Activation of the contractile mechanism in the anterior byssal retractor muscle of Mytilus edulis.

1. The electrical and mechanical responses of the anterior byssal retractor muscle (ABRM) of Mytilus edulis to acetylcholine (ACh), high [K]O or the removal of external Ca were examined under a variety of conditions. 2. ACh (10(-6)--10(-3)M) produced contracture tensions larger than those produced by high [K]O (30-300 mM) for a given amount of depolarization. In Ca-free solution, the rate of decline of ACh-contractures was much smaller than that of K contractures, though both ACh- and K-contractures eventually disappeared. 3. 5-HT (10(-4)M) of procaine (1 mM) markedly reduced the height of ACh-contractures, but had little or no effect on K-contractures. The height of K contractures was markedly decreased by Mn ions (20 mM) or low pH (4-5), while ACh-contractures remained unaffected. 4. Partial replacement of [Na]o by choline (30-100 mM) reduced both ACh-induced depolarization and contracture tension, whereas K-contractures remained unchanged even after total replacement of [Na]o by choline. 5. ACh could produce little or no tension when applied during the relaxation phase of K-contractures, while high [K]o produced the maximal contracture tension when applied during the relaxation phase of ACh-contractures. 6. Following the removal of external Ca from solutions containing less than 10 mM-Mg, the ABRM showed a marked tension development associated with repetitive electrical activity superimposed on a gradual decline of membrane potential. 7. These results suggest that ACh-contractures are mainly due to the release of intracellularly stored Ca, while K-contractures are mainly associated with the inward movement of external Ca.