Sex differences in lung and adrenal neurosympathetic development in rabbits

To assess the possible mechanism for the increased incidence and severity of the respiratory distress syndrome (RDS) in male versus female infants, we studied neurosympathetic development in the lung and adrenal glands in male and female fetal and newborn rabbits. Tissue levels of catecholamines, norepinephrine (NE), epinephrine (E), and dopamine (DA) were measured using a sensitive radioenzymatic assay. Beta adrenergic receptor development in the lung was studied using the tritiated radioligand dihydroalprenolol. Neurosympathetic innervation of the lung was assessed by measuring tissue NE levels, which increased gradually from 27-day fetuses to 8-day-old newborns. Sex differences were not significant. Lung beta-receptor number was significantly elevated in females as compared with males at each gestational age and throughout the neonatal period (p less than 0.005). Adrenal gland content of E, NE, and DA increased exponentially with advancing developmental age (each p less than 0.005). Adrenal E was significantly (p less than 0.01 elevated in female as compared with male fetuses, as was the proportion of E. Adrenal NE and DA were similar in male and female fetuses. Sex differences were not significant in the newborn animals. The significant relative delay in adrenal medullary and lung beta-receptor matration may relate to the male susceptibility to neonatal morbidity and neonatal RDS.