Increased number of thromboxane A2-prostaglandin H2 platelet receptors in active unstable angina and causative role of enhanced thrombin formation.

The current study was designed to investigate the number and affinity of plateletthromboxane A2/prostaglandinH2 () receptors in patients with unstableangina and, if any, the role played by the increasedthrombinformation that is a common finding in these patients. Measurements taken during activeunstableangina but not those taken during inactive angina showed an increasednumber (p < 0.001), without changes in affinity, of plateletreceptors, evaluated as the binding capacity of iodine 125PTAOH, a stable TxA2 analogue. Moreover patients with activeangina had higher plasma concentrations of fibrinopeptide A (FPA) (p < 0.0001), which were significantly related to the number of plateletreceptors (r = 0.76; p < 0.01) Heparin infusion but not aspirin treatment promptly normalized the number of receptors and significantly reduced plasma FPA concentrations. In an in-vitro study thrombin in a concentration similar to that found in vivo significantly increased the number of plateletreceptors (p < 0.01), whereas heparin did not affect receptors. These results have important therapeutic implications and indicate the preferential use of heparin rather than aspirin during the acute phase of unstableangina.