The pharmacology of a molluscan smooth muscle.

The effects of a number of pharmacologically active substances on contraction and on membrane polarization of the anterior byssal retractor muscle of Mytilus edulis, L., have been studied. Tetramethylammonium bromide, trimethyl(4-oxopentyl)ammonium chloride and nicotine, like acetylcholine, produced depolarization and sustained contraction. Nicotine, on repeated application, lost acetylcholine-like activity and effectively blocked acetylcholine. In order of decreasing potency, methanthelinium, tubocurarine, benzoquinonium, tetraethylammonium, atropine, pentamethonium, and decamethonium blocked acetylcholine action. These agents did not show initial acetylcholine-like action and did not relax sustained contractions. Adrenaline, noradrenaline, tyramine, dibenamine, phentolamine, and lysergic acid diethylamide relaxed sustained contractions without reducing initial depolarization and tension development in response to acetylcholine or electrical stimuli. Adrenaline and noradrenaline often caused depolarization and contraction when first applied, and displayed relaxing action on subsequent application.