Ca-coupling in the anterior byssal retractor muscle of Mytilus edulis L.

1. Experiments designed to elucidate the role of Ca in the excitation-contraction coupling of the anterior byssus retractor muscle (ABRM) were carried out. Ca influences membrane depolarization and provides for coupling of the contraction in response to repetitive electrical stimulation as well as of ACh and KCl contracture. Depriving ABRM of Ca results in two closely correlated events: disappearance of action potential and of the contraction in response to repetitive electrical stimulation.2. A sigmoid increase in tension with the log-Ca concentration in artificial medium was observed whereas, over the same range of concentrations, the tension remnant decreased.3. Induction of relaxation by 5-HT is Ca dependent. Either thiourea inactivation or Ca deprivation results in failure to relax. Low concentrations of 5-HT (10(-7) g/ml.) bring about increase in peak tension of the contraction in response to repetitive electrical stimulation, whereas higher concentrations (10(-5) g/ml.) undermine peak tension.4. Frequencies exceeding 40 cycles evoke a contraction accompanied by tension remnant, which is eliminated with 5-HT.5. Dropwise addition of Ca on a trypsin window in the muscle induces a latency relaxation before onset of Ca-contracture.