Abstract-Arachidonic acid is metabolized by the 15-lipoxygenase-1 pathway to the vasodilatory eicosanoids hydroxyepoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid. We determined the in vitro and in vivo effects of the 15-lipoxygenase-1 metabolites in rabbits infected with adenovirus containing cDNA for human 15-lipoxygenase-1 (Ad-15-LO-1). Forty hours after intravenous adenoviral injection, 15-lipoxygenase-1 expression increased in liver and mesenteric arteries 10-fold and 3-fold, respectively. Expression of 15-LO-1 was limited to the endothelium of mesenteric arteries. Overexpression did not occur in tissues from rabbits infected with a ␤-galactosidase containing adenovirus. Trihydroxyeicosatrienoic acid and hydroxy-epoxyeicosatrienoic acid synthesis per milligram of tissue increased by 2.1-and 1.5-fold, respectively, in mesenteric arteries from Ad-15-LO-1-infected rabbits compared with normal rabbits. Pretreatment with a 15-lipoxygenase inhibitor BW755C inhibited the synthesis. NO and prostaglandin-independent, maximal relaxations to acetylcholine were greater in mesenteric arteries from Ad-15-LO-1-infected rabbits (98.3Ϯ1.7%) compared with normal (60.93Ϯ10.5%) and ␤-galactosidase containing adenovirus-infected rabbits (68.3Ϯ7.7%). Pretreatment with BW755C decreased these relaxations. Mean arterial pressure and heart rate did not differ in Ad-15-LO-1-infected rabbits compared with normal or ␤-galactosidase containing adenovirus-infected rabbits.The hypotensive responses to acetylcholine in the presence and absence of inhibition of NO and prostaglandins were greater in Ad-15-LO-1-infected rabbits (Ϫ52Ϯ2% and Ϫ47Ϯ2%) compared with normal (Ϫ31Ϯ3% and Ϫ25Ϯ5%) or ␤-galactosidase containing adenovirus-infected rabbits (Ϫ38Ϯ2% and Ϫ30Ϯ3%). Therefore, increased expression of 15-LO-1 increases acetylcholine relaxation in arteries and hypotensive responses in rabbits because of increased hydroxy-epoxyeicosatrienoic acid and trihydroxyeicosatrienoic acid synthesis. (Hypertension. 2008;51:246-251.)Key Words: endothelium-derived hyperpolarizing factors Ⅲ endothelium Ⅲ adenovirus Ⅲ hypotension Ⅲ mean arterial pressure 1 5-Lipoxygenase-1 (15-LO-1) is expressed in the vascular endothelium. 1 It metabolizes arachidonic acid (AA) to 15-hydroperoxyeicosatetraenoic acid. 15-Hydroperoxyeicosatetraenoic acid is reduced to 15-hydroxyeicosatetraenoic acid or rearranged to 15-hydroxy-11,12-epoxyeicosatrienoic acid and 11-hydroxy-14,15-epoxyeicosatrienoic acid. Hydroxyepoxyeicosatrienoic acids (HEETAs) are further hydrolyzed to 11,12,15-trihydroxyeicosatrienoic acid and 11,14,15-trihydroxyeicosatrienoic acid. 2 Trihydroxyeicosatrienoic acid (THETA) and HEETA relax rabbit arteries. 2,3 In the presence of NO synthase and cyclooxygenase (COX) inhibition, acetylcholine (ACH) and AA relaxed preconstricted rabbit arteries. These relaxations were inhibited by 15-lipoxygenase inhibitors. 3,4 Thus, 15-LO-1 metabolites mediate relaxations to ACH and AA. 15-Hydroxyeicosatetraenoic acid is inactive in rabbit aorta. 5,6 However, 11,12,15-...