Coping With the Cold: Effect of Hibernation on Pulmonary Surfactant in the Thirteen-Lined Ground Squirrel

Abstract

Pulmonary surfactant, a mixture of phospholipids, cholesterol, and proteins, stabilizes the lung by reducing surface tension at the air-water interface of the alveoli. We hypothesized that lowering body temperature to approximately 5oC during hibernation would require compositional changes in surfactant lipids and perhaps proteins in order to maintain lung function. Large aggregate (LA) fractions were obtained by centrifuging lung lavage at 40,000 g for 15 min, with small aggregates (SA) remaining in the supernatant. Because hibernating animals have lower body masses, surfactant levels per animal were compared. Hibernation resulted in an increase in total surfactant due to increased LA with little change in SA. Cholesterol at 8 wt% PL was not altered. Hibernation was accompanied by a small (∼15%) decrease in disaturated phosphatidylcholine and phosphatidylglycerol. Decreases were observed in the mRNAs for the surfactant proteins (SP-). Western analysis revealed levels of all SP-s decreased to approximately 10% (SP-A), 50% (SP-B), 90% (SP-C), and 50% (SP-D) of warm active levels. We speculate that the changes in surfactant LA levels might reflect the much lower breathing rate during hibernation. Furthermore, the compositional changes could arise, in part, from a slowing of metabolism. The decrease in disaturated phospholipid levels could lead to enhanced adsorption due to increased fluidity, but this suggestion will have to be confirmed experimentally.