Calcium channels and intracellular calcium release are pharmacologically different in frog skeletal muscle.

Abstract

The pharmacology of Ca2+ channels and intracellular Ca2+ release from the sarcoplasmic reticulum (SR) were compared by injecting Ca2+ channel blockers into the cytoplasm and observing contraction under voltage clamp of frog skeletal muscle fibers, a preparation that contracts only in response to Ca2+ release from the SR. A method for quantifying intracellular injections by co-injecting a fluorescent dye is described. Nifedipine injected into cells blocks Ca2+ current through the cell membrane showing that nifedipine is active when applied to the cytoplasmic side of the membrane in which Ca2+ channels are located. Neither the presence of Ca2+ channel blockers in the extracellular medium nor 24 h incubation in nifedipine and D-600 [methoxyverapamil] affect contraction. Nifedipine and D-600 injected to intracellular concentrations much greater than necessary to block Ca2+ channels do not affect contraction. The presence of 30 .mu.M-D-600 during K+ contractures caused paralysis but 20 .mu.M-nifedipine did not. Thus, contracture-dependent D-600 paralysis is not due to blockade of the transverse tubule Ca2+ channel. A functioning Ca2+ channel on the cell membrane is not necessary to trigger Ca2+ release from the SR. SR Ca2+ release and Ca2+ channels are pharmacologically different.