Effects of postnatal dexamethasone and exogenous surfactant on surfactant protein B mobilization in preterm rabbits

Surfactant protein B (SP-B) is critical for surfactant function. SP-B mobilization from lamellar bodies to the airspaces of the lungs may facilitate postnatal adaptation. The effects of postnatal dexamethasone (Dex) and exogenous surfactant on postnatal mobilization to the airspaces of preterm rabbits were studied. Design: Fetuses were delivered at 29 days gestation and sequentially assigned to no ventilation (postmortem lavage at birth), spontaneous breathing (± 0.5 mg/kg intravascular Dex, postmortem lavage after 8 & 16 hours), or mechanical ventilation (± Dex, ± 100 mg/kg exogenous Survanta, postmortem lavage after 8 & 16 hours). Spontaneously breathing rabbits were maintained in an isolette with 50% O₂. Mechanically ventilated rabbits were maintained with 50% O₂ in individual temperature-controlled plethysmographs by adjusting peak inspiratory pressures to regulate tidal volumes. At necropsy, airspaces were lavaged with saline and samples were quantified for SP-B content using a solid-phase, adsorption-based ELISA and for phosphatidylcholine (PC) content using an enzymatic colorimetric assay following chemical extraction. Results: PC airspace pools were unaltered by Dex, and neither SP-B nor PC contents were significantly changed by up to 16h of mechanical ventilation. Postnatal Dex significantly elevated SP-B content in airspace lavage at 16h in both spontaneously breathing (p<.001) and mechanically ventilated (p<.01) preterm rabbits. Surfactant-treated animals following 16h of mechanical ventilation had greater than predicted SP-B levels based on the estimated content of SP-B in exogenous surfactant. Conclusions: Early postnatal Dex mobilizes SP-B to the airspaces via either incriased SP-B secretion or decreased reuptake/clearance. Increased recycling of surfactant following instillation of treatment doses may stimulate subsequent SP-B mobilization.