Oxygen is involved in cell signaling through oxygenases and oxidases and this applies especially for the vascular system. Nitric oxide ((NO)-N-center dot) and epoxyarachidonic acids are P450-dependent monooxygenase products and prostacyclin is formed via cyclooxygenase and a heme-thiolate isomerase. The corresponding vasorelaxant mechanisms are counteracted by superoxide which not only traps (NO)-N-center dot but through the resulting peroxynitrite blocks prostacyclin synthase by nitration of an active site tyrosine residue. In a model of septic shock, this leads to vessel constriction by activation of the thromboxane A(2)-prostaglandin endoperoxide H-2 receptor. This sequence of events is part of endothelial dysfunction in which the activated vascular smooth muscle counteracts and regenerates vessel tone by cyclooxygenase-2-dependent prostacyclin synthesis. Peroxynitrite was found to activate cyclooxygenases by providing the peroxide tone at nanomotar concentrations. Such new insights into the control of vascular function have allowed us to postulate a concept of redox regulation in which a progressive increase of superoxide production by NADPH-oxidase, mitochondria, xanthine oxidase, and even uncoupled NO-synthase triggers a network of signals originating from an interaction of (NO)-N-center dot with superoxide. (c) 2005 Elsevier Inc. All rights reserved.