Relaxation of catch in a molluscan smooth muscle—I. Effects of drugs which act on the adenyl cyclase system

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1. 1. A method is described which permits quantitative evaluation of the effectiveness of drugs in relaxing prolonged catch contractions in the anterior byssus retractor muscle of Mytilus edulis L. 2. 2. At 10°C (or 15°C) adenosine cyclic 3',5'-monophosphate (referred to as cyclic AMP) and N 6,O 2-dibutyryl cyclic AMP (dibutyryl cyclic AMP), 2'-adenosine monophosphate (2'-AMP) and adenosine 5'-diphosphate (ADP) did not relax catch. At 5°C, cyclic AMP did not relax but dibutyryl cyclic AMP (5 × 10 −4 M) relaxed catch in some preparations. 3. 3. Theophylline (10 −2 M) relaxed catch and slightly increased relaxation by serotonin (5-HT). Sodium fluoride (10 −2 M) did not relax but first increased and then decreased relaxation by serotonin. These agents have been said to increase intracellular levels of cyclic AMP. 4. 4. Nicotinic acid, an agent which may decrease cyclic AMP levels, decreased relaxation by serotonin. 5. 5. Metabolic inhibitors such as sodium azide (10 −2 M) and sodium cyanide (10 −2 M) did not relax or affect relaxation by serotonin. 6. 6. Adrenergic blocking agents, both α. and β, were found to be without effect on serotonin relaxation of catch. Isoproterenol (10 −4 M), a β-adrenergic agent, relaxed weakly; adrenaline and noradrenaline (10 −4 M) relaxed more effectively, although less than serotonin. 7. 7. These pharmacological results do not strongly indicate a role of cyclic AMP but are generally consistent with the hypothesis that cyclic AMP may mediate relaxation of catch.