THE RELATIONSHIP BETWEEN INTRACELLULAR AND EXTRACELLULAR SURFACTANT PHOSPHOLIPIDS IN THE LUNGS OF RABBITS AND THE EFFECTS OF SILICA-INDUCED LUNG INJURY

Extensive homogenization of lung tissue by nitrogen decompression in a Parr disruption bomb increased by 5-fold the yields of low-density phospholipid (d = 1.06) achieved by other methods. This intracellular phospholipid preparation was high in phosphatidylcholines (84.3%), particularly disaturated phosphatidylcholine (51.2%). On the basis of its low density, composition, and morphological appearance, we concluded that this phospholipid was derived from the intracellular compartment of pulmonary surfactant. We examined the relationship between intra- and extra-cellular surfactant pools according to age, gender and silica-induced pulmonary injury. In normal animals the intracellular pool of surfactant phospholipids increased from 1.54 .+-. 0.14 mg at 1 day after birth to 62.30 .+-. 4.50 mg per pair of lungs after 31 months, and over the same time period the extracellular pool increased from 1.04 .+-. 0.15 mg to 27.45 .+-. 2.30 mg per pair of lungs. The ratio between the extracellular and intracellular pools of surfactant increased from 1.50 .+-. 0.19 at 1 day after birth to 2.28 .+-. 0.23 after 31 months of age. The ratio between the two pools was not influenced by gender, but was changed by the intratracheal injection of silica into the lungs. Intratracheal injection of silica dust increased the levels of surfactant in both compartments, but not to the same extent, indicating that the ratio between the pools could be changed by toxic materials. These data suggest the existence of a size relationship between the intra- and the extra-cellular pools of surfactant, a relationship which implies a common regulatory mechanism that can be disturbed during pulmonary injury.