Effect of Tempol and apocynin on vasodilation response to ACh. Experiments were performed on aortic rings isolated from control rats (A, B), AIA rats (C, D) and nor-NOHA AIA rats (E, F) 21 days after the onset of arthritis. Cumulative concentration-responses curves with Ach were obtained after a 60-minute incubation period with Tempol at 10-4 moles/liter (A, C, E) or with apocynin at 3 × 10-4 moles/liter (B, D, F). Values are the mean ± SEM from 8 to 12 rings. ** = P < 0.01.