PULMONARY-TYPE SURFACTANTS IN THE LUNGS OF TERRESTRIAL AND AQUATIC AMPHIBIANS

We examined the composition and function of pulmonary surfactants in a mphibians inhabiting aquatic and terrestrial habitats with particular regard to the influences of(l) variations in body temperature, (2) ext ernal hydrostatic pressure and (3) breathing pattern. Two fully aquati c salamanders, and the completely terrestrial cane toad Bufo marinus ( all maintained at 21-23 degrees C) were selected. Whereas one of the s alamanders (Siren intermedin) possessed gills and lungs, Amphiuma trid actylum only possessed lungs. We determined the amounts of cholesterol (Chol), disaturated phospholipids (DSP) and total phospholipid (PL) i n lavage of all three species, and also determined the types of phosph olipids of B. marinus and A. tridactylum. DSP lowers surface tension a t the air-water interface in the lung, while Chol and unsaturated phos pholipids assist spreading and maintain the DSP in its disordered, liq uid-crystalline state at high lung volumes. All three species had sign ificant amounts of pulmonary-type surfactant. The two aquatic salamand ers had identical ratios of both Chol/PL and DSP/PL both of which in t urn were nearly twice those of B. mirrors. All three species had simil ar Chol/DSP ratios. Aquatic salamanders sustain high external hydrosta tic pressures exerted by the aquatic environment and tend to collapse their lungs during expiration. We hypothesize that these salamanders m ight require a DSP-rich surfactant to prevent the epithelial surfaces from adhering and large amounts of Chol to keep the DSP fluid. The ter restrial B. marinus has less DSP, suggesting a surfactant which is flu id over a large range of temperatures. Possibly, cane toads do not req uire a DSP rich surfactant as they neither collapse their lungs on def lation, nor experience external hydrostatic pressures promoting lung c ollapse. The PL profile of B. marinus lavage was similar to that of ot her frogs and mammals, containing phosphatidylcholine (PC) as the pred ominant phospholipid together with substantial amounts of phosphatidyl glycerol (PG). On the other hand, although A. tridactylum exhibited hi gh levels of PC, it contained phosphatidylinositol (PI) in place of PG , a pattern typical of reptiles and birds.