Role of calcium in drug action on smooth muscle.

1. Calcium is essential for the stimulating action of ACh, serotonin, histamine and oxytocin on smooth muscle, both in normal Ringer and K-Ringer.2. In contrast with the above drugs, barium does not require the presence of calcium in Ringer or K-Ringer for its stimulatory action. Calcium rather depresses the barium effect.3. These phenomena are discussed in relation to a concept that calcium and barium act directly on contractile protein of smooth muscle.