An experimental protocol of adenosine diphosphate-(ADP) induced platelet aggregation in the mouse was designed in order to study the roles played by endothelin-A (ETA) and endothelin-B (ETB) receptors in the ET-1-induced inhibition of ex vivo platelet aggregation. The pressor effects of ET-1 or IRL-1620 were firstly determined in vivo in anesthetized (ketamine/xylazine) CD-1 mice (males and females; 25-30 g). All agents were administered intravenously (via the jugular vein) and blood samples were collected from the carotid artery into heparinized Eppendorfs (15 U/ml). To obtain platelet-rich plasma (PRP) the blood was immediately centrifuged for 12 min at low speed (1100g). Plateletpoor plasma (PPP) was then prepared by centrifugation of the whole blood sample at high speed (3700 g) for 30 min. PPP was used to calibrate the aggregometer at 100% transmission. Platelet aggregation was monitored ex vivo as a change in light transmission through PRP following the injection of ADP (5 μM). ET-1 (0.01-2.0 nmol/kg) induced a significant and dose-dependent inhibition of platelet aggregation ex vivo (12-84%). The selective ETB agonist, IRL-1620 (0.05-2.0 nmol/kg), also triggered a marked inhibition of platelet aggregation. Indomethacin (10 mg/kg), a nonselective cyclooxygenase inhibitor, abolished the inhibitory effect of ET-1. The selective ETA antagonist, BQ-123 (1 nmol/kg), abolished the in vivo pressor effect of exogenous ET-1, without affecting its anti-aggregatory activity. The selective ETB antagonist, BQ-788 (0.5 nmol/kg), did not modify the elevation of blood pressure produced by the ET-1; however, it did abrogate dose-dependently the inhibitory effect of ET-1 on platelet aggregation. These results suggest that the anti-aggregatory effect of ET-1, in anesthetized CD-1 mice, relies mainly upon the activation of ETB receptors. The mechanism whereby ET-1 exerts this effect, is partially indirect and requires at least the production and the release of prostanoids (possibly PGI2) into the blood stream.
*Institute of Pharmacology, †Department of Anatomy and Cell Biology, Medical School, University of Sherbrooke, Sherbrooke, Québec, Canada
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