Three types of acetylcholine response in bivalve heart muscle cells

Abstract

The acetylcholine (ACh) responses of cardiac muscle cells from 3 spp. of bivalves were studied by intracellular recording and ACh ionophoresis. Heart muscle contraction was abolished by bathing in artificial seawater in which Mn2+ was substituted for Ca2+. Three different types of membrane potential changes were observed in response to ACh pulses: a slow hyperpolarization in the clam Mercenaria mercenaria, a rapid depolarization which was sometimes followed by a slower hyperpolarization in the muscle Mytilus edulis, and a biphasic response consisting of both a rapid depolarization and slower hyperpolarization in the oyster Crassostrea virginica. All responses were accompanied by an increase in membrane conductance, as measured by passing constant current pulses with an extracellualr suction electrode. The hyperpolarizing response in all 3 spp. was blocked effectively by methylxylocholine and not effectively by tubocurarine or hexamethonium. The depolarizing response in Mytilus was blocked preferentially by tubocurarine and hexamethonium, while the depolarizing response in Crassostrea was blocked effectively by tubocurarine only. The hyperpolarizing response in all 3 spp. was not altered by Cl--free or Na+-free seawater, but was affected by changes in external K+. The rate of change of the inversion potential of this response with change in [K+]o was 59 mV/10-fold concentration change. The depolarizing response of Mytilus was not altered by changes in external Cl- or K+, but was abolished in Na+-free seawater. The depolarizing response of Crassostrea was insensitive to external Na+ or K+, inverted at -31 mV in normal seawater, and was altered by changes in [Cl-]o. Substituting for Cl- with sulfate shifted the inversion potential at a rate of 56-61 mV/10-fold Cl- concentration change. The 3 types of ACh response in bivalve heart muscle are similar with respect to time course, pharmacological sensitivity and ionic mechanism to the 3 types of ACh response described in Aplysia central neurons. Analogies can be drawn with vertebrate ACh responses.