Acetylcholine causes endothelium-dependent contraction of mouse arteries

The 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^-/-) 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 μM. After inhibition of NO synthase [with N^ω-nitro-L-arginine methyl ester (L-NAME), 1 mM], acetylcholine (0.1-10 μM) 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 μM) or by a thromboxane A₂ (TxA₂) and/or prostaglandin H₂ receptor antagonist SQ-29548 (1 μM) and was associated with endothelium-dependent generation of the TxA₂ metabolite TxB ₂. Also, SQ-29548 (1 μM) abolished the reversal in relaxation evoked by 0.3-3 μM acetylcholine and subsequently enhanced the relaxation to the agonist. The magnitude of the endothelium-dependent contraction to acetylcholine (0.1-10 μM) was similar in aortas from WT mice treated in vitro with L-NAME and from eNOS^-/- mice. In addition, we found that acetylcholine (10 μM) also caused endothelium-dependent contraction in carotid and femoral arteries of eNOS^-/- mice. These results suggest that acetylcholine initiates two competing responses in mouse arteries: endothelium-dependent relaxation mediated predominantly by NO and endothelium-dependent contraction mediated most likely by TxA₂.