The release of internal Ca from saponin-treated skinned smooth muscle of guinea pig taenia caecum was studied. The amount of Ca released was estimated by the area under the contraction curve during treatment with 25 mM caffeine in the presence of 0.1 mM EGTA. The magnitude of the caffeine response in skinned muscle, after loading with 10(-6) M Ca for 3 min, was similar to that in the depolarized muscle in the presence of EGTA before treatment with saponin. This suggests that Ca in the skinned muscle was in a physiological range after loading. The release of Ca from the storage site could be facilitated by Ca itself when the skinned muscle was exposed to Ca above 3 x 10(-6) M. An increase in environmental MG concentration suppressed the Ca-induced Ca release mechanism. Sudden replacement of propionate with Cl in the bathing solution made it possible to release Ca from the storage site. This "depolarization"-induced Ca release occurred only immediately after the application of Cl; thereafter, the Ca release mechanism seemed to be inactivated by the prolonged presence of Cl. These results suggest that two mechanisms of Ca release operate in smooth muscle: (a) release induced by Ca itself, and (b) release by "depolarization".

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