Surfactant phospholipid metabolism in 3-day and 3-day postmature rabbits in vivo

Three-day-old normal and 3-day postmature rabbits (prepared by human chorionic gonadotropin treatment of the does) were given injections of the radiolabeled phospholipid precursors palmitic acid, choline, and glycerol. The specific activities as cpm/mumol phospholipid phosphate were measured for phosphatidylcholine, disaturated phosphatidylcholine, phosphatidylglycerol, phosphatidylinositol, and phosphatidylethanolamine from lung and alveolar wash fraction lipid extracts of 3-day-old rabbits. Specific activities similarly were measured for phosphatidylcholine and disaturated phosphatidylcholine from postmature rabbits. Curves describing the appearance of labeled phospholipids into the alveolar wash fraction demonstrated that in both 3-day and postmature rabbits, phospholipid secretion was delayed for 3 to 4 hr. Thereafter, there was a rapid accumulation of labeled phospholipids until 12 to 16 hr after labeled precursor injection. The curves were similar for all measured phospholipids and all labeled precursors studied. The phospholipid compositions of lung and alveolar wash fraction lipid extracts were determined by analysis of the various phospholipids separated by two-dimensional thin-layer chromatography. Forty percent of lung samples and 35% of alveolar wash samples from postmature rabbits contained no detectable phosphatidylglycerol, whereas all 22 lung and surfactant samples from 3-day-old rabbits contained this phospholipid.