Figure 7. Phenylephrine (10 sup -6 M)-induced sustained vasoconstriction and relaxation by acetylcholine (ACh, 0.03–10 micro Meter) in (A) an intact rat mesenteric resistance artery and (B) an endothelium-denuded resistance artery. ACh (1 micro Meter) produces complete relaxation of the phenylephrine constriction in the intact vessel, whereas ACh (up to 10 micro Meter) produces little response after removal of the endothelium, which also resulted in attenuation of the phenylephrine-induced oscillations in diameter. ID = inside diameter of the vessel; OD = outside diameter of the vessel.