Caffeine and quinine increase the release of sarcoplasmic enzymes from frog skeletal muscles.

1. The rates of release of two sarcoplasmic enzymes, creatine kinase and lactate dehydrogenase, from frog skeletal muscle were increased by caffeine and by quinine at concentrations that induced muscle contractures. 2. Pre-treatment with procaine blocked both the contractures and the increased enzyme loss induced by these alkaloids. 3. It is suggested that the prolonged increases of the Ca2+ concentration in the myofibrillar space during the drug-induced contractures lead to membrane injury and to an increase in the loss of sarcoplasmic enzymes from the muscle.