Ciliary frequency and orientational responses to clamped voltage steps inParamecium

Abstract

1. Simultaneous voltage clamping and microcinematography were used to examine the behavior of cilia in response to prolonged hyperpolarizing and depolarizing steps in the membrane potential ofParamecium caudatum. In the absence of stimulation the cilia beat at less than 20 cycles per second with the power stroke directed toward the posterior and somewhat to the right (i.e. 4 o'clock) of the cell. 2. Hyperpolarization of the membrane results in a graded increase in frequency and a slight clockwise shift in orientation of the power stroke to a more posteriad orientation (i.e. toward 6 o'clock); peak frequencies of up to 50/sec are reached in about 4 sec, after which the beating settles to a lower steady frequency. 3. Responses to depolarization are more complex; below +3 to +5 mV the frequency drops toward a minimum. Further depolarization up to +20 mV produces a stimulus graded increase in frequency with a graded counterclockwise shift in orientation of the power stroke. 4. During maintained depolarizations the frequency of reversed beating increases with time over several seconds and then gradually decreases in frequency over a period of 30 to 60 sec during which time the orientation also relaxes toward normally directed beating. 5. With potential shifts of more than +60 mV the frequency of reversed beating becomes less, reaching a minimum between +80 and +100mV; at greater positive potential steps normal beating is reestablished. 6. During 1- to 2-sec steps of either small (1–4 mV) or large (60 to 100 mV) positive potential steps there is a progressive slowing of normally directed beating until the cilia stop and finally exhibit reversed beating. 7. In all instances ciliary orientation and frequency change together in parallel as a function of membrane voltage. Following the first second of stimulation these two parameters of movement also exhibit changes which parallel in time slow changes in current. This suggests a common regulating substance or process. 8. Evidence is discussed which indicates that it is [Ca]in, regulated by the voltage-sensitive calcium conductance of the surface membrane, which is the common agent controlling both the frequency and the direction of ciliary beating.