The effect of pressure on the lipid microviscosity and phase transition of lung surfactant

The effect of pressure on the lipid dynamics of the rat lung surfactant was studied in liposomes made of the natural lung surfactant of the rat and of model phospholipid mixture. The determined parameter was the lipid microviscosity, monitored by the fluorescence polarization of the probe 1,6-diphenyl-1,3,5-hexatriene. Osmotic pressure of up to 47 atm, as well as hydrostatic pressure of up to 1.4 kbar, were applied at a constant temperature. The effect of pressure was monitored by the change in the lipid microviscosity of the system. The maximal change achieved with osmotic pressure at a constant temperature was only 30%. This suggests that the conversion of melted lipid to its solid phase above the lipid critical temperature requires several hundred atmospheres. Similarly, measurements of lipid microviscosity under increased hydrostatic pressure revealed transitions which occurred at above 400 atm. Since such pressures are far beyond the physiological scale, it excludes the possibility that pressure alone can be responsible for a full phase transition of the lung surfactant during respiration. Upon decompression, microviscosity of the examined lipid system was found to return to its original values, confirming the reversibility of the process.