Exposure of fibrinogen-binding sites on stored platelets on stimulation by aggregating agents--an electron microscopic study.

We studied the function of stored platelets by immunoelectron microscopy. The distribution of glycoprotein (GP) IIb/IIIa complex was demonstrated by the binding of a monoclonal anti-GP IIb/IIIa complex antibody (NNKY1-32) to platelets, and the binding of fibrinogen to platelets by use of an anti-fibrinogen antibody. The binding of NNKY1-32 decreased significantly in unstimulated platelets after storage for 5 days. When platelet aggregation was tested, 5-day-stored platelets did not aggregate in the presence of either ADP or epinephrine alone (40 microM each) or in combination (8 microM each), but did aggregate on stimulation with the combination of collagen and epinephrine (1 micrograms/ml and 1 microM, respectively). The 5-day-stored platelets stimulated with the combination of ADP and epinephrine scarcely bound fibrinogen, but did bind NNKY1-32. On stimulation with collagen and epinephrine, a large amount of fibrinogen bound to the surface membrane of 3- and 5-day-stored platelets, and the binding of NNKY1-32 also increased, reaching the same level seen in stored platelets stimulated with ADP and epinephrine. Our analysis of stored platelets revealed that the binding of NNKY1-32 decreased after storage for 5 days, but the binding increased even with subaggregating stimulation. The binding of fibrinogen correlated well with the rate of maximal platelet aggregation.