Cardiac Responsiveness to Chronotropic Agents in the Cobra, Naja naja L.: Effect of Temperature and Thyroid Hormones

Cardiac responsiveness to isoproterenol, phenylephrine and parathyroid hormone have been studied in isolated cobra snake atria. The positive chronotropic responses to these agents are increased under elevated metabolic conditions: high temperature and tri-iodothyronine treatment. The enhanced cardiac reactivity provides greater cardiac output in animals with increased metabolic demands. In view of the different receptors being stimulated by these agents, theenhanced cardiac responsiveness is a general one and, most likely, a result of increased metabolism.