ADRENERGIC-RECEPTORS AND THE REGULATION OF VASCULAR-RESISTANCE IN BULLFROG TADPOLES (RANA-CATESBEIANA)

Vascular adrenergic sensitivity to exogenous catecholamines was examined in tadpoles of the American bullfrog (Rana catesbeiana), ranging from stage III to XIV. Central arterial blood pressure was measured in decerebrate bullfrog tadpoles to determine a reasonable initial infusion pressure. Solutions of epinephrine and phenylephrine were infused into the vasculature of pithed tadpoles, and the resulting changes in vascular resistance (R(v)) were used to construct log dose-response relationships. Epinephrine infusion produced a dose-dependent increase in R(v), (EC50 = 5.3 . 10(-7) M), which could be reversed by sodium nitroprusside (a smooth muscle relaxant) and blocked by phenoxybenzamine (an alpha-adrenergic antagonist). Larval R(v) also increased with infusion of the alpha-agonist phenylephrine (EC50 = 7.4 . 10(-8) M). Infusion of 10(-6) M isoproterenol (a beta-agonist) largely reversed the phenylephrine-induced increase in R(v). These results indicate that the capacity exists for both a-mediated vasoconstriction and beta-mediated vasodilation early in bullfrog ontogeny. Neither initial R(v) nor the responses to infused epinephrine or phenylephrine were significantly correlated to development over the range of larval stages used in this study.