Clearance of intra-amniotic lung surfactant: uptake and utilization by the fetal rabbit lung

To investigate the metabolism of intra-amniotic surfactant, surfactant containing double-labeled dipalmitoylphosphatidylcholine (DPPC) was injected in amniotic fluid on days 23-27 of gestation. Within 44 h, DPPC was distributed to the gastrointestinal tract (45.9%), fetal membranes and placenta (8.2%), fetal lung (6.6%), and liver (1.9%). DPPC uptake was higher in the upper than in the lower lung lobes. The mixture of phosphatidylglycerol and DPPC increased the uptake of DPPC that was not saturable (range 15-60 mg phospholipid). There was no detectable metabolism of DPPC taken up by the fetal lung. Surfactant protein A, originating from intra-amniotic heterplogous surfactant, was detected immunohistochemically in alveolar epithelium. Intra-amniotic surfactants did not affect the expression of surfactant protein mRNAs. Intra-amniotic surfactant (1,500-2,000 mg/kg on day 25.3) improved lung compliance of ventilated 27.0-day premature rabbits less than intratracheal surfactant at birth (75-100 mg/kg). Reutilization by the alveolar epithelium of surfactant secreted to future airspaces, airways, and amniotic fluid may be a mechanism that increases intracellular surfactant pool before birth.