SP-A, SP-B, AND SP-C IN SURFACTANT SUBTYPES AROUND BIRTH - REEXAMINATION OF ALVEOLAR LIFE-CYCLE OF SURFACTANT

Transformations of surfactant after secretion are incompletely underst ood. To clarify them, we lavaged lungs in fetuses and in newborn rabbi ts, fractionated the lavage fluid by differential and density gradient centrifugation, and analyzed the distribution of surfactant protein ( SP) phospholipids, SP-A, SP-B, and SP-C. Furthermore, we administered into trachea of newborn rabbits labeled surfactant and compared the al veolar clearance of SP-A, SP-B, SP-C and saturated phosphatidylcholine . We found that, in the fetus, secreted lamellar bodies contain all co mponents of surfactant, except a small amount of SP-A. As breathing st arts and new surfactant subtypes are generated, the proteins are mostl y associated with dense subtypes, but SP-B and SP-C are especially con centrated in dense materials that contain minor amounts of phospholipi ds and SP-A. Furthermore, we found that, during breathing, alveolar su rfactant is degraded into more than one type of remnant, that the lava ge fluid contains a pool of SP-A not associated with membranes, and th at SP-A, SP-B, and SP-C are all turned over at a faster rate than satu rated phosphatidylcholine.