Ultrasound-induced transport across lipid bilayers: Influence of phase behavior

This paper examines the role of lipid membrane phase behavior on the kinetics and mechanism of ultrasound-induced transport. We present a quantitative study of low frequency ultrasound (LFUS)-induced release of calcein from the aqueous core of large unilamellar vesicles (LUVs) comprising a wide range of lipid compositions. The LUVs comprise of the lipids 1,2-dioleoyl-phosphocholine (DOPC), 1,2-dipalmitoyl-phosphocholine (DPPC), and cholesterol, as these species constitute a proven model membrane system for which the phase behavior is well described. Samples from different regions in the composition space were exposed to 20 kHz, continuous wave ultrasound and steady-state fluorescence spectroscopy was used to quantify leakage. Results are presented in the form of a "release map"; that is, leakage results are superimposed onto a phase diagram. Additionally, release kinetics are fit with simple mathematical models that account for diffusion and bilayer destruction. As membrane phase changes toward liquid-ordered, the membrane becomes increasingly resistant to destruction such that the rate of diffusion decreases. Two mechanisms of release are evident in l(d) samples, diffusion and destruction. l(o) samples on the other hand, do not exhibit vesicle destruction and fit well to diffusion-only model. The phase of the membrane, rather than the cholesterol mole fraction per se, has a stronger influence on membrane permeability and destruction potential. Specifically the variation in permeability among different phases, but whose cholesterol mole fraction is identical, is roughly twice the variation in permeability observed as cholesterol mole fraction is varied within the a given phase. The difference in membrane thickness between l(d) and l(o) phases does not account for the observed difference in permeability; thus influence of phase behavior is not trivial. The correlation of permeability with phase behavior might prove useful in designing and developing therapies based on ultrasound and membrane interactions. (C) 2011 Elsevier B.V. All rights reserved.