Surfactant in the Lung and Tracheal Fluid of the Fetal Lamb and Acceleration of Its Appearance by Dexamethasone

We measured tracheal fluid production rate and pulmonary surfactant flux in 12 sets of twin fetal lambs of 102 to 135 days' gestation. In nine of these sets we also measured surfactant flux and the concentration of saturated lecithin in the lungs before and during dexamethasone infusion into one of the twins. The average tracheal fluid flow rate was 3.25 ml/kg/hr (SD 1.6) and, relative to body weight, did not change between 103 and 135 days' gestation or during the infusion of dexamethasone. In untreated fetuses, surfactant was detected between 108 and 130 days and its flux gradually increased but remained less than 150mug/kg/hr at 135 days' gestation; the amount of disaturated lecithin in the lung relative to body weight increased 13-fold from 108 to 134 days. From these data we calculated that the minimal rate of synthesis of disaturated lecithin was increased about 4-fold by dexamethasone from 108 to 120 days; this enhancement fell to about 1.8-fold by 135 days. The rate of secretion of disaturated lecithin, as estimated by surfactant flux in tracheal fluid, was increased by dexamethasone throughout the interval of the study.