Reference: Biol. Bull. 188: 179-185. (April. 1995) The Role of the Cardioregulatory Nerves in Mediating Heart Rate Responses to Locomotion, Reduced Stroke Volume, and Neurohormones in Homarus americanus M. S. GUIRGUIS AND J. L. WILKENS* Department of Biological Sciences, University of Calgary, Calgary, Alberta, Canada T2N 1N4 Abstract. Control of decapod crustacean heart activity is believed to be dependent on the regulation of the cardiac ganglion by external input from the central nervous system as well as by circulating neurohormones. This study in-vestigated the roles of these inputs on the heart rates of lobsters exercising on a treadmill. Heart rate increased rapidly at the onset of walking in control animals. This rapid phase was lost after the regulatory nerves were cut, but small increases still occurred. When stroke volume was reduced by cutting alary ligaments, the animals com-pensated by increasing heart rate: this compensation was lost when the regulatory nerves were cut. In resting ani-mals, injection of serotonin, octopamine, and dopamine induced increases in heart rate. After the regulatory nerves were cut, only dopamine and serotonin injections caused increases in heart rate, suggesting that these amines act on the cardiac ganglion as independent effectors. Introduction The neurogenic decapod crustacean heart consists of a single ventricle suspended in the pericardial sinus by an array of alary ligaments. The energy stored as these liga-ments are stretched during systole is recovered to re-ex-pand the heart during diastole. The heart fills by means of valved ostia and supplies hemolymph to seven arteries. There is no direct venous supply to the heart. The basic contraction rhythm of the heart arises from the bursting discharges of the nine-cell cardiac ganglion located on the inner dorsal wall of the heart (Alexan-drowicz, 1932). The cardiac ganglion receives extrinsic Received 2 September 1994; accepted 8 December 1994. * To whom correspondence should be addressed. Abbreviations: f H , heart rate; DA, dopamine; OA, octopamine; 5-HT, 5-hydroxytryptamine. nerve fibers via the paired dorsal nerves that arise from the central nervous system (Alexandrowicz, 1932). Each dorsal nerve contains two accelerator axons and one in-hibitory axon. In isolated hearts, stimulation of the ac-celerator nerves speeds the contraction rate, and stimu-lation of the inhibitory nerves slows or stops the heart (Maynard, 1953; Florey, 1960: Wilkens and Walker, 1992). En passant recordings from the dorsal nerves in semi-intact animals reveal periodic increases in inhibitory nerve firing rates: these increases cause bradycardia (Field and Larimer, 1975; Young, 1978). The role of this au-tonomic-like control system in regulating heart rate re-sponses in intact animals has not been studied. Heart rate (/ H ) is also modified by all of the neurohor-mones that have been identified in the pericardial organs (see reviews by Wilkens, 1987; Wilkens and McMahon. 1992). Each of the aminergic neurohormones, dopamine (DA), octopamine (OA) and serotonin (5-HT), trigger tachycardia in isolated hearts (Cooke, 1966; Florey and Rathmayer, 1978; Grega and Sherman. 1975) and in in-tact animals (Wilkens el a/., 1985). In intact animals the possibility cannot be discounted that the neurohormones act indirectly via the nervous system as well as directly (Berlind et ill. 1970). The experiments reported here were designed to test the hypothesis that / H in intact lobsters is under continuous modulatory control by cardioregulatory nerves and pericardial aminergic neurohormones. Materials and Methods Source ami holding condition of lobsters Homarus americanus of 490-670 g mass were obtained from a commercial supplier and maintained in flowing artificial seawater at 1 2C. No differences in performance were observed between males and females. 179
The Role of the Cardioregulatory Nerves in Mediating Heart Rate Responses to Locomotion, Reduced Stroke Volume, and Neurohormones in Homarus americanus
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