THE COMPOSITION AND FUNCTION OF THE PULMONARY SURFACTANT SYSTEM DURING METAMORPHOSIS IN THE TIGER SALAMANDER AMBYSTOMA-TIGRINUM

Mammalian lungs secrete a mixture of surface-active lipids (surfactant ), which greatly reduces the surface tension of the fluid coating the inner lung surface, thereby reducing the risk of collapse upon deflati on and increasing compliance upon inflation. During foetal lung matura tion, these lipids become enriched in the primary and active ingredien t, a disaturated phopholipid. However, disaturated phospholipids exist in their inactive gellike form at temperatures below 37 degrees C and thus are inappropriate for controlling surface tension in the lungs o f many ectotherms. We examined the development of the composition and function of the surfactant system of the tiger salamander (Ambystoma t igrinum) during metamorphosis from the fully aquatic larva (termed sta ge I) through an intermediate air-breathing larval form (stage IV) to the terrestrial adult (stage VII). Biochemical analysis of lung washin gs from these three life stages revealed a decrease in the percentage of disaturated phospholipid per total phospholipid (23.03 versus 15.92 %) with lung maturity. The relative cholesterol content remained const ant. The increased level of phospholipid saturation in the fully aquat ic larvae may reflect their generally higher body temperature and the higher external hydrostatic compression forces exerted on the lungs, c ompared to the terrestrial adults. Opening pressure (pressure required for initial lung opening) prior to lavage decreased from larval to ad ult salamanders (7.96 versus 4.69 cm H2O), indicating a decrease in re sistance to opening with lung development. Opening pressure increased after lavage in older aquatic (stage IV) larvae (5.36 versus 9.80 cm H 2O) and in the adults (4.69 versus 7.65 cm H2O), indicating that the s urfactant system in salamanders may have an antiglue function which pr events apposing epithelial surfaces from adhering together.