Changes in contractile force by barium in the frog skeletal muscle

Effects of Ba2+ ions on the contractility were investigated in the frog skeletal muscle under the current clamp condition. The membrane potential was depolarized by 10 to 20 mV by perfusing 2 mM Ba2+ for 5 to 10 min. Membrane resistance was first increased and then decreased. The mechanical threshold examined in TTX-containing solution was shifted to more positive potential by applying Ba2+. The electrically induced contracture in TTX-solution as well as twitch in normal Ringer were enhanced by superfusing Ba2+. In both types of contraction an afterpotentiation was observed on washing out of Ba2+ ions, reflecting the existence of an inhibitory action during Ba2+ perfusion which could be masked by potentiation. Caffeine was able to induce contracture even in muscles soaked in "zero" Ca2+ solution for 24 to 48 h. In these muscles Ba2+ was more effective than Ca2+ to potentiate the contracture tension presumably by releasing Ca2+ ions from the sarcoplasmic reticulum whereas Mg2+ inhibited it.