Excitatory and inhibitory effects of acetylcholine on the anterior aorta muscle of Aplysia.

The acetylcholine (ACh) responses of anterior aorta muscle cells of Aplysia kurodai were studied by intracellular recording and ACh iontophoresis. Three different types of membrane potential change were observed in response to ACh pulses: a rapid Na+-mediated depolarizing response, a slow, mainly Cl-, in part K+-mediated, hyperpolarizing response, and Na+ (for depolarizing component) and Cl- (for hyperpolarizing component)-mediated biphasic response. The depolarizing response and the depolarizing component of the biphasic response were effectively blocked by hexamethonium or atropine. The hyperpolarizing response and the hyperpolarizing component of the biphasic response were completely blocked by tubocurarine. Low concentrations of ACh inhibited and relaxed the anterior aorta muscle; high concentrations of ACh excited the muscle and initiated contraction. The biphasic responses to ACh are likely due to the presence of more than one ACh receptor type on the same cell membrane of the anterior aorta muscle fiber.