Co2+, low Ca2+, and verapamil reduce mechanical activity in rat skeletal muscles

We have studied the effects of Co2+ (5 mM), low-Ca2+ solution [0 added CaCl2, 5 mM ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid, 3 mM MgCl2 Ringer], and verapamil (0.1 mM) on mechanical and electrical properties of rat soleus muscle in vitro at 34 degrees C. Muscle fibers had normal resting potentials in Co2+ and verapamil solutions. Low-Ca2+ solution produces a depolarization of approximately 4 mV. The action potentials are smaller and have a slower time course when exposed to test solutions. Iterative generation of action potentials in the presence of Co2+ and low-Ca2+ solution is not modified. In the presence of Co2+ or low-Ca2+ solution, the mechanical output, twitch and tetanus tensions, and caffeine contracture are reduced significantly. Verapamil produces a decrease in the twitch and tetanic tensions but does not modify the caffeine contracture tension. The effect of verapamil on the twitch becomes more manifest when the muscle is stimulated at 3-5 Hz. We suggest that changes in the action potential characteristics or the inhibition of a Ca2+ current are responsible for the mechanical changes observed in the presence of the drugs.