Dependence of ciliary reversal inParamecium on extracellular Ca²⁺ concentration

Summary

The locomotor behaviour ofParamecium caudatum was studied in solutions of various low Ca²⁺ concentration, adjusted by EGTA, in the presence of K⁺, Na⁺and other substances, which under normal conditions induce ciliary reversal.

A distinct [Ca²⁺]₀ of ca. 6·10⁻⁸mol/l was found below which ciliary reversal failed to occur. Monovalent cations shifted this threshold towards higher [Ca²⁺]₀ values. Previous incubation at different [K⁺ ₀ and [Ca²⁺]₀ did not influence the threshold concentration. At low [Ca²⁺]₀ monovalent cations caused the surface membrane to disrupt, and the organisms swam backward until they became immobile. The threshold [Ca²⁺]₀ for this phenomenon induced by 67 mmol/l NaCl was 2·10⁻⁷ mol/l. The critical Ca²⁺ activity required for reversal was slightly lower for live paramecia than for those whose membrane was disintegrated. The intraciliary Ca²⁺ concentration, [Ca²⁺]_i; at rest was estimated to be less than 5·10⁻⁸ mol/l.

The results support the assumption that the increment in [Ca²⁺]_i underlying ciliary reversal is caused by passive Ca²⁺ influx through voltagesensitive Ca channels in the ciliary membrane. The effect of monovalent cations on the critical [Ca²⁺] is explained mainly by their depolarizing action which lowers the effective voltage for Ca²⁺ entry.