Abstract
1. Ca 2+-antagonists counteract the muscular activity of the sea urchin pluteus. Agents that block rapid Na +-channels have no effect. 2. High muscular activity is induced by increasing the sea water concentration of Ca 2+ or K + and by a Ca 2+-ionophore. The stimulatory effects tend to decline. 3. Muscarinic agents counteract the effects of Ca 2+ and K +. 4. Variation in the concentration of Ca 2+ or K + has profound effects on the response to nicotinic agents. 5. It is suggested that Ca 2+ plays the role as a charge-carrier and in the release of monoamines from an inner source, and that an excessive Ca 2+-influx induces an outflux of K + leading to hyperpolarization and abolition of the impulse activity.
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