The surfactant film under thermal stress: Insights to a new paradigm for surfactant composition & function

Abstract

Pulmonary surfactant comprises a lipid-protein film that lines the air-liquid interface of the lung of all animals and functions to regulate the surface tension with changing lung volume. Temperature changes can alter the physical state and packing density of the lipids at an air-water interface and significantly alters the surface activity of surfactant. Several species of heterothermic mammal demonstrate modest increases in the fraction of surfactant cholesterol, although no significant differences in phospholipid molecular species occur between warm-active and torpid individuals. On the other hand, surface film structure as determined by atomic force microscopy differs between warm-active and torpid individuals. We investigated the possibility that the composition of heterotherm surfactant may be to some extent “pre-prepared” for thermoregulatory plasticity. However, no consistent difference in the pattern of molecular species was observed between heterothermic and homeothermic species. Out of 12 mammalian species, no 2 species had the same molecular species composition, with 20-fold differences observed in the purportedly’ major’ surfactant lipid component PC16:0/16:0. However, allometric analyses revealed a positive correlation between body mass and molecular species concentration. Moreover, there was a negative correlation with breathing frequency. Hence, body mass and its effect on metabolism and respiratory variables may be an important determinant of phospholipid molecular species composition, such that it is uniquely optimised to the physiology of each animal species and individual.