Contribution of intracellular stored calcium to contractile activation in contractures of stomach circular muscle of Bufo vulgaris formosus.

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Abstract
Contractile responses of stomach circular muscle of Bufo to high-K, to acetylcholine (ACh) in normal Ringer or in high-K solution, and to Ca in Ca-free high-K solution showed a phasic contraction which relaxed completely in 30-45 s. K-induced contracture was abolished in Ca-free solution containing 1 mM ethylene glycol bis(.beta.-aminoethyl ether)-N,N''-tetraacetic acid (EGTA) within 20 s while ACh-induced contracture was not abolished and 10-25% of control tension was kept up to 40 s. This response increased to 40-50% when all extracellular Na was replaced with tris(hydroxymethyl)aminomethane (Tris). K-induced contracture was inhibited completely by 1 mM La. ACh-induced contracture in the muscle depolarized by high-K solution was dependent on the depolarization time, 0-10, 60-70, and nearly 100% of control after 1, 3, and 10 min depolarization, respectively. These ACh-induced contractures were not inhibited by 1 mM EGTA or La. These contractures were markedly inhibited by 5 mM procaine. Apparently, activation of both contractures induced by high-K and ACh were, at least partly, dependent on the Ca at the intracellular Ca storage sites. Ca-induced contracture was dependent on depolarization time as was ACh-induced contracture, when the muscle was depolarized by Ca-free high-K solution without pretreatment with Ca-free Ringer solution. Activation of Ca-contracture may also be dependent on intracellular stored Ca.