THE CHANGING STATE OF SURFACTANT LIPIDS - NEW INSIGHTS FROM ANCIENT ANIMALS

Pulmonary surfactant is a mixture of phospholipids (including disatura ted phospholipids), cholesterol and proteins lining the air-liquid int erface within the lung. Surfactant acts to reduce surface tension, the reby increasing lung compliance and also preventing edema. The saccula r lungs, or other gas-holding structures, of nonmammals have 7-70% mor e surfactant/cm(2) of surface than lungs of mammals. Nonmammalian surf actant acts as an antiglue that decreases the inflation pressures of c ollapsed lungs by reducing the adherence of apposing epithelial surfac es. The autonomic nervous system appears to be the primary system cont rolling release of surfactant in nonmammals. The lipid composition is highly conserved within the vertebrates, except that surfactant of tel eost fish is dominated by cholesterol whereas tetrapod surfactant cons ists primarily of disaturated phospholipids (DSP), The dipnoan Neocera todus forsteri demonstrates a ''fish-type'' surfactant profile while t he other derived dipnoans demonstrate a surfactant profile similar to that of tetrapods, Homology of the surfactant protein SP-A within the vertebrates points to a single evolutionary origin for the system and indicates that fish surfactant is a ''protosurfactant''. Amongst the t etrapods, the relative proportions of DSP and cholesterol vary in resp onse to lung structure, habitat, and body temperature (T-b) but not in relation to phylogeny, The cholesterol content of surfactant is eleva ted in species with simple saccular lungs, in aquatic species, and in species with low T-b, The DSP content is highest in complex lungs, par ticularly of aquatic species or species with high T-b, The cholesterol content of surfactant also increases in response to acute decreases i n T-b in lizards and torpid marsupials, presumably to maintain fluidit y of the lipid mixture.