Intrinsic cardiac rate regulation in elasmobranchs: The horned shark, Heterodontus francisci, and thornback ray, Platyrhinoidis triseriata

Abstract

1. 1. Isolated spontaneously beating hearts of five horned sharks, Heterodontus francisci, and four thornback rays, Playtrhinoidis triseriata, have been perfused in vitro at various pressures with a saline medium in order to establish whether or not the pacemakers of these elasmobranchs contained an intrinsic pressure-sensitive rate regulatory mechanism. 2. 2. In the shark hearts 77 per cent and in the ray hearts 88 per cent of the observations yielded positive results. A total of 230 studies were performed on the nine hear from both species. 3. 3. In numerous instances, spontaneously occurring atrioventricular block or arrhythmias, which were observed at low perfusion pressures, were converted to a regular beat when the pressure was augmented sufficiently. 4. 4. The data consistently indicate that the pacemakers of both shark and ray hearts exhibit the fundamental property of intrinsic rate regulation. 5. 5. The data presented in this paper on elasmobranch hearts further substantiate the earlier conclusion that intrinsic cardiac rate regulation represents an ancient and independent phylogenetic development in the pacemakers of many species of animals.