A Regenerative Calcium Response in Paramecium

ABSTRACT Standard intracellular current-passing and recording techniques were used to investigate the electrical properties of the membrane of Paramecium caudatum. The surface membrane produces regenerative depolarizations graded in amplitude and rate of rise with the intensity of applied outward current pulses. In a solution of 1 mM-CaCl2, 1 mM-KCl and 1 mM Tris-HCl at pH 7·2 and 18 °C the overshoot reaches a maximum amplitude of +5 to +15 mV, and shows a maximum rate of rise of about 7 V/sec. The overshoot is insensitive to Li+, Na+, Cl−, TTX and procaine, but increases slightly (up to 10 mV/10-fold rise in ion concentration) in the presence of K+, Rb+, Cs+,, Mg2+ and tetraethylammonium. The overshoot increases 22–25 mV for each 10-fold increase in external Ca. Ba2+ and Sr2+ convert the graded response to one that is all-or-none. Repolarization following the peak of the response is more rapid than expected from the time constant of the resting membrane, suggesting delayed rectification. A conditioning depolarization causes inactivation of the regenerative response, and hyperpolarization depresses the response to a given stimulus current. Refractoriness persists for up to 200 msec after each response. The regenerative response of the membrane is consistent with a transient increase in conductance to Ca2+ (Ba2+, Sr2+) with a consequent inwardly directed calcium current. This agrees with recent evidence that ciliary reversal (which accompanies depolarization) is produced by an influx of Ca2+.