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.