The effects of external potassium, multivalent cations and temperature on caffeine contractures in rat skeletal muscle

1 The effects of multivalent cations, membrane potential and temperature on caffeine contractures of rat soleus and extensor digitorus longus (e.d.l.) muscles were investigated. 2 The amplitude of the caffeine contracture was depressed by the removal of calcium and by the addition of a high concentration (1 mm) of lanthanum. Low concentrations of lanthanum (0.1–0.5 mm) augmented the caffeine contracture. 3 Low levels of depolarization by potassium (10–40 mm) augmented the amplitude of the caffeine contracture, while higher concentrations of potassium depressed the contracture. Maximum augmentation of the caffeine contracture occurred with a higher concentration of potassium (20 mm vs 10 mm) in the e.d.l. than in the soleus muscle. 4 The amplitude of contractures was directly related to temperature between 22 and 37°C and inversely related to temperature below 22°C. 5 The effects of caffeine in rat skeletal muscle are suggested to be exerted on the sarcolemma and the mechanisms of action are by modification of the processes of activation and inactivation.