goal of this study was to determine whether acetylcholine evokes endothelium-dependent contraction in mouse arteries and to define the mechanisms involved in regulating this response. Arterial rings isolated from wild-type (WT) and endothelial nitric oxide (NO) synthase knockout (eNOS Ϫ/Ϫ ) mice were suspended for isometric tension recording. In abdominal aorta from WT mice contracted with phenylephrine, acetylcholine caused a relaxation that reversed at the concentration of 0.3-3 M. After

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... ition of NO synthase [with N -nitro-L-arginine methyl ester (L-NAME), 1 mM], acetylcholine (0.1-10 M) caused contraction under basal conditions or during constriction to phenylephrine, which was abolished by endothelial denudation. This contraction was inhibited by the cyclooxygenase inhibitor indomethacin (1 M) or by a thromboxane A 2 (TxA2) and/or prostaglandin H 2 receptor antagonist SQ-29548 (1 M) and was associated with endothelium-dependent generation of the TxA2 metabolite TxB2. Also, SQ-29548 (1 M) abolished the reversal in relaxation evoked by 0.3-3 M acetylcholine and subsequently enhanced the relaxation to the agonist. The magnitude of the endothelium-dependent contraction to acetylcholine (0.1-10 M) was similar in aortas from WT mice treated in vitro with L-NAME and from eNOS Ϫ/Ϫ mice. In addition, we found that acetylcholine (10 M) also caused endothelium-dependent contraction in carotid and femoral arteries of eNOS Ϫ/Ϫ mice. These results suggest that acetylcholine initiates two competing responses in mouse arteries: endotheliumdependent relaxation mediated predominantly by NO and endothelium-dependent contraction mediated most likely by TxA2. endothelial nitric oxide synthase; gene knockout; cyclooxygenase; endothelium-derived contracting factor