CALCIUM CURRENT ACTIVATED BY COOLING IN PARAMECIUM

The cooling of deciliated Paramecium cells induced a transient Ca curr ent and its amplitude depended on the rate of the temperature drop. Th e amplitude of the Ca current was increased by the addition of Ca2+ to the bath solution in a concentration-dependent manner, whereas Ni2+, Co2+, Mn2+ and Mg2+ each reversibly inhibited the Ca current in a conc entration-dependent manner with apparent dissociation constants of 0.5 2, 0.66, 0.67 and 2.17 mmol.l(-1), respectively. The Ca current was al so inhibited reversibly by amiloride, with a dissociation constant of 0.32 mmol.l(-1). The Ca current was desensitized by repetitive cooling . The amplitude of the Ca current at the second cooling was smaller th an that at the first cooling when the interval was short, but recovere d as the interval increased. Replacing extracellular Ca2+ with equimol ar Sr2+ or Ba2+ did not significantly affect the amplitude of the curr ent response to cooling, but it accelerated the rate of recovery from desensitization and slowed the decay of the current response. These re sults suggest that the desensitization and the inactivation of the Ca current may involve a Ca2+-dependent pathway.