The phospholipid composition of the 150-g pellet containing macrophages from neonatal lung lavages resembles that of surfactant. To study whether this composition reflects the surfactant content of the macrophage, we isolated the alveolar phospholipids and macrophages from the lavage fluids of fetal, newborn, young, and adult Wistar rats. The alveolar surfactant phospholipids increased from fetal levels of 2.8 nmol/mg dry lung weight (DLW) to 39 nmol/mg DLW at day 1, decreased sharply within the first week, and stabilized at a level of 2-4 nmol/mg DLW after day 15. The number of alveolar macrophages increased significantly during the first postnatal day from approximately 750 to more than 5000 (per mg DLW), decreased during the next 4 days, and varied strongly at older ages. We estimated the surfactant content in the macrophages semiquantitatively by polarization microscopy. Birefringence augmented significantly during the first 1.5 days after birth and decreased after that concurrently with the amount of alveolar surfactant. However, only cells without birefringent inclusions sedimented at 150g, whereas the phospholipid composition of the pellets falsely suggested that large amounts of intracellular surfactant were present in its cells. At least two populations of macrophages (surfactant-rich and surfactant-poor) are present in the growing animal. We suggest that differences in function of these various types of macrophages also might depend on surfactant congestion.

Isolation of alveolar surfactant from human cadaver lungs might be ineffective because of postmortem effects. We studied therefore in the rat the effect of autolysis on the yield and composition of alveolar surfactant at different intervals after death. The total amount of phospholipids in the lavage fluids decreased at 4 h postmortem and increased thereafter again. Increased amounts of proteins, significant deviations from the normal phospholipid composition of surfactant and decreased surface activity were already present from 2 h onwards. However, a normal alveolar surfactant can be obtained up to 16 h after death by using a sucrose gradient centrifugation procedure. With this procedure it is possible to isolate a surfactant with adequate surface activity from mildly, but not from severely autolytic rat lungs