Flow cytometric analysis of the platelet surface area and surface density of glycoprotein IIb-IIIa of unactivated human platelets of various sizes.

Large (L) activated platelets exhibit greater aggregability and express more activated glycoprotein IIb-IIIa (GPIIb-IIIa) per cell and per unit surface area than small (S) activated platelets. We studied the binding of CD61 monoclonal antibody to GPIIb-IIIa on resting platelets and developed a new method for determining platelet surface area by flow cytometry. Using this method, we found that resting L platelets contain two times more GPIIb-IIIa per cell than S platelets but the same amount of GPIIb-IIIa per unit surface area. The data suggest that the greater aggregability of L platelets is likely to be due to increased activation and/ or expression of GPIIb-IIIa rather than to elevated density of unactivated GPIIb-IIIa on resting L platelets.