Three types of acetylcholine response in bivalve heart muscle cells.

1. The acetylcholine (ACh) responses of cardiac muscle cells from three species of bivalves were studied by intracellular recording and ACh ionophoresis. Heart muscle contraction was abolished by bathing in artificial sea water in which Mn(2+) had been substituted for Ca(2+).2. 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 extracellular suction electrode.3. The hyperpolarizing response in all three species was blocked most effectively by methylxylocholine and not very 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.4. The hyperpolarizing response in all three species was not altered by Cl(-)-free or Na(+)-free sea water, but was affected by changes in external K(+). The rate of change of the inversion potential of this response with change in K(+) was 59 mV per tenfold concentration change. The depolarizing response of Mytilus was not altered by changes in external Cl(-) or K(+), but was abolished in Na(+)-free sea water. The depolarizing response of Crassostrea was insensitive to external Na(+) or K(+), inverted at -31 mV in normal sea-water, and was altered by changes in Cl(-). Substituting for Cl(-) with sulphate shifted the inversion potential at a rate of 56-61 mV per tenfold Cl(-) concentration change.5. The three types of ACh response seen in bivalve heart muscle are similar with respect to time course, pharmacological sensitivity and ionic mechanism to the three types of ACh response described in Aplysia central neurones. Analogies can also be drawn with vertebrate ACh responses.