Lateral Heterogeneity in Human Platelet Plasma Membrane and Lipids from the Time-resolved Fluorescence of Trans-parinaric Acid

We have investigated the complex behaviour of the time resolved fluorescence intensity and anisotropy of trans-parinaric acid, incorporated into fragments of the plasma membrane of human platelets and in multibilayers of lipids extracted from that membrane. It is shown that the observation of anisotropies that increase at long times can be satisfactorily interpreted by assuming two populations of the fluorescence probe with distinct life-times, rotational relaxation times and order parameters. The heterogeneous probe distribution was correlated with a similar heterogeneity in the lipid composition of the bilayer, modulated by temperature. Below 35 degrees C an important fraction of the lipids of the plasma membrane are apparently in the form of solid-like domains (20% at 20 degrees C). However, in the physiological temperature range that solid/fluid heterogeneity is almost negligible. Since these effects were also observed in multibilayers of lipids from the platelet membrane, the formation of solid-like clusters appears to arise from lipid-lipid interactions only, and most probably involving cholesterol. These results support the previous finding of a lateral phase separation for temperatures less than 37 degrees C described by Gordon et al. (1983) in a spin-probe study of the platelet plasma membrane.