Morphine-potentiated platelet aggregation in in vitro and platelet plug formation in in vivo experiments.

The detailed mechanisms underlying morphine-signaling pathways in platelets remain obscure. Therefore, we systematically examined the influence of morphine on washed human platelets. In this study, washed human platelet suspensions were used for in vitro studies. Furthermore, platelet thrombus formation induced by irradiation of mesenteric venules with filtered light in mice pretreated with fluorescein sodium was used for an in vivo thrombotic study. Morphine concentration dependently (0.6, 1, and 5 microM) potentiated platelet aggregation and the ATP release reaction stimulated by agonists (i.e., collagen and U46619) in washed human platelets. Yohimbine (0.1 microM), a specific alpha(2)-adrenoceptor antagonist, markedly abolished the potentiation of morphine in platelet aggregation stimulated by agonists. Morphine also potentiated phosphoinositide breakdown and intracellular Ca(2+) mobilization in human platelets stimulated by collagen (1 microg/ml). Moreover, morphine (0.6-5 microM) markedly inhibited prostaglandin E(1) (10 microM)-induced cyclic AMP formation in human platelets, while yohimbine (0.1 microM) significantly reversed the inhibition of cyclic AMP by morphine (0.6 and 1 microM) in this study. The thrombin-evoked increase in pH(i) was markedly potentiated in the presence of morphine (1 and 5 microM). Morphine (2 and 5 mg/g) significantly shortened the time require to induce platelet plug formation in mesenteric venules. We concluded that morphine may exert its potentiation in platelet aggregation by binding to alpha(2)-adrenoceptors in human platelets, with a resulting inhibition of adenylate cyclase, thereby reducing intracellular cyclic AMP formation followed by increased activation of phospholipase C and the Na(+)/H(+) exchanger. This leads to increased intracellular Ca(2+) mobilization, and finally potentiation of platelet aggregation and of the ATP release reaction.

Morphine-potentiated agonist-induced platelet aggregation through alpha2-adrenoceptors in human platelets.

Morphine dose-dependently (0.6, 1, and 5 microM) potentiated platelet aggregation and ATP release stimulated by agonists (i.e., collagen and U46619) in washed human platelets. Furthermore, morphine (1 and 5 microM) markedly potentiated collagen (1 microg/ml) evoked an increase of intracellular Ca2+ mobilization in fura 2-AM loading human platelets. Morphine (1 and 5 microM) did not influence the binding of fluorescein isothiocyanate-triflavin to platelet glycoprotein IIb/IIIa complex. Yohimbine (0.1 microM), a specific alpha2-adrenoceptor antagonist, markedly abolished the potentiation of morphine in platelet aggregation stimulated by collagen. Moreover, morphine (0.6-5 microM) markedly inhibited prostaglandin E1 (10 microM)-induced cAMP formation in human platelets, and yohimbine (0.1 microM) significantly reversed the inhibition of cAMP by morphine (0.6 and 1 microM) in this study. Morphine (1 and 5 microM) significantly potentiated thromboxane B2 formation stimulated by collagen in human platelets, and yohimbine also reversed this effect of morphine in this study. In addition, morphine (1 and 5 microM) did not significantly affect nitrate production in human platelets. Morphine may exert its potentiation in platelet aggregation by binding to alpha2-adrenoceptors in human platelets, which leads to reduced cAMP formation and subsequently to increased intracellular Ca2+ mobilization; this, in turn, is followed by increased thromboxane A formation and finally potentiates platelet aggregation and ATP release.