Effects of norepinephrine on lung fluid flow rate in the chronically catheterized fetal lamb

The influence of norepinephrine on lung fluid flow rate was studied in seven chronically catheterized fetal lambs. Lung fluid was collected through a catheter inserted into the fetal trachea. In six experiments, continuous saline solution infusion for 1 hour followed by equivolumnar norepinephrine (3.0 micrograms/min) for 1 hour via the fetal jugular vein was carried out. In three additional experiments (two fetuses), a 3.0 mg intravenous bolus of propranolol was administered before norepinephrine infusion. Saline solution infusion did not influence the lung fluid flow rate or fetal biophysical and biochemical parameters. Norepinephrine increased fetal blood pressure by 22% and decreased lung fluid flow by 45% from control values. Lung fluid flow rate recovered within 2 hours after the end of the norepinephrine infusion. Pretreatment with propranolol did not alter the results. In addition to previous reports that demonstrated beta-sympathomimetic agonist-enhanced reabsorption of lung fluid from fetal alveolar epithelial cells, our present results suggest that a reduction in fetal lung fluid secretion may be implemented as well through stimulation of alpha-sympathomimetic receptor sites in the pulmonary vasculature.