Antagonism of membrane compression effects by high pressure gas mixtures in a phospholipid bilayer system

Binary mixtures of helium with nitrogen, xenon or nitrous oxide were applied to suspensions of phosphatidylcholine-cholesterol vesicles to determine those mixtures of lipid soluble gases which would exactly antagonize the membrane rigidifying effect of 100 ATA compression. A previous study has shown that the initial application of 100 ATA compression by gas produces a significant reduction in the fluidity of the phospholipid bilayer. However, as the high pressure gas dissolves into the lipid region it creates disorder and increases fluidity. Fluidity of the bilayer at equilibrium represents the sum of the compression-ordering and dissolved-gas disordering effects and is dependent on the gas/lipid partition coefficient of the particular gas. The beneficial effect of a narcotic gas added to Trimix mixtures to ameliorate HPNS in deep divers may be due to a balance of compression-ordering and solubility-disordering effects achieved within the nerve membrane. It is therefore valuable to determine those gas mixtures which achieve balance of these two effects and result in zero net change in phospholipid bilayer fluidity at an established pressure of 100 ATA. Binary mixtures of helium with 88% nitrogen, 3.8% xenon or 2.8% nitrous oxide resulted in zero net change in bilayer fluidity with our model system at 100 ATA. A graph of the percent of narcotic gas needed to produce zero net effect as a function of pressure, however, was nonlinear. This would suggest the ratio of gases in Trimix must be varied as a function of pressure. While the phosphatidylcholine-cholesterol bilayer is a good model for certain components of the nerve membrane, it does not allow for study of protein-lipid or gas-protein interactions. The data presented thus aid in our understanding of HPNS but are yet incomplete for precise use in predicting diving mixtures.