysis of other cells, notably neurons, whose response to veratridine is of particular and continous interest.