The action of acetylcholine and catecholamines on an intracellular calcium store in the smooth muscle cells of the guinea‐pig taenia coli.

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Abstract
The role of an intracellular Ca store in excitation-contraction coupling was studied by recording isometric tension development of thin strips (100-150 .mu.m in diameter) of tenia coli incubated in Ca-free solution containing 2 mM-EGTA [ethylene glycol-bis(.beta.-aminoethylether)N,N,N'',N''-tetraacetic acid]. The smooth muscle cells of tenia coli do not contract during exposure to a K+-rich and Ca-free solution. A contractile response can be elicited by acetylcholine or carbachol at concentrations exceeding 10-6 M. These contractions are probably induced by a release of intracellular Ca. Ca is also released from the same store, although less effectively, by histamine and caffeine. The amount of Ca in the intracellular store, as revealed by the magnitude of the carbachol contraction in Ca-free solution, increases after contractions have been induced by high (K+)o or by solutions containing low concentrations of carbachol. This contraction amplitude decreases after stimulation with a high concentration of carbachol. The amount of Ca in the filled store is sufficient for a near-maximal contraction. The activation of .beta.-receptors during a K+-depolarization reduces the height of the contracture and induces a carbachol response in Ca-free solution which is higher than that obtained after a preceding K+ depolarization without isoprenaline. An increased uptake of Ca into the carbachol-sensitive store may contribute to the relaxing effect of .beta.-agonists. In the tissues loaded with 45Ca in a K+-depolarizing solution, a release of Ca into Ca-free solution is observed when the muscle stimulated with carbachol, but not when it is stimulated with Ca-free high K+. The release is larger when isoprenaline is present during the loading with 45Ca. The removal of Na+ from the solution exerts a complex and unexplained action on the Ca store. Substitution of Na+ by Tris+ and by K+ have similar effects. The smooth muscle cells of the guinea pig tenia coli may have an intracellular store of Ca which participates in excitation-contraction coupling. The store is sensitive to muscarinic agonists, .beta.-agonists and monovalent ions, but is not affected by depolarization of the outer membrane.