Inhibition of cyclic flow variations in stenosed canine coronary arteries by thromboxane A2/prostaglandin H2 receptor antagonists.

We tested the hypothesis that thromboxane A2 and thromboxane A2/PGH2 receptor occupation are important in mediating cyclical reductions in coronary blood flow (CFVs) in concentrically narrowed canine coronary arteries. Two potent and selective thromboxane A2/PGH2 receptor antagonists, SQ29,548 and SQ28,668 eliminated CFVs and restored a normal pattern of blood flow through the severely narrowed vessels in 77 and 75% of the dogs, respectively. CFVs were eliminated within several minutes of an intravenous bolus injection of SQ29,548 or SQ28,688. A continuous infusion of SQ29,548 (0.2 mg/kg X min) prevented the recurrence of CFVs throughout the duration of its infusion. Left atrial infusion of the thromboxane A2 mimetic, U46619, restored CFVs in 5 of 8 SQ29,548-treated and in 5 of 7 SQ28,668-treated dogs. Circulating concentrations of the stable metabolites of TxA2 and PGI2, TxB2 and 6-keto-PGF1 alpha, respectively, were unaffected by administration of SQ29,548. However, stenosed vascular segments of the left anterior descending coronary artery (LAD) of SQ29,548-treated dogs produced significantly less thromboxane A2 than comparable segments from untreated dogs. Morphologic studies showed that stenosed coronary arteries in which CFVs had been abolished by either SQ29,548 or SQ28,668 had relatively few adherent platelets, whereas comparable coronary segments removed from untreated animals had relatively large, platelet-rich mural thrombi. SQ29,548 did not alter the synthesis of TxB2 by platelets. 6-keto-PGF1 alpha concentrations in the stenosed LAD and nonstenosed circumflex coronary arteries were not altered by SQ29,548 administration. These data suggest that the thromboxane A2/PGH2 receptor antagonists, SQ29,548 and SQ28,688, inhibit cyclic reductions in coronary blood flow in this model by preventing the accumulation of platelets at the site of a critical coronary arterial stenosis. The data also suggest that TxA2 is important in mediating the interaction between platelets and the constricted coronary artery that is responsible for the development and maintenance of CFVs in this experimental model.