Role of the thromboxane A(2) receptor in the vasoactive response to ischemia-reperfusion injury

Neutrophil-endothelial adhesion in venules and progressive vasoconstriction in arterioles seem to be important microcirculatory events contributing to the low flow state associated with ischemia-reperfusion injury of skeletal muscle. Although the neutrophil CD-18 adherence function has been shown to be a prerequisite to the vasoconstrictive response, the vasoactive substan ces involved remain unknown. The purpose of this study was to evaluate the role of thromboxane A(2) receptor in the arteriole vasoactive response to i schemia-reperfusion injury. An in vivo microscopy preparation of transillum inated gracilis muscle in male Wistar rats (175 +/- 9 g) (n = 12) was used for this experiment. Three experimental groups were evaluated in this study : (1) sham, flap raised, no ischemia (20 venules, 20 arterioles), (2) 4 hou rs of global ischemia only (19 venules, 22 arterioles), and (3) 4 hours of global ischemia + thromboxane A(2) receptor antagonist (ONO-3708) (17 venul es, 20 arterioles). ONO-3708 (5 mg/kg), a specific competitive antagonist o f thromboxane A(2) receptor, was infused at a rate of 0.04 ml/minute into t he contralateral femoral rein 30 minutes before reperfusion. Mean arterial blood pressure was not changed at this dose of ONO-3708 (88 +/- 6 mmHg befo re infusion, 81 +/- 4 mmHg after infusion, n = 3). The number of leukocytes rolling and adherent to endothelium (15-sec observation) were counted in 1 00-mu m venular segments, and arteriole diameters were measured at 5, 15, 3 0, 60, and 120 minutes of reperfusion. Leukocyte counts and arteriole diame ters were analyzed with two-way factorial analysis of variance for repeated measures and Duncan's post hoc mean comparison. Statistical significance w as indicated by a p less than or equal to 0.05. The ischemia-reperfusion-in duced vasoconstriction was significantly reduced by the thromboxane A(2) re ceptor antagonist (ONO-3708). The mean arteriole diameters at 30, 60, and 1 20 minutes reperfusion were significantly greater in the treated animals th an in the ischemia-reperfusion controls. Despite a significant increase in treated mean arteriole diameters, 30 percent of arterioles still demonstrat ed vasoconstriction. Neutrophil-endothelial adherence was not reduced by ON O-3708. Thromboxane A(2) receptor blockade significantly reduces but does n ot eliminate ischemia-reperfusion-induced vasoconstriction in this model. T his finding suggests that additional and perhaps more important vasoactive mediators contribute to vasoconstriction. Furthermore, thromboxane A(2) rec eptor blockade has no effect on polymorphonuclear endothelial adherence.