title: Activation of the contractile mechanism in the anterior byssal retractor muscle of Mytilus edulis.
abstract: 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.
mesh_terms: D000109:Acetylcholine / Q000494:pharmacology / Q000502:physiology*; D000818:Animals; D049872:Bivalvia / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D002794:Choline / Q000494:pharmacology; D066298:In Vitro Techniques; D008564:Membrane Potentials; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000502:physiology*; D011188:Potassium / Q000378:metabolism; D011343:Procaine / Q000494:pharmacology; D012701:Serotonin / Q000494:pharmacology
chemical_list: D012701:Serotonin; D011343:Procaine; D002794:Choline; D000109:Acetylcholine; D011188:Potassium; D002118:Calcium