Adaptations to hibernation in lung surfactant composition of 13-lined ground squirrels influence surfactant lipid phase segregation properties

Abstract

Pulmonary surfactant lines the entire alveolar surface, serving primarily to reduce the surface tension at the air–liquid interface. Surfactant films adsorb as a monolayer interspersed with multilayers with surfactant lipids segregating into different phases or domains. Temperature variation, which influences lipid physical properties, affects both the lipid phase segregation and the surface activity of surfactants. In hibernating animals, such as 13-lined ground squirrels, which vary their body temperature, surfactant must be functional over a wide range of temperatures. We hypothesised that surfactant from the 13-lined ground squirrel, Ictidomys tridecemlineatus, would undergo appropriate lipid structural re-arrangements at air–water interfaces to generate phase separation, sufficient to attain the low surface tensions required to remain stable at both low and high body temperatures. Here, we examined pressure–area isotherms at 10, 25 and 37°C and found that surfactant films from both hibernating and summer-active squirrels reached their highest surface pressure on the Wilhelmy–Langmuir balance at 10°C. Epifluorescence microscopy demonstrated that films of hibernating squirrel surfactant display different lipid micro-domain organisation characteristics than surfactant from summer-active squirrels. These differences were also reflected at the nanoscale as determined by atomic force microscopy. Such re-arrangement of lipid domains in the relatively more fluid surfactant films of hibernating squirrels may contribute to overcoming collapse pressures and support low surface tension during the normal breathing cycle at low body temperatures.