Pharmacological studies of charge movement in frog skeletal muscle.

Abstract

Charge movements in frog twitch fibers were studied using the three-microelectrode voltage-clamp technique. In a solution made moderately hypertonic with 350 mM-sucrose, fiber contraction was effectively blocked and a secondary hump appeared in the decay phase of the on part of charge movement. At small depolarizations, the hump (Q.gamma.) is small and slow. As depolarization is increased, Q.gamma. becomes larger in magnitude and faster in kinetics until it merges with the main part of charge movement (Q.beta.). As the fiber is perfused extracelluarly with a test solution saturated with dantrolene sodium, Q.gamma. disappears in .apprx. 30 min, whereas the kinetics of Q.beta. are slowed down. After equilibration in the dantrolene sodium solution, the total moveable charge is reduced by .apprx. 20%, which could very well be the charge carried by Q.gamma.. Tetracaine suppresses Q.gamma. but does not seem to have any effect on the kinetics of Q.beta.. The suppression of Q.gamma. appears to be dose-dependent, with complete abolition occurring at .apprx. 4 mM-tetracaine. Dissection of charge movement with tetracaine indicates that Q.gamma. might be bell-shaped and capacitive in nature. Q.beta. and Q.gamma. might be 2 distinct species of charge and Q.gamma. would probably be more closely associated with Ca release from the sarcoplasmic reticulum.