The composition and function of reptilian pulmonary surfactant

In mammals, the surface tension of the fluid lining the inner lung greatly contributes to the work of breathing. Surface tension is modified by the secretion of a mixture of surface active lipids and proteins (termed pulmonary surfactant). A disaturated phospholipid (DSP), predominantly dipalmitoylphosphatidylcholine (DPPC), can eliminate surface tension under high dynamic compression. Cholesterol (CHOL) and unsaturated phospholipids (USP) promote respreading upon inflation by converting DPPC to the disordered liquid-crystalline state. It appeared to us that a surfactant rich in DPPC, which has a high phase transition temperature of 41 degrees C, is likely to be of only limited use in the lungs of reptiles, many of which have preferred body temperatures between 20 and 30 degrees C. We review here the presence and composition of surfactant in species from the three subclasses of the Reptilia and relate these to lung structure and function, phylogeny and environmental selection pressures such as body temperature. We also discuss the function of reptilian surfactant and the factors which control surfactant turnover. Large amounts of pulmonary surfactant have been found in all reptiles so far examined. In general, warmer reptiles have greater amounts of surfactant which is also relatively enriched in DSP. Cold lizards (18 degrees C) have significantly elevated levels of surfactant cholesterol. As in all vertebrates, PC is always the dominant phospholipid (60-80%). Unlike mammals, phosphatidylglycerol (PG) is absent, with the exception of one species. The remaining phospholipid groups are present to varying degrees. The saturated fatty acid, palmitic acid (16:0) is the dominant acyl group, oleic acid (18:1) is the dominant mono-unsaturated fatty acid, and polyunsaturates comprise only about 20% of the total fatty acid profile. For two species of dragon lizards, short term changes in temperature do not affect the fatty acids, but protracted periods of cold significantly decrease the presence of 16:0 in turtle lavage (Lau and Keough, Can.J. Biochem. 59: 208-219, 1981). Surfactant appears to function as an antiglue in most reptiles, serving to lower opening pressure, and decrease the work of breathing. However, surface tension forces generally do not influence reptilian lung compliance, suggesting that the primary functions of mammalian surfactant are not necessarily relevant to reptiles.