PHOSPHORYLATION OF THE OUTER SURFACE OF PLATELETS ENHANCES THE EFFECTS OF COLLAGEN ON PLATELET AGGREGATION, ATP RELEASE, CALCIUM TRANSLOCATION AND PHOSPHOINOSITIDE HYDROLYSIS

We have recently isolated and purified from human plasma two isomeric forms of protein kinase, both of which can phosphorylate the outer surface proteins of human platelets. One of the proteins phosphorylated is the platelet collagen receptor. The phosphorylation of the outer surface proteins of human platelets increased their functional responsiveness to collagen. Collagen-stimulated platelet aggregation, release of ATP and calcium translocation were all enhanced by pretreatment with plasma protein kinase in the presence of ATP. The mechanism by which phosphorylated platelets become hypersensitive to collagen is not established. In the present study, we have used [³H]myo-inositol-labeled human platelets to investigate the possible role of phosphoinositide metabolism in mediating this hypersensitivity. Formation of inositol mono-, bis-, and trisphosphate in response to collagen was more pronounced in phosphorylated platelets than controls. these results indicate that enhanced phosphoinositide hydrolysis in phosphorylated platelets correlate with the increased functional responses to collagen.