Binding of 45Ca by live Paramecium caudatum was determined under various external ionic conditions. It was found that calcium uptake was separable into at least two components, a rapid and a slow one. The rapid component was influenced by the presence of certain other ions in a manner which agrees with the law of mass action. It appears that an ion exchange system may be involved in a binding equilibrium established between Paramecium, Ca++, and certain other ions. K+, Rb+, and Ba++ in the equilibrium medium are among those ions which inhibit calcium uptake. It is proposed that liberation of Ca++ from binding sites on Paramecium by an exchange reaction with competing ions is the first step in the mechanism of ciliary reversal in the response to external application of these ions.

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