the aim of this study was to determine the relative importance of Ca(2+) sensitization, ion channels, and intracellular Ca(2+) ([Ca(2+)](i)) in the mixed constrictor/relaxation actions of superoxide anion on systemic and pulmonary arteries. pulmonary and mesenteric arteries were obtained from rat. Superoxide was generated in arteries and cells with 6-anilino-5,8-quinolinequinone (LY83583). Following pre-constriction with U46619, 10 μmol/L LY83583 caused constriction in pulmonary and relaxation in mesenteric arteries. Both constrictor and relaxant actions of LY83583 were inhibited by superoxide dismutase and catalase. LY83583 caused Rho-kinase-dependent constriction in α-toxin-permeabilized pulmonary but not mesenteric arteries. Phosphorylation of myosin phosphatase-targeting subunit-1 (MYPT-1; as determined by western blot), was enhanced by LY83583 in pulmonary artery only. However, in both artery types, changes in tension were closely correlated with changes in phosphorylation of the 20 kDa myosin light chain as well as changes in [Ca(2+)](i) (as measured with Fura PE-3), with LY83583 causing increases in pulmonary and decreases in mesenteric arteries. When U46619 was replaced by 30 mmol/L K(+), all changes in [Ca(2+)](i) were abolished and LY83583 constricted both artery types. The K(V) channel inhibitor 4-aminopyridine abolished the LY83583-induced relaxation in mesenteric artery without affecting constriction in pulmonary artery. However, LY83583 caused a similar hyperpolarizing shift in the steady-state activation of K(V) current in isolated smooth muscle cells of both artery types. superoxide only causes Rho-kinase-dependent Ca(2+) sensitization in pulmonary artery, resulting in constriction, and whilst it opens K(V) channels in both artery types, this only results in relaxation in mesenteric.