Adrenomedullary catecholamine release in the fetus and newborn: secretory mechanisms and their role in stress and survival.

Catecholamines released by the adrenal medulla during birth play a key role in the adaptation of the newborn to extrauterine life. Respiratory, metabolic and cardiovascular adaptations to the hypoxia and other stresses associated with delivery are dependent upon a profound surge of adrenomedullary activity which occurs despite the immaturity of connections between the central nervous system and the adrenal. The "non-neurogenic" response seen in the fetus and neonate is thus essential to survival, and any interference either with catecholamine release or with catecholamine actions at adrenergic targets results in loss of the ability to survive hypoxia or other stressors. The immature secretory mechanism disappears as a result of development of neural connections, and factors which accelerate ontogeny of neural competence thus lead to premature loss of non-neurogenic secretory capabilities and a consequent increase in vulnerability. The fetus and neonate also have unusual proportions of adrenergic receptor subtypes in many tissues; these confer reactivity to specific stimuli associated with birth and with periods in which tissue differentiation may be under adrenergic control. Again, the ontogenetic switchover of receptor-mediated mechanisms appears to be a function of the development of neuronal competence, but in this case an important role may be played by a secondary surge in sympathetic tone occurring during the postnatal period. Through specialized mechanisms mediating catecholamine secretion and adrenergic responses, the adrenal medulla thus appears to provide both physiological and trophic signals to the fetus and neonate.