INFLUENCE OF LABOR AND NEONATAL HYPOXIA ON SYMPATHOADRENAL ACTIVATIONAND METHIONINE-ENKEPHALIN RELEASE IN CALVES

Labor and delivery stimulate increased release of catecholamines and e ndogenous opioid peptides in neonates. Catecholamines promote adaptati on to the extrauterine environment after birth. Enkephalins are stored together with catecholamines in the adrenal medulla and have an inhib itory effect on catecholamine release. We investigated the influence o f labor and neonatal hypoxia on epinephrine, norepinephrine, and met-e nkephalin release in calves. Blood samples were taken from the umbilic al artery before rupture of the umbilical cord and from the jugular ve in repeatedly after birth. Highest plasma norepinephrine concentration was found in calves delivered at the end of gestation (term calves) b efore umbilical cord rupture. In calves delivered before the physiolog ic end of gestation (preterm calves), norepinephrine values increased after cord rupture, but remained lower than values in term calves. Epi nephrine release followed a similar pattern, but norepinephrine was cl early predominant. In term calves, met-enkephalin values were signific antly higher than values in preterm calves. In calves of both groups, met-enkephalin release increased after cord rupture. During birth, the increase in catecholamine release seems to take place earlier than th at of enkephalins. Norepinephrine-dominated stimulation during expulsi on of the calf might be followed by increasing enkephalinergic inhibit ion after cord rupture and onset of respiration. Reduced release of ca techolamines and enkephalins in preterm calves may be connected with d elayed adaptation to the extrauterine environment.