Heparin-induced platelet activation: The role of thromboxane A2 synthesis and the extent of platelet granule release in two patients

Abstract

Heparin-induced thrombosis is due to an immune-mediated activation of circulating platelets and has significant clinical implications for patients with vascular disease. The purpose of this article was (1) to define the biochemical mechanisms of heparin-induced platelet activation (HIPA) and (2) to determine the relationship between thromboxane A2 (TxA2) synthesis and platelet granule release. In two patients with confirmed HIPA, heparin (3 U/ml) induced extensive platelet aggregation (61.5%), release of 14C-serotonin (81.5% of releasable 14C-serotonin, a dense granule marker) and platelet factor 4 (63.7% of releasable platelet factor 4, an alpha granule marker) and generation of TxB2, a stable metabolite of TxA2 (100% relative to serum control). In one patient heparin did not induce release of n-acetyl-beta-glucosaminadase (N-AC, a lysosomal granule marker), and aspirin (4 mmol/L), which abolished TxA2 synthesis, prevented aggregation and granule release. In the second patient heparin did induce release of N-AC (39.7% of releasable N-AC) and aspirin, despite abolishing TxA2 synthesis, did not prevent aggregation or granule release. In contrast, by elevating intracellular cyclic adenosine monophosphate, iloprost (0.01 mumol/L), a stable prostacyclin analogue, prevented heparin-induced aggregation, granule release, and TxB2 generation in both patients. Thus we show (1) HIPA can proceed independently of TxA2 synthesis; (2) heparin in certain patients can release lysosomal hydrolases, thus mimicking strong platelet agonists such as thrombin; and (3) iloprost but not aspirin prevents HIPA regardless of the biochemical pathways involved.

In Vitro Efficacy of Platelet Glycoprotein IIb/IIIa Antagonist in Blocking Platelet Function in Plasma of Patients with Heparin-induced Thrombocytopenia

“…In this study, heparin resulted in P-selectin expression, dense granule release and microparticle formation from donor platelets in the presence of plasma from patients with HIT. These results confirm previous studies in which platelet granule release by HIT-associated immunoglobulin and heparin and platelet-derived microparticle formation have been observed (9,22,23). In this study, heparin resulted in a slight increase in aggregation and P-selectin expression in control samples, confirming the direct platelet-activating effects of heparin (22).…”

The Pathogenesis of the Antiphospholipid Syndrome: a Hypothesis Based on Parallelisms with Heparin-induced Thrombocytopenia

“…These authors showed that serum and purified IgG from two patients with HIT induced aggregation of normal plate lets in the presence of therapeutic concentrations of heparin. That hepa rin-dependent antibodies are potent platelet activators was confirmed by several authors and it was shown that these antibodies also cause thromboxane A2 generation and platelet granule release (35)(36)(37)(38). This last property formed the basis for the development of a sensitive and specific method for the laboratory diagnosis of heparin-dependent anti bodies (39).…”

The Clinical Significance and Risk Factors of Anti-Platelet Factor 4/heparin Antibody on Maintenance Hemodialysis Patients: A Two-Year Prospective Follow-up

“…Based on the fundamental in vitro and in vivo experiments, antiplatelet drugs might prevent the occurrence of HIT [38], [39]; by contrast, several clinical trials have confirmed that antiplatelet drugs do not effectively suppress the occurrence of HIT [40], and antiplatelet drugs do not reduce the occurrence and degree of thrombocytopenia [41]. However, these studies were limited to case reports or research using a small sample size.…”