The ontogeny of cardiovascular regulatory mechanisms in the bullfrog (Rana catesbeiana)

Although the study of cardiovascular regulation in adult amphibians began over a century ago, almost nothing is known about the ontogeny of cardiovascular regulatory mechanisms in the Amphibia. The development of circulatory regulation in anuran amphibians is particularly interesting, since metamorphosis from the aquatic larval stage to the more terrestrial adult stage involves extensive modifications in both morphology and physiology. In this dissertation, mechanisms for the regulation of cardiac output and blood pressure were examined in bullfrog larvae (Rana catesbeiana), with particular reference to the effects of development and metamorphosis. Transmural electrical stimulation of isolated, spontaneously beating cardiac preparations revealed the existence of both inhibitory cholinergic and excitatory adrenergic nerve fibers in the heart in the earliest larval stages examined (Stage III). The effects of transmural cardiac stimulation became stronger in later stages, indicating that neural control of the heart may improve during larval development. Early larval stages also possess cardiac receptors capable of responding to exogenous epinephrine and acetylcholine, with dose-response characteristics not significantly different from the relationships in older larvae. Cardiac preparations from adult bullfrogs showed qualitatively similar responses to transmural stimulation and to the application of exogenous epinephrine and acetylcholine The infusion of solutions of epinephrine and phenylephrine into cannulated larvae provided evidence of vascular adrenergic receptors mediating vasoconstriction and vasodilation in larvae as early as Stage III. Vascular receptor sensitivity did not appear to change over the course of larval development. MS-222, known to be a sympathetic cardiac stimulant in adult anurans, produced a tachycardia of sympathetic origin in all larval stages examined, including Stage I larvae weighing less than a quarter of a gram. These results indicate that some degree of regulation is possible in both the heart and peripheral vasculature in bullfrog larvae, almost from the time of hatching. The capacity for cardiovascular regulation (cardiac nerve fibers, cardiac and vascular receptors) remains relatively unchanged, at least prior to the important metamorphic events beginning with larval Stage XVIII. In addition, the mechanisms of cardiovascular regulation are essentially the same in larval and adult bullfrogs. The differences between the two are largely in degree, and not in the basic pattern of responses