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

1. The role of an intracellular Ca store in excitation-contraction coupling was studied by recording isometric tension development of thin strips (100-150 micron in diameter) of taenia coli incubated in Ca-free solution containing 2 mM-EGTA. 2. The smooth muscle cells of taenia coli do not contract during exposure to a K+-rich and Ca-free solution. However 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. 3. The amount 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+)0 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. 4. 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. This observation indicates that an increased uptake of Ca into the carbachol-sensitive store contributes to the relaxing effect of beta-agonists. 5. In the tissues which have been loaded with 45Ca in a K+-depolarizing solution, a release of Ca into Ca-free solution is observed when the muscle is stimulated with carbachol, but not when it is stimulated with Ca-free high K+. The release is larger when isoprenaline was present during the loading with 45Ca. 6. 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. 7. It is concluded that the smooth muscle cells of the guinea-pig taenia coli 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.