Cholesterol effect on the physical state of lipid multibilayers from the platelet plasma membrane by time-resolved fluorescence.

There are indications that the plasma membrane lipid composition and, in particular, the cholesterol/phospholipid (C/PL) ratio, affects platelet function. As a first approximation to the molecular characterization of the effect of cholesterol on the order, fluidity and lateral heterogeneity of the platelet plasma membrane, the steady-state and time-resolved fluorescence of 1,6-diphenyl-1,3,5-hexatriene (DPH) and trans-parinaric acid (tPnA) has been studied in multibilayer vesicles of phospholipids extracted from human platelet plasma membrane with different cholesterol/phospholipid molar ratios modified in vitro from 0.07 to 0.9. The DPH studies show that the increased presence of cholesterol has a stronger effect on the order than on the fluidity of the bilayer, as has been previously observed in other lipid membranes. On the other hand, from the analysis of the fluorescence kinetics of tPnA we conclude that a higher cholesterol content gives rise to an increase of the heterogeneity of the bilayer, due to a larger fraction of solid-like lipid domains. These domains contain a cholesterol concentration much higher than the macroscopic average value.