Role of Platelet Microparticles in the Production of Thromboxane by Rabbit Pulmonary Artery

Abstract

This study examined the role of platelet microparticles in thromboxane A₂ (TXA₂) production. Incubation of microparticles with [¹⁴C]arachidonic acid and A23187 produced ¹⁴C-labeled TXB₂, the stable metabolite of TXA₂. To investigate the possibility that endothelial cells (ECs) transfer arachidonic acid to platelet microparticles and promote TXB₂ synthesis, ECs with their cellular lipids prelabeled with tritiated arachidonic acid were incubated with microparticles. In the absence of microparticles, there was no production of tritiated TXB₂ by the ECs. However, when microparticles were coincubated with prelabeled ECs, tritiated arachidonic acid was metabolized to tritiated TXB₂. Aspirin was then used to inhibit cyclooxygenase. ECs coincubated with aspirin-treated platelet microparticles did not produce TXB₂, as measured by radioimmunoassay. In contrast, aspirin-treated ECs coincubated with microparticles produced TXB₂, and its production was enhanced by methacholine (10⁻⁴ mol/L), indicating that endothelially derived arachidonic acid, and not endothelially derived prostaglandin endoperoxide, was transferred to the microparticle and further metabolized to TXA₂. Additional studies with rabbit aorta and pulmonary artery investigated whether microparticles contributed to vascular contractions. Preincubation with microparticles enhanced arachidonic acid–induced contractions in the aorta and methacholine-induced contractions in the pulmonary artery. The thromboxane receptor antagonist SQ29548 and the thromboxane synthase inhibitor dazoxiben blocked these effects. Because TXA₂ is an important mediator in various pathophysiologic states, including hypertension, the ability of platelet microparticles to act as a cellular source of TXA₂ might provide new insight into the role of platelets and platelet microparticles in the control of vascular tone.