Microenvironment changes of human blood platelet membranes associated with fibrinogen binding

Alterations in the membrane organization caused by fibrinogen binding to human blood platelets and their isolated membranes were analyzed by fluorescence and electron spin resonance measurements. The degree of fluorescent anisotropy of DPH, ANS and fluorescamine increased significantly when fibrinogen reacted with its membrane receptors. Both fluorescence and ESR analyses showed that fibrinogen binding to platelet membranes is accompanied by an increase of the membrane lipid rigidity. This effect seems to be indirect in nature and is mediated by altered membrane protein interactions. As it has been shown that an increased membrane lipid rigidity leads to a greater exposure of membrane proteins, including fibrinogen receptors, this might facilitate a formation of molecular linkages between neighboring platelets. On the other hand, changes of fluorescence anisotropy of membrane tryptophans and N-(3-pyrene) maleimide suggest the augmented mobility of the membrane proteins. Evidence is presented which indicated that the binding of fibrinogen to the membrane receptors is not accompanied by any changes in the fluorescence intensity of ANS attached to the membranes. It may suggest that the covering of platelets with fibrinogen does not influence the surface membrane charge. In contrast to fibrinogen, calcium ions caused an increase of the fluorescence intensity resulting from the more efficient binding of ANS to the platelet membranes.