Uptake of pulmonary surfactant protein C into adult rat lung lamellar bodies

Previous studies have provided evidence that a large proportion of secreted surfactant lipids is taken up from the alveolar air space by type II cells, incorporated into lamellar bodies, and resecreted. Our goal was to characterize the clearance of exogenously administered recombinant surfactant protein C (SP-C) and to determine if SP-C is taken up by type II cells and incorporated into lamellar bodies. SP-C was radiolabeled by alkylation with [3H]iodoacetic acid and retained its ability to enhance phospholipid adsorption to an air-liquid interface. A mixture of 100 micrograms phospholipid radiolabeled with [14C]dipalmitoylphosphatidylcholine and 10 micrograms SP-C was instilled into the lungs of spontaneously breathing anesthetized adult rats. At later times, the lungs were lavaged and subcellular organelles were isolated. The radioactivity of both phospholipids and SP-C (expressed as disintegrations per minute per microgram phospholipid) in lamellar body fractions increased up to 4 h postinstillation and began to decline after approximately 4 h. The results of this study suggest that SP-C and dipalmitoylphosphatidylcholine are taken up promptly from the alveolar air space and are incorporated into lamellar bodies with time courses that do not differ greatly.