[Postnatal development of the sympathoadrenergic system in premature and newborn infants]

Autonomic regulatory mechanisms and some metabolic functions are predominantly influenced by the sympathetic nervous system. Premature and mature newborns show a high variability and a low adaptability of the sympathetic nervous system. In order to investigate whether sympathetic systems are completely developed at birth or underly a postnatal maturation process we have determined plasma levels of adrenaline and noradrenaline as well as the density and affinity of alpha- and beta-adrenoceptors on thrombocytes and lymphocytes in pre- and mature newborns and adults. Catecholamines were determined by means of a radioenzymatic method, number and affinity of adrenoceptors by use of the radioactive labelled antagonists 3-H-Yohimbine and 125-I-Cyano-Pindolol. Beta-adrenoceptor responsiveness was assessed by measurement of cyclic AMP in lymphocytes before and after stimulation of beta-adrenoceptors by isoprenaline. A linear relationship occurred between the gestational age and the number of adrenoceptors on lymphocytes, whereas the alpha-adrenoceptors on thrombocytes showed no age dependency. The basal content of cyclic AMP and the accumulation in response to beta-adrenoceptor stimulation by isoprenaline was significantly lower in newborns than in adults. Since noradrenaline and adrenaline plasma levels were not significantly different in newborns and adults it is assumed that the low density of beta-adrenoceptors in premature and mature newborns is due to a postnatal maturation and not to a "down regulation" by circulating catecholamines. Our results suggest that an immaturity of beta-adrenoceptors is involved in the poorly developed adaptive control in newborns.