Effects of Raised [K+]o And [Ca2+]o on Conjugation and on Electrophysiological Parameters in the Marine Ciliate Euplotes Vannus

Co-stimulation of mixed complementary mating types of Euplotes vannus was blocked if [K+]o was raised to at least 60 mmol l−1. This inhibition could be cancelled by simultaneously raising [Ca2+]o. An electrophysiological investigation showed that elevation of either [K+]o or [Ca2+]o depolarized the cell. The effect of potassium was observed at concentrations above 50 mmol l−1, with a slope of about 38 mV/decade change, and was accompanied by a reduction in membrane resistance. Calcium had an effect at lower concentrations than potassium, but the slope was lower, and was associated with a rise in resistance. When potassium and calcium were applied together in ratios at which the calcium level was just sufficient to annul the K+-induced block of conjugation, the membrane was depolarized only slightly more than in potassium alone, and resting resistance was near the control level. It is therefore proposed that high [K+]o blocks co-stimulation by acting upon membrane resting conductance.