Low epinephrine levels and selective deficiency of β2-adrenoceptor vasodilation at birth

Epinephrine is unique among biogenic catecholamines as a potent agonist of β2-adrenoceptors. The β2-adrenoceptor mediated effects during development might be linked to the increase of epinephrine synthesis. Our purpose was to characterize β-adrenoceptor-mediated relaxation in the aorta of newborn and young rabbits (3 to 4months old), and to relate those responses with the epinephrine content of the adrenal gland.

The epinephrine levels and the tyrosine hydroxylase activity were determined in adrenal glands of newborn and young rabbits. Also, concentration-response curves to phenylephrine (selective α1-adrenoceptor agonist), dobutamine (selective β1-adrenoceptor agonist), terbutaline (selective β2-adrenoceptor agonist), and CL 316243 (selective β3-adrenoceptor agonist) were determined in isolated aortic rings obtained from both groups.

The adrenal gland content and the plasma concentrations of epinephrine were lower in newborn than in young rabbits. In contrast, the tyrosine hydroxylase activity was higher in newborn than in young rabbits. On the other hand, the maximal response to phenylephrine was lower in newborn than in young rabbits. Terbutaline at concentrations selective for β2-adrenoceptors had no relaxing effects in neonates, in contrast to young rabbits. The potency and the maximal response of neither dobutamine nor CL 316243 were significantly different between the two groups.

In rabbits, as well as in humans, β2-adrenoceptor-mediated responses and epinephrine synthesis are both immature at birth. On the other hand, the β1 and β3-adrenoceptor-mediated responses are fully developed. We conclude that epinephrine may influence the development of the β2-adrenoceptor-mediated responses at birth and the rabbit is an excellent model to study these issues.