A solid state transition in the tetragonal lipid bilayer structure at the lung alveolar surface

According to our recent results the alveolar surface is formed by a coherent phase, not a monolayer as has been assumed earlier. This surface phase is a tetragonal organization of the lipid bilayer which seems to follow the CLP minimal surface structure. As lipid bilayers at cooling will undergo a transition from the liquid-crystalline type of structure into a solid state structure, we have followed the changes in the X-ray scattering curves versus temperature of a sample of lung lavage from rabbit. There are significant changes in the range 15-25 degreesC indicating a solid/liquid bilayer transition. The size of the X-ray scattering changes indicate that only parts of the bilayers are involved. As indicated by similar studies of lung surfactant extracts, there are cholesterol-rich regions that remain in the liquid-like disordered conformation at cooling through this transition. The bilayer-embedded proteins in the alveolar CLP; structure are proposed to be located in the "corners" of the CLP-structure. This surface-phase structure was also examined by conventional electron microscopy with fixation at room temperature, and it was found to exhibit quite planar bilayer regions. On the other side, the same-sample seen in cryo-transmission electron microscopy vitrified from 40 degreesC was more disordered. These observations are consistent with a partial solid state transition of the bilayer on cooling, with segregation of lipids within the bilayers into cholesterol-rich regions remaining in a liquid-like disordered state and cholesterol-poor regions crystallizing in the range 25-15 degreesC. Physiological consequences-of such a partial solidification are finally discussed.