Evidence for a synaptically mediated decrease in conductance in a crustacean myocardium.

1. In the neurogenic heart of the isopod Porcellio dilatatus, electrical stimulation of the cardio-regulatory nerves at rates greater than 20-25 pulses/s elicited inhibitory junctional potentials (IJPs) in the myocardium. Its cessation was followed by a long lasting hyperpolarization of myocardial membrane (post-stimulus hyperpolarization = PSH). 2. During the PSH the membrane resistance of the heart muscle increased. The PSH was enhanced by myocardium hyperpolarization, decreased by depolarization and reversed around -50 mV. 3. Picrotoxin inhibited the summated IJPs elicited by the stimulation and thus caused the membrane to maximally hyperpolarize during inhibitory train, thus suggesting a composite nature of the inhibitory processes. 4. The PSH was reversibly reduced in K+-free saline or in ouabain containing saline but partial restoration was obtained by injection of inward current to the myocardium. 5. The PSH was abolished in lithium saline and reduced in Na+-deficient (choline) solution. Cl-deficient solution that markedly affected the summated IJPs shortly after its introduction did not affect the PSH. 6. It is proposed that the PSH results from a decrease in conductance, presumably to both Na+ and K+. The implication of such a mechanism as a component of the inhibitory regulation of this crustacean heart is discussed.