Alterations in composition and function of surfactant associated with torpor in Sminthopsis crassicaudata

Cold profoundly influences lung compliance in homeothermic mammals. Much of this effect has traditionally been attributed to the inactivation of the surfactant system. However, many mammals undergo large fluctuations in body temperature (heterothermic mammals). Here, the surfactant lipid composition and lung compliance of warm-active dunnarts (Sminthopsis crassicaudata) and the homeothermic mouse (Mus musculus) [body temperature (Tb) = 35-37 degrees C] were compared with those of dunnarts killed after 1,4 or 8 h of torpor (Tb < 20 degrees C). Lung compliance was measured before and after the removal of surfactant, and tissue compliance was determined by inflating the lung with saline. Relative to total phospholipid (PL), mouse surfactant contained proportionately less phosphatidylinositol but more cholesterol (Chol) and phosphatidylglycerol than that of the dunnart. Lung compliance was lower in dunnarts than in mice, consistent with an allometric effect. Surfactant levels, including total PL, Chol, and disaturated phospholipid (DSP) increased during torpor. The relative proportions of Chol and DSP increased after 4 and 8 h, respectively. In marked contrast to previous studies on the behavior of isolated lungs from homeothermic mammals, in our study the lung compliance of dunnarts remained unchanged throughout torpor. Tissue compliance decreased at 1 and 4 h of torpor, but this decrease was abolished by 8 h. It appears that the surfactant of the dunnarts counteracted the negative effect of tissue compliance at 1 and 4 h, an effect not present in homeothermic mammals. However, because lung compliance was maintained at 1 h of torpor in the absence of a compositional change in surfactant lipids, the changes in lipid composition observed at 4 and 8 h of torpor are thought to relate to functions of surfactant other than that of maintaining lung compliance.