SURFACTANT COMPOSITION AND FUNCTION IN LUNGS OF AIR-BREATHING FISHES

Examination of lung washings from primitive air-breathing fishes (rope fish, bichirs, and gar) revealed a lipid-based surfactant with an aver age disaturated phospholipid-to-total phospholipid ratio five times lo wer than in mammals. The lung lavage of fishes was exceptionally rich in cholesterol, resulting in average cholesterol-to-phospholipid ratio s three times higher, and cholesterol-to-disaturated phospholipid rati os nearly 15 times higher, than those of mammals. Removal of lung surf actant doubled the pressures necessary to initially open the anterior regions of collapsed lungs in all three fish species but had little or no effect on pressures required to fill the lung (i.e., compliance) a fter the initial opening. The elevated cholesterol content found in pu lmonary surfactant of these fishes is consistent with such findings in other ectotherms, suggesting that the proportional elevation of chole sterol may serve to stabilize the fluidity of the lung surfactant over broader temperature ranges. The influence of surfactant on lung openi ng pressures rather than on compliance contrasts with that seen in mam mals and supports an ''antiglue'' role of pulmonary surfactant in the simpler open-design lungs of lower vertebrates.