The effect of physiologically relevant pressures on the phase transition of liposomes containing dipalmitoyl phosphatidylcholine

The increase in melting temperature (t_m) of lipid phase transition upon application of pressure (P) obeys the empirical dependence dt_m dP ∼ 0.02°C/atm. This temperature-pressure equivalence of lipid phase transition predicts a significant increase in lipid microviscosity when pressure of only several atmospheres is imposed on lipid bilayers around their melting point. In this study we have examined this possibility with sonicated liposomes made of dipalmitoyl phosphatidylcholine (DPPC) and of mixtures containing either DPPC, egg phosphatidylcholine (PC), and egg phosphatidylglycerol (PG) (54:36:10 mole%) or DPPC and PG (90:10 mole%). The mixtures served as models for the lung surfactant. The lipid microviscosity, η, as function of temperature or pressure, was monitored by steady-state fluorescence depolarization of the fluidity probe 1,6-diphenyl-1,3,5-hexatriene (DPH). Hydrostatic pressure of up to 10 atm, or osmotic pressure of up to 24 atm, at temperatures around the lipid phase transition of DPPC (∼40°C), induced significant increase in η which approximately obeyed the predicted dependence dη dP ∼ 7 cP/atm.