Contractile activation in scorpion striated muscle fibers. Dependence on voltage and external calcium

Excitation-contraction coupling was characterized in scorpion striated muscle fibers using standard microelectrode techniques as employed in studies on vertebrate skeletal muscle. The action potential of scorpion muscle consists of two phases of regenerative activity. A relatively fast, overshooting initial spike is followed by a prolonged after- discharge of smaller, repetitive spikes. This after-discharge is accompanied by a twitch that relaxes promptly upon repolarization. Twitches fail in Na-free, tetrodotoxin (TTX)-containing, or Ca-free media. However, caffeine causes contractures in muscles paralyzed by Na- and Ca-free solutions. Experiments on muscle fibers voltage-clamped at a point with two microelectrodes in Na-free or TTX-containing media indicate that: (a) the strength-duration relation for threshold contractions has a shape similar to that in frog muscle, but mean values are displaced approximately 20 mV in the positive direction; (b) tetracaine exerts a parallel effect on strength-duration curves from scorpion and frog; (c) contractile activation in scorpion is abolished in Ca-free media; and (d) the contractile threshold is highly correlated with the occurrence of inward Ca current for pulses of all durations. Thus, the voltage dependence of contractile activation in scorpion and frog muscle is similar. However, the preparations differ in their dependence on extracellular Ca for contraction. These results are discussed in relation to possible mechanisms coupling tubular depolarization to Ca release from the sarcoplasmic reticulum in vertebrate and invertebrate skeletal muscle.