EXTRACELLULAR CALCIUM AND THE INOTROPIC EFFECT OF COOLING ON ISOLATEDSKELETAL-MUSCLE

1. The maximal isometric twitch and tetanic tensions of isolated frog sartorii muscles have been studied at 10 and 20-degrees-C. 2. Reducing incubation temperature from 20 to 10-degrees-C caused a large (53.1 % ) increase in twitch tension and a smaller (20.4%) decrease in tetanic tension. 3. Thermal ratios (R10) for twitch and tetanic tension were 0.65 and 1.26, respectively. 4. Addition of the Ca2+-channel antagonis t diltiazem (3 x 10(-5) M) to the incubation medium had no effect on t witch or tetanic tension at 20-degrees-C. 5. Diltiazem significantly d epressed the potentiating effect of reduced temperature on twitch tens ion but had no effect on the depressing effect of reduced temperature on tetanic tension. 6. These results suggest that the positive inotrop ic effect of cooling on isometric twitch tension is, at least, partial ly dependent on extracellular Ca2+ entry during activation.