Role of calcium in initiation of contraction in smooth muscle

¹ Department of Physiology, Ohio State University, Columbus, Ohio

Extracellular Ca is not essential for the responses of depolarized aortic muscle of the turtle and the stomach muscle of the frog to acetylcholine. The drug caused contractions after the muscles were washed in KCl or K₂SO₄ solutions for many hours. Muscles became unresponsive in Ca-free Ringer's solution but they gave responses again after being transferred into Ca-free K solutions. These responses were temporarily increased by a brief treatment with solutions of Na salts. A net uptake of Ca by the fibers under most conditions did not induce contraction. Ca induced a contraction in muscles immersed only in K, but not in Na solutions, although Ca has been found to penetrate equally well in both types of solutions. Therefore, the responses of depolarized muscle to acetylcholine and Ca appear to depend on an increase in sensitivity of the myoplasm produced by K solutions. It is concluded that unresponsiveness in Ca-free Na solutions is mainly due to an interference with electrogenic processes. The presence of Ca in low concentrations diminished responses to acetylcholine and, vice versa, acetylcholine depressed Ca contractions.

Key Words: action of Ca in smooth muscle • smooth muscle responses in Ca-free solutions • acetylcholine, action on smooth muscle • acetylcholine action on depolarized muscle • acetylcholine action on aortic muscle • epinephrine and Ca action in smooth muscle

Submitted on April 5, 1963