Analysis of vasodilator responses to peroxynitrite in the hindlimb vascular bed of the cat

The free radical peroxynitrite (ONOO-) is formed in biological systems from the reaction of nitric oxide (NO) with superoxide (O-2(-)) and can react with protein and nonprotein thiol groups to produce tissue injury. However, these pathologic actions of ONOO- may have been overemphasized, in that ONOO- has vasorelaxant properties through activation of soluble guanylate cyclase; inhibits leukocyte-endothelial cell interactions; and reduces ischemia-reperfusion injury in the heart, lung, and liver. It has been reported that tolerance develops to the vasodilator actions of ONOO- and that ONOO- impairs vascular function. However, little, if anything, is known about responses to ONOO- in the hindlimb circulation of the cat. To better understand the effects of ONOO- on responses to vasoactive agonists and the mechanism by which ONOO- induces vasodilation, the effects of short-term exposure to ONOO- were investigated under constant-flow conditions in the hindlimb vascular bed of the cat. In these studies, direct intraarterial injections of ONOO- produced dose-dependent decreases in hindquarters perfusion pressure. The vasodilator responses to ONOO- were rapid in onset, were short in duration, and could be repeated without exhibiting tachyphylaxis. Vasodilator responses to ONOO- were not changed in the presence of inhibitors of nitric-oxide synthase, cyclooxygenase, or K+-ATP (adenosine triphosphate-sensitive potassium) channels. Furthermore, responses to ONOO- were enhanced in duration by the type 5-cGMP (cyclic guanosine monophosphate) phosphodiesterase inhibitor zaprinast, whereas rolipram, a type 4-cGMP phosphodiesterase inhibitor, was without effect. Repeated administration of ONOO- had no significant effect on responses to vasoconstrictor or to vasodilator agents including acetylcholine. These results show that ONOO- has significant vasodilator activity in the hindlimb vascular bed of the cat and suggest that the response is mediated by a cGMP-dependent mechanism. The results of experiments with repeated injections of ONOO- indicate that ONOO- does not impair vasoconstrictor and endothelium-dependent or endothelium-independent vasodilator responses. Furthermore, tolerance did not develop with repeated short-term exposure to ONOO-. Moreover, the results of experiments with inhibitors suggest that responses to ONOO- are not dependent on K+-ATP (adenosine triphosphate-sensitive potassium) channel activation, increased NOS activity, or the formation of products in the cyclooxygenase pathway. The results of these studies are consistent with the hypothesis that ONOO- is rapidly converted in the hindlimb circulation to a substance that has the properties of an NO donor. These studies suggest that under physiologic conditions, the cytotoxic effects of ONOO- on a short-term basis may have been overemphasized.