ALTERATIONS IN COMPOSITION AND FUNCTION OF SURFACTANT-ASSOCIATED WITHTORPOR IN SMINTHOPSIS-CRASSICAUDATA

Cold profoundly influences lung compliance in homeothermic mammals. Mu ch of this effect has traditionally been attributed to the inactivatio n of the surfactant system. However, many mammals undergo large fluctu ations in body temperature (heterothermic mammals). Here, the surfacta nt lipid composition and lung compliance of warm-active dunnarts (Smin thopsis crassicaudata) and the homeothermic mouse (Mus musculus) [body temperature (T-b) = 35-37 degrees C] were compared with those of dunn arts killed after 1, 4, or 8 h of torpor (T-b < 20 degrees C). Lung co mpliance was measured before and after the removal of surfactant, and tissue compliance was determined by inflating the lung with saline. Re lative to total phospholipid (PL), mouse surfactant contained proporti onately less phosphatidylinositol but more cholesterol (Chol) and phos phatidylglycerol than that of the dunnart. Lung compliance was lower i n dunnarts than in mice, consistent with an allometric effect. Surfact ant levels, including total FL, Chol, and disaturated phospholipid (DS P) increased during torpor. The relative proportions of Chol and DSP i ncreased 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 throug hout torpor. Tissue compliance decreased at 1 and 4 h of torpor, but t his decrease was abolished by 8 h. It appears that the surfactant of t he dunnarts counteracted the negative effect of tissue compliance at 1 and 4 h, an effect not present in homeothermic mammals. However, beca use lung compliance was maintained at 1 h of torpor in the absence of a compositional change in surfactant Lipids, the changes in lipid comp osition observed at 4 and 8 h of torpor are thought to relate to funct ions of surfactant other than that of maintaining lung compliance.