Proctolin affects the activity of the cardiac ganglion, myocardium, and cardioarterial valves in Carcinus maenas hearts

Abstract

The decapod crustacean heartbeat is initiated by the cardiac ganglion and is regulated by a variety of neuronal and hormonal inputs. In this paper we examine the effects of the peptide hormone proctolin which appears to have multiple sites of action in the shore crab, Carcinus maenas. To examine some of the potential sites of proctolin action we used three heart preparations: in situ intact and open hearts, and isolated hearts. We provide evidence in support of the hypothesis that proctolin affects cardiac activity at many levels. It acts at the cardiac ganglion to modulate burst rate and at the myocardium to alter contractile force. We calculated the relationship between contractility and ganglionic output of in situ hearts as the ratio of ventricular pressure or tension to amplitude of the electromyogram or intracellular excitatory junction potential. Large proctolin-induced changes in this ratio, which could not be accounted for by ganglionic output, membrane potential or input resistance suggest direct action on the myocardium. The greater increases in ventricular pressure than in tension in the in situ hearts may reflect proctolin-induced contraction of the cardioarterial valves. Finally, proctolin can possibly influence heart rate by action on the cardioregulatory nerves of the central nervous system.