Membrane Control of Ciliary Activity in the Protozoan Euplotes

Abstract

ABSTRACT Membrane control of ciliary activity in the protozoan Euplotes was investigated by a combination of electrophysiological and cinematographic techniques. The anal cirri, which are quiescent in the absence of stimulation, were selected for this study. Membrane depolarization by means of injected current produced a reversal of the direction of beating (i.e. towards the cell anterior so as to make the ciliate swim backwards). Depolarization also increased the frequency of beating. Increasing depolarizations resulted in an increased number of reversed beats and increased frequency. When the membrane potential was shifted beyond + 70 mV, reversed beating did not occur until after the current pulse ended. Depolarization did not evoke reversed beating when the external calcium (Ca) concentration was reduced to 10−6 M with EGTA. Hyperpolarization caused the cirri to beat in a normal direction (i.e. towards the rear of the ciliate so as to cause the animal to swim forward). Increasing hyperpolarizations resulted in an increased number of forward beats and an increased frequency. The cell was treated with the detergent Triton X-100 to permit Ca, Mg and ATP direct access through the extracted membrane to the cell interior. At Ca concentrations below 10−7 M, Mg-ATP-reactivated cilia of Triton-extracted cells beat normally. At Ca concentrations above approximately 10−7 M the reactivated beat resembled the reversed beat in the living cell. The evidence suggests that membrane-regulated concentrations of intracellular Ca control the direction of ciliary beating. Thus, stimuli which produce an adequate Ca influx lead to ciliary reversal.