Spontaneous fluctuation of the resting membrane potential inParamecium: amplification caused by intracellular Ca2+

Abstract

SUMMARYThe ciliated protozoan Paramecium spontaneously changes its swimming direction in the absence of external stimuli. Such behavior is based on resting potential fluctuations, the amplitudes of which reach a few mV. When the resting potential fluctuation is positive and large, a spike-like depolarization is frequently elicited that reverses the beating of the cilia associated with directional changes during swimming. We aimed to study how the resting potential fluctuation is amplified. Simultaneous measurements of the resting potential and intracellular Ca2+([Ca2+]i) from a deciliated cell showed that positive potential fluctuations were frequently accompanied by a small increase in[Ca2+]i. This result suggests that Ca2+influx through the somatic membrane occurs during the resting state. The mean amplitude of the resting potential fluctuation was largely decreased by either an intracellular injection of a calcium chelater (BAPTA) or by an extracellular addition of Ba2+. Hence, a small increase in[Ca2+]i amplifies the resting potential fluctuation. Simulation analysis of the potential fluctuation was made by assuming that Ca2+ and K+ channels of surface membrane are fluctuating between open and closed states. The simulated fluctuation increased to exhibit almost the same amplitude as the measured fluctuation using the assumption that a small Ca2+ influx activates Ca2+ channels in a positive feedback manner.