Collagen But Not Fibrinogen Surfaces Induce Bleb Formation, Exposure of Phosphatidylserine, and Procoagulant Activity of Adherent Platelets: Evidence for Regulation by Protein Tyrosine Kinase-Dependent Ca Responses

With a combined phase-contrast and fluorescence video implatelets to convert prothrombin into thrombin. Thrombin aging system, changes in morphology and cytosolic [Ca]i addition accelerated the onset of the Ca signals and the were investigated of fura-2–loaded platelets during adheappearance of surface-exposed PS. Collagen-induced PS exsion to fibrinogen or collagen matrices. The Ca signals posure was slightly reduced by treatment of the platelets were, on the level of single platelets, compared to the secrewith aspirin, and strongly inhibited by suppression of the tion and procoagulant responses, using fluorescent-labeled Ca responses with prostaglandin E1 or the Ca chelator, AK-6 antibody against P-selectin and labeled annexin V for dimethyl-BAPTA. Inhibition of protein tyrosine phosphoryladetection of surface-exposed phosphatidylserine (PS), retion with genistein, U73343, or wortmannin resulted in spikspectively. Platelets in contact with fibrinogen developed ing Ca responses in many of the platelets and in almost filapods and spread over the matrix, in most of the cells complete reduction of bleb formation and PS exposure. In without detectable Ca signal. Thrombin induced repetitive contrast, genistein did not suppress bleb formation and PS spiking in [Ca]i , followed by the expression of P-selectin exposure of platelets stimulated with the Ca ionophore but not of PS on the platelet surface. Platelet interaction A23187. We conclude that a collagen but not fibrinogen mawith collagen resulted in spreading and transformation of trix acts as a potent activator of the procoagulant response the cells into blebbing, ‘‘balloon’’-like structures (diameter through activation of tyrosine kinases and subsequent genabout 5 mm). The latter morphological changes were accomeration of sustained intracellular Ca signals. panied by high and prolonged increases in [Ca]i , by the q 1997 by The American Society of Hematology. exposure of both P-selectin and PS, and by the ability of the