The effect of parathion on crustacean skeletal muscle—I. The mechanical threshold and dependence on Ca 2+ ions

Abstract

1. 1. The threshold for potassium-induced contractures in Carcinus skeletal muscle is 70 mM, tension being maximal above 100 mM KCl. 2. 2. Parathion, at a concentration of 0·5 mM, induced irreversible tonic contractures, these contractures being enhanced to well beyond peak tetanus tension by increasing external K +. 3. 3. In parathion-containing salines, the mechanical threshold of the muscle fibres was considerably lowered, but unlike the caffeine effect tension was always generated, even at low K + levels. 4. 4. The parathion/K + contractures were reduced at low and at very high Ca 2+ levels, although measureable tension was still developed in zero Ca 2+ salines. Potassium-induced contractures were abolished at low Ca 2+ levels but were not reduced at very high Ca 2+ levels. 5. 5. Parathion does not appear to activate muscle by a transient depolarization, and although it modifies the mechanical threshold its major action appears to be on the internal calcium control system of the muscle fibre.