Blockade of Ca2+ channels inhibits K+ contractures but not twitches in skeletal muscle

Abstract

The effects of the voltage-sensitive, calcium channel blocking agents, D-600 and verapamil, on twitches and K⁺-induced contractures were studied using frog's toe muscles. K⁺-contracture tension was reduced by concentrations as low as 10⁻⁸ M and the contractures were blocked by 10⁻⁶ M. There was no significant difference in the effects of the two drugs. Twitches were potentiated by 5 × 10⁻⁵ M D-600 and blocked only at 3 × 10⁻⁴ M. The latter concentration also produced contractures in the toe muscles. As shown by other workers, the higher concentration also blocks action potential production and this is probably the way in which it blocks the twitch. Raising the bathing solution Ca²⁺ concentration from 1.08 to 10 or 20 mM, produced only a small, inconsistent, noncompetitive antagonism of the D-600 block of K⁺ contractures.