Effect of euplotin C on swimming behavior in Paramecium

The effect of euplotin C—a lipophilic toxic metabolite produced by the protist ciliate Euplotes crassus—on the swimming behavior was studied in a single-celled system represented by the ciliate Paramecium primaurelia. When in P. primaurelia internal Ca2+ concentration coupled to membrane depolarization increases, a reversal in the direction of ciliary beating and consequently in swimming direction occurs. The ciliary reversal is correlated to Ca2+ influx amount. In this study, evidence was given that continuous ciliary reversal (CCR) duration, induced by high external KCl concentrations, is longer in euplotin C-treated cells than in control cells. To test the hypothesis that euplotin C increases CCR duration by modulating a specific subtype of Ca2+ channel, selective Ca2+ channel blockers were used. Blocking L-type channels by nimodipine and verapamil, N- and Q-type channels by Ω-conotoxins, fractions GVIA and MVIIC, significantly reduced the CCR duration evoked by membrane depolarization, suggesting an involvement of these channels in ciliary reversal in Paramecium. The effect of euplotin C on CCR duration persisted when Ω-conotoxin GVIA or Ω-conotoxin MVIIC were applied, conversely, it disappeared when L-type channel blockers were used. The magnitude of the block by nimodipine and verapamil in the presence of euplotin C was comparable with that observed in the absence of euplotin C, suggesting that the Ca2+ channels modulated by euplotin C were dihydropyridine-sensible calcium channels similar to L-type channels found in mammalian cells.