Neonatal adaptation: naloxone increases the catecholamine surge at birth.

A marked increase in plasma catecholamines at birth has been described in animals and man. Because the factors that regulate catecholamine secretion are incompletely understood and because it has recently been suggested that endogenous opiates are important in the regulation of catecholamine secretion, we designed studies to determine the influence of opiate receptor blockade prior to delivery on the increase in plasma catecholamines at birth. Term fetal sheep were delivered by cesarean section and randomly assigned to receive naloxone or vehicle. Naloxone was given just prior to umbilical cord cutting as a 2 mg/kg bolus followed by 2 mg/kg/h. Naloxone administration resulted in significantly greater peak levels of plasma norepinephrine (peak levels of 1.5 +/- 0.4 versus 0.9 +/- 0.1 ng/ml) and epinephrine (peak levels of 1.4 +/- 0.7 versus 0.9 +/- 0.3 ng/ml) and higher norepinephrine values throughout the study period. Naloxone administration was associated with significantly elevated heart rate (peak 184 +/- 12 versus 207 +/- 13 beats per min) and blood pressure (peak 95 +/- 6 versus 88 +/- 2 mm Hg). These studies demonstrate that opiate receptor blockade from birth markedly augments the neonatal sympathoadrenal response in the term newborn lamb.