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Some effects of caffeine and quinidine on sarcoplasmic reticulum of skeletal and cardiac muscle.

Sarcoplasmic reticulum (SR) was prepared from the gastrocnemius muscle and the heart of freshly killed rabbits. It was found that the skeletal SR actively bound significantly more calcium than did the cardiac SR under the same experimental conditions. The effect of caffeine and quinidine on the release of calcium actively bound by both cardiac and skeletal SR was studied. Quinidine (10−3 M) released 4.1% of the calcium bound by skeletal SR and 27.7% of that bound by cardiac SR. Similarly, caffeine (20 mM) released 10.5% and 34.3% of the calcium bound by skeletal and cardiac SR, respectively. It is suggested that both caffeine and quinidine could produce contracture of skeletal muscle by acting on the SR and that caffeine could stimulate myocardial contractility through its action on the cardiac SR. However, it is unlikely that quinidine exerts its negative inotropic effect on the heart through its calcium releasing action on the cardiac SR.