Blockade or reversal of the contraction induced by calcium and adrenaline in depolarized arterial smooth muscle.

1. Mesenteric arteries immersed in a depolarizing solution contract in the presence of calcium. These contractions are proportional to the calcium concentration and are reversible.2. Mesenteric arteries immersed in a calcium-free depolarizing solution contract in the presence of adrenaline. Under the experimental conditions reported here, this response develops only about one-third of the contractile tension developed in polarizing solution (modified Krebs bicarbonate).3. Cinnarizine and chlorpromazine inhibit the contractile response to calcium and induce relaxation of depolarized muscle previously contracted by calcium; cinnarizine was 4 times more potent than chlorpromazine in such activity.4. Chlorpromazine inhibits the response to adrenaline in both polarizing and calcium-free depolarizing solutions, whereas cinnarizine inhibits the response in polarizing solution but not that in calcium-free depolarizing solution.5. The significance of these results is discussed.