The relationship of the calcium content of smooth muscle to its contractility in response to different modes of stimulation

The Ca2+ and Mg2+ contents of strips of the circular smooth muscle from the stomach of the toad (Bufo marinus) were measured after varying lengths of exposure to NaCl-Ringer and K2SO4 depolarizing Ringer solutions in which the Ca2+ and Mg2+ concentration was varied. The Ca2+ content of the smooth muscle was correlated with the contractility of the muscle when the tissue was first depleted of Ca2+ to abolish responses and then controlled replacements made. In Ca2+-free depolarizing K2SO4-Ringer the Ca2+ content fell from 0.95 ± 0.02 (mean ± S.E. of mean) to 0.14 ± 0.01 μmole per g wet weight before contractility to acetylcholine was lost. When only 9% of the tissue Ca2+ was replaced by exposure to 0.08 mM Ca2+ in the Ringer solution, full contractility was restored, the muscle then containing 0.23 ± 0.01 μmole per g wet weight. However, contractions produced by alternating current applied in the longitudinal direction of the fibres were practically eliminated when the tissue still contained 0.31 ± 0.01 μmole of Ca2+ per g wet weight. 26% of the maximal contractility was restored when the tissue contained 0.43 ± 0.01 μmole per g wet weight and 42% response when the tissue contained 0.53 ± 0.02 μmole per g wet weight. When the tissue contained supranormal amounts of Ca2+, the contractile response to maximum electrical stimulus equalled that produced by a maximal dose of acetylcholine. The experiments suggest that acetylcholine response depends on a small fraction of Ca2+ located in the cortex of the cell while the effect of alternating current is to displace Ca2+ from myoplasmic sites.