Soluble Poison of Pseudokeronopsis rubra Affects Inward Current in Euplotes vannus

. The marine ciliate Euplotes vannus was used to lest whether keronopsin, the water-soluble component of (he toxic pigment complex from Pseudokeronopsis rubra, narcotizes cells by affecting ion conductance. Keronopsin delays the rate of rise in the regenerative component of action potentials under constant current injection. In depolarizing voltage steps it reduces the rate of rise and the final amplitude of the initial Ca2+ inward current. Normal traits are restored after change to keronopsin-free medium. Effects on locomotory behavior, spontaneous membrane depolarization, action potential, and inward current, all provide evidence that keronopsin acts on cells by obstructing Ca2+ influx.