Catecholamine effects on blood pressure and heart rate in the American bullfrog, Rana catesbeiana.

The effects of catecholamines and adrenergic blocking agents were studied in vivo on the blood pressure and heart rate of the unanaesthetized American bullfrog, Rana catesbeiana. Bullfrogs were chronically cannulated with a T cannula in the right sciatic artery. The mean systemic arterial blood pressure prior to the infusion of catecholamines was 18.5 +/- 1.5 mm Hg. Mean preinfusion heart rate was 30.9 +/- 2.0 beats/min. Epinephrine elicited the largest increase in blood pressure, with an accompanying decrease in heart rate. Norepinephrine and phenylephrine were less effective. Isoproterenol was the only catecholamine tested which elevated heart rate in a dose-dependent manner. It had no effect on blood pressure. The beta adrenergic antagonist, propranolol, blocked the increase in heart rate elicited by isoproterenol but had no effect on the blood pressure increases elicited by the other catecholamines. The alpha adrenergic antagonist, phentolamine, partially blocked the blood pressure increase by epinephrine, norepinephrine, and phenylephrine as well as the elevation of heart rate by isoproterenol. Atropine alone elevated heart rate 19 +/- 3 beats/min, and prevented slowing of the heart due to epinephrine, norepinephrine, and phenylephrine. Stimulatory effects of epinephrine on heart rate were observed only after atropine had been administered. Beta adrenergic receptors, therefore, appear to function in heart rate regulation; however, the predominant effect of catecholamines is reflex slowing of the heart due to stimulation of the vagus nerve. In contrast, the alpha receptor, stimulated by epinephrine, appears to be the main adrenergic receptor controlling blood pressure changes.