Increase in membrane conductance by 5-hydroxytryptamine and acetylcholine on the hearts of Modiolus demissus demissus and Mytilus edulis (Mytilidae, Bivalvia)

Abstract

1. 1. The effects of 5-hydroxytryptamine (5HT) and acetylcholine (ACh) on mussel myocardia were studied. Membrane resistance and membrane potential were measured, and spontaneous electrical and mechanical activity monitored, by means of a double sucrose gap apparatus. 2. 2. 5HT (10 −6 M), while producing a small hyperpolarization (2–3 mV), reduced the resistance of the Modiolus myocardium only about twofold. Concomitantly, excitability decreased markedly and spontaneous activity was abolished. Doses 1000 times greater than the threshold for arrest were required to reduce the relative membrane resistance sixfold. 3. 3. ACh decreased the relative membrane resistance of the Modiolus myocardium, and this effect was dose dependent. Only a slight hyperpolarization occurred during the arrest of spontaneous activity. 4. 4. Both ACh and 5HT excited the Mytilus myocardium. 5HT could induce or augment rhythmical activity while producing only a small depolarization and no detectable change in relative membrane resistance. ACh markedly depolarized the Mytilus heart, and this effect was accompanied by a large decrease in membrane resistance. 5. 5. While the ACh reversal potential in Modiolus hearts is only about 3 mV higher than the resting level, that of 5HT is about 15 mV higher. 6. 6. The action of 5HT on Modiolus myocardia, including the small relative resistance decrease, was unaffected by the replacement of Na + with Tris, or Cl − with propionate, in the perfusion fluid. ACh inhibition was dependent on Cl −; in Cl −-free sea water, ACh was excitatory. The action of ACh was unaffected by a lack of Na +.