Acetylcholine (ACh) is widely used as a standard test substance for nitric oxide (NO)-mediated vasodilation. However, it also augments the release of prostaglandins, a group of other endothelium-derived smooth muscle relaxants. Using intravital microscopy in the cremaster muscle of anesthetized hamsters, we studied the relative roles of NO and prostaglandins in mediating ACh-induced dilation and in the control of basal vessel tone (253 arterioles in 31 experiments). N(omega)-nitro-L-arginine (L-NNA), a competitive inhibitor of NO synthase, significantly reduced ACh-induced vasodilation (by 42-73%), irrespective of whether it was applied intravenously (30 mg/kg) or topically (30 muM. Additional indomethacin (3 muM, topical) nearly abolished the dilator response. In contrast, the vascular responses to the endothelium-independent dilator sodium nitroprusside were not affected. The resting diameters (range: 6-114 muM) were significantly (p < 0.05) reduced after L-NNA or indomethacin by 10.2 and 16.6% of control diameter, respectively. The constriction induced by L-NNA was stronger in larger (>50 mum) than in smaller (<50 mum) vessels, whereas indomethacin was equipotent in both groups. Thus, in addition to NO, dilating prostaglandins are important mediators of the ACh-induced dilation and contribute to the control of resting arteriolar diameter in the hamster cremaster microcirculation in vivo.
