Veratridine-Mediated Ca2+ Dynamics and Exocytosis in Paramecium Cells |
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| Authors: | M.-P. Blanchard N. Klauke S. Zitzmann H. Plattner |
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| Affiliation: | Universit?t Konstanz, Fakult?t für Biologie, P.O. Box 5560, D-78434 Konstanz, Germany, DE
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| Abstract: | 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°C and trichocyst-free strain ``trichless' (tl), 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 μm or 1 mm). In 7S cells which are representative for a normal reaction, at [Ca2+] o = 30 nm (c.f. [Ca2+] rest i =∼50 to 100 nm), veratridine produced only a small cortical [Ca2+] i transient. This increased in size and spatial distribution at [Ca2+] o = 50 μm 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 μm to 1 mm, the established membrane labilizing effect of veratridine may activate not only subplasmalemmal stores but also Ca2+ o influx from the medium via so far unidentified (anteriorly enriched) channels. Visibility of these phenomena is best in tl 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 Ca2+ o influx may be impeded by the known membrane stabilizing effect of Ca2+ o 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. Received: 8 December 1998/Revised: 2 March 1999 |
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| Keywords: | : Calcium — Exocytosis — Membrane fusion — Paramecium— Secretion — Veratridine |
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