K+-induced Ca2+ conductance responsible for the prolonged backward swimming in K+-agitated mutant of Paramecium caudatum.

The K+-agitated (Kag) mutant of Paramecium caudatum shows prolonged backward swimming in K+-rich solution. To understand the regulation mechanisms of the ciliary motility in P. caudatum, we examined the membrane electrical properties of the Kag mutant. The duration of the backward swimming of the Kag in K+-rich solution was about 10 times longer than that of the wild type. In response to an injection of the outward current, the wild type produced an initial action potential and a subsequent membrane depolarization due to I-R potential drop, while the Kag exhibited repetitive action potentials during the depolarization. Under voltage-clamp conditions, the depolarization-activated transient inward current exhibited by the Kag was slightly smaller than that exhibited by the wild type. In response to an application of K+-rich solution, both the wild type and the Kag exhibited a depolarizing afterpotential representing the activation of the K+-induced Ca2+ conductance. The inactivation time course of the K+-induced Ca2+ conductance of Kag was about 10 times longer than that of the wild type. This difference corresponds well with the difference in behavioral responses between Kag and wild type to K+-rich solution. We conclude that the overreaction of the Kag mutant to the K+-rich solution is caused by slowing down of the inactivation of the K+-induced Ca2+ conductance.