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P eroxynitrite (ONOO-) anion, formed by the interaction of superoxide with nitric oxide (NO), has previously been implicated as a cytotoxic agent. However, the effects of this free radical species on neutrophil (PMN)-endothelial cell interactions is largely unknown. We investigated the direct actions of ONOO- on PMN adhesion to endothelial cells in vitro and in vivo, as well as the effects of ONOO- on PMN-mediated myocardial ischemia-reperfusion injury. In vitro, peroxynitrite (100-1,000 nM) inhibited the adhesion of rat PMNs to the endothelium of isolated thrombin- or H2O2-stimulated rat mesenteric artery (P < 0.01 vs. thrombin or H2O2 alone). In vivo, in the rat mesentery, thrombin (0.5 U/ml) or N(G)-nitro-L-arginine-methyl ester (50 microM) significantly increased venular leukocyte rolling and adherence, which were also significantly (P < 0.01) attenuated by ONOO (800 nM) accompanied by reduced P-selectin expression on the endothelial cell surface. Isolated perfused rat hearts were subjected to global ischemia and reperfusion with rat PMNs (10(8) cells), which resulted in profound cardiac depression (i.e., a marked reduction in left ventricular developed pressure and maximal rate of development of left ventricular pressure). Infusion of ONOO- reversed the myocardial contractile dysfunction of ischemic-reperfused rat hearts to near baseline levels, and markedly attenuated the accumulation of PMNs in the postischemic heart. The present study provides strong evidence that nanomolar concentrations of ONOO- both inhibit leukocyte-endothelial cell interactions and exert cytoprotective effects in myocardial ischemia-reperfusion injury. Furthermore, our results suggest that the inhibition of P-selectin expression by peroxynitrite is a key mechanism of the modulatory actions of ONOO- on leukocyte-endothelial cell interactions.