Abstract
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.
