Elevated platelet-derived microparticle levels during unstable angina.

OBJECTIVE: To use fluorescent-activated flow cytometry coupled with activation-dependent and -independent platelet-specific monoclonal antibodies in a pilot study to assess the degree and time course of platelet activation events in patients presenting within 24 h of onset of Canadian Cardiovascular Society class 4 angina.
BACKGROUND: Although activated platelets play a key role in the pathogenesis of unstable angina, the development of simple assays to quantify platelet activation events directly is lacking.
METHODS: Blood samples were drawn from six unstable angina patients from the coronary care every 4 h over a 24 h period into a fixative and analyzed the following day. All patients were on acetylsalicylic acid and heparin. Comparisons were made with six healthy, medication-free volunteers. Platelets were defined by flow cytometry as positive for fluorochrome-labelled monoclonal antibody to glycoprotein Ib (AP1) and within the single intact platelet window defined by scatter characteristics. The presence of the fluorochrome-labelled activation-specific monoclonal antibody (KC4.1 for anti-P-selectin, PAC-1 for activated glycoprotein IIb/IIIa) was used to determine the percentage of activated platelets. Platelet activation-dependent microparticles were identified by gating on AP1-positive events and defining microparticles (percentage of total platelet events) as being smaller (forward size scatter) than single intact platelets.
RESULTS: There was a marked, sixfold increase in microparticle generation (17 +/- 7% versus 2.8 +/- 1.4%) in the unstable angina patients (P = 0.001) compared with healthy volunteers. Further assessment of six coronary care unit patients with nonischemic cardiac disorders demonstrated a highly variable intermediate level of microparticle generation (11 +/- 7%). No differences in activated glycoprotein IIb/IIIa expression were noted for the various groups and P-selectin expression was lower in the unstable angina patients (6 +/- 2% versus 12 +/- 3%, (P = 0.007).
CONCLUSIONS: This pilot study suggests that measuring circulating platelet microparticle levels is a simple yet useful parameter for the assessment of platelet activation in unstable angina compared with activation markers on intact whole platelets. Despite antiplatelet and antithrombin therapy, significant platelet activation occurred in these patients over the initial 24 h. Flow cytometry may be a useful tool in assessing the efficacy of newer therapeutic modalities.