The influence of water on the phase transition of sheep lung surfactant. A possible mechanism for surfactant phase transitions in vivo.

The effect of water on the thermal properties of sheep lung surfactant lipids was determined by differential scanning calorimetry. Dry surfactant exhibited a phase transition with an upper limit of about 54 degrees C, whereas that of the fully hydrated surfactant was about 30 degrees C. The effect of water was confined to a range of hydration values from 0 to 25%. The results indicate that pulmonary surfactant lipids are capable of undergoing both thermotropic and lyotropic mesomorphism in vitro. The degree of hydration of the surfactant could influence its in vivo biophysical role in alveolar dynamics. Indeed, small changes in the surfactant to water ratio induced by regional differences in the surfactant concentration at the alveolar surface during alveolar expansion and contraction could be sufficient to trigger isothermal phase transitions in the surfactant lipids. This would allow changes to occur in the equilibrium between solidus and fluidus surfactant during the respiratory cycle.