Phospholipase A2 activity in platelets of patients with primary thrombocythemia

Abstract

Patients with primary thrombocythemia (PT) have both, bleeding and thrombotic events. Although platelet aggregation tests are usually abnormal, synthesis of thromboxane B2 (TxB2) by platelets is increased. This feature could be the consequence of an increased phospholipase activity or a facilitated metabolism of arachidonate by prostaglandin synthetase pathway. We studied the activity of phospholipase A2 as well the arachidonate metabolism in platelets of patients suffering from PT. Eleven patients and 11 controls were included. Platelets were labelled with [14C]arachidonic acid ([14C]AA). Lost of radioactivity from phospholipids and new radioactive prostanoids were evaluated in calcium ionophore A23187 activated platelets, to explore phospholipase A2 activity. This assay was also carried out in aspirin-incubated platelets. We also studied the formation of prostanoids in platelets activated by radioactive free arachidonic acid. Platelet aggregation studies of patients were abnormal. [14C]AA incorporation in platelet phospholipids was normal. Ionophore activated platelets from patients and controls lost 26.1 ± 8.3% and 24.1 ± 10.5% of radioactivity, respectively, mainly from phosphatidylcholine. The main arachidonate metabolite was 12-L-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE), which comprised 14.1 ± 5.1% of the radioactivity released from phospholipids in patients, and a similar amount in the controls (14.4 ± 7.5%). Formation of TxB2 was also similar in patients (5.5 ± 1.2%) and controls (4.9 ± 2.9%). Formation of 12-L-hydroxy-5,8,10-heptadecatrienoic acid (HHT) was also normal. Ionophore A23187 activation of aspirinized platelets of patients released 19.5 ± 7.4% of radioactivity from phospholipids, which was completely metabolized to HETE. Formation of prostanoids HETE, HHT and TxB2 by arachidonic acid activated platelets of patients was normal. Phospholipase A2 activity as well both cyclooxygenase and lipoxygenase activities in platelets of patients with PT were found to be normal.