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Dependence of contractile potentiation by calcium antagonists on external calcium was investigated on the frog's twitch muscle. Low concentrations (10(-6) to 10(-5) M) of nicardipine and verapamil enhanced the peak tension of both twitch and electrically induced contracture in the presence of calcium. In the calcium-free media the drugs suppressed the contractile tension. Caffeine contracture was inhibited by the calcium antagonists at 20 degrees C. This inhibition was caused by an early onset of relaxation, which was not observed at 7 degrees C. The results suggest that some interaction between calcium ions and drug molecules at the voltage sensor on the transverse tubular membrane, which regulates E-C coupling but is not directly related to the functional calcium channel, may play an important role for the phenomena. The inhibitory action of calcium antagonists on the caffeine contracture in the presence of calcium is probably independent of the potentiating effect seen in the depolarization-induced contractions.

Dependence of contractile responses by some calcium antagonists on external calcium in the skeletal muscle.