Veratridine triggers exocytosis in Paramecium cells by activating somatic Ca channels

Abstract

Paramecium tetraurelia wild-type (7S) cells respond to 2.5 mM veratridine by immediate trichocyst exocytosis, provided [Ca2+]o (extracellular Ca2+ concentration) is between about 10(-4) to 10(-3) M as in the culture medium. Exocytosis was analyzed by light scattering, light and electron microscopy following quenched-flow/freeze-fracture analysis. Defined time-dependent stages occurred, i.e., from focal (10 nm) membrane fusion to resealing, all within 1 sec. Veratridine triggers exocytosis also with deciliated 7S cells and with pawn mutants (without functional ciliary Ca channels). Both chelation of Ca2+o or increasing [Ca2+]o to 10(-2) M inhibit exocytotic membrane fusion. Veratridine does not release Ca2+ from isolated storage compartments and it is inefficient when microinjected. Substitution of Na+o for N-methylglucamine does not inhibit the trigger effect of veratridine which also cannot be mimicked by aconitine or batrachotoxin. We conclude that, in Paramecium cells, veratridine activates Ca channels (sensitive to high [Ca2+]o) in the somatic, i.e., nonciliary cell membrane and that a Ca2+ influx triggers exocytotic membrane fusion. The type of Ca channels involved remains to be established.

Veratridine-Mediated Ca 2+ Dynamics and Exocytosis in Paramecium Cells

“…As previously described [14] (Fig. 2), while exocytosis is strongly inhibited under these conditions (Table 1), just as in our previous freeze-fracture studies [36].…”

“…For these reasons, the reaction of nd9-28°C may be in between tl and 7S cells. (iv) Veratridine induces trichocyst exocytosis in 7S cells (as it does in d4-500r; data not shown), as previously analyzed [36], while it causes internal decondensation of trichocyst contents without external release (exocytosis) in nd9-28°C cells which are incompetent for membrane fusion.…”

“…In Paramecium, veratridine induces trichocyst exocytosis [15,29,36], though less efficiently than the polyamine secretagogue, aminoethyldextrane (AED) [35,38] or caffeine [15,27]. For both these agents we demonstrated, also by fluorochrome analysis, the occurrence of a cortical [Ca 2+ ] i transient orginating primarily from cortical pools ("alveolar sacs" [49]) and reinforced by superimposed Ca 2+ influx from the medium [14,15,26,27].…”

“…In our previous work with veratridine we combined quenched-flow and freeze-fracture to analyze quantitatively exocytosis by ultrastructural changes occurring at performed exocytosis sites (typical of trichocyst docking sites in Paramecium cells) during membrane fusion, resealing and endocytosis [36]. Paramecium has no voltage-dependent Na channels [32,41] Our findings may have some bearing also on the analysis of some other cells, to which veratridine is frequently used as a cell membrane channel agonist.…”

“…Just like in some other cell types [2,8,21,42], including neuronal systems [1,7,12,13,17,31,51], veratridine induces exocytosis in Paramecium cells [15,24,29,36]. Depending on the cell type analyzed, mechanisms involved in veratridine activation may range from depolarization by activation of Na channels to Na o + -independent mechanisms.…”