The mechanism mediating epoxyeicosatrienoic acid (EET)-induced contraction of intralobar pulmonary arteries (PA) is currently unknown. EET-induced contraction of PA has been reported to require intact endothelium and activation of the thromboxane/endoperoxide (TP) receptor. Because TP receptor occupation with the thromboxane mimetic U-46619 contracts pulmonary artery via Rho-kinase activation, we examined the hypothesis that 5,6-EET-induced contraction of intralobar rabbit pulmonary arteries is

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... mediated by a Rho-kinase-dependent signaling pathway. In isolated rings of second-order intralobar PA (1-2 mm OD) at basal tension, 5,6-EET (0.3-10 M) induced increases in active tension that were inhibited by Y-27632 (1 M) and HA-1077 (10 M), selective inhibitors of Rho-kinase activity. In PA in which smooth muscle intracellular Ca 2ϩ concentration ([Ca 2ϩ ]i) was increased with KCl (25 mM) to produce a submaximal contraction, 5,6-EET (1 M) induced a contraction that was 7.0 Ϯ 1.6 times greater than without KCl. 5,6-EET (10 M) also contracted ␤-escin permeabilized PA in which [Ca 2ϩ ]i was clamped at a concentration resulting in a submaximal contraction. Y-27632 inhibited the 5,6-EET-induced contraction in permeabilized PA. 5,6-EET (10 M) increased phosphorylation of myosin light chain (MLC), increasing the ratio of phosphorylated MLC/total MLC from 0.10 Ϯ 0.03 to 0.30 Ϯ 0.02. Y-27632 prevented this increase in MLC phosphorylation. These data suggest that 5,6-EET induces contraction in intralobar PA by increasing Rho-kinase activity, phosphorylating MLC, and increasing the Ca 2ϩ sensitivity of the contractile apparatus. cytochrome P-450; lung; isolated vessels; Y-27632; U-46619; myosin light chain phosphatase EPOXYEICOSATRIENOIC ACIDS (EETs) are arachidonic acid metabolites of cytochrome P-450 (CYP450) epoxygenase activity that have been demonstrated to be vasoactive in both the systemic and pulmonary circulations (27). In the systemic vasculature, EETs are most commonly described as vasodilators. The mechanism of EET-induced vasodilation has been studied extensively and depends on hyperpolarization of vascular smooth muscle cells (2, 4, 8, 10) resulting from EETinduced opening of smooth muscle cell potassium channels (2, 4, 7-9, 12, 23, 42). Although EETs also reduce tension in nonresistance extralobar pulmonary artery (PA) segments (30, 32, 33) , in small-diameter intralobar PA segments of rabbits and rats (33, 37, 41) all four EET regioisomers (5, 8, 11, and 14, were observed to be vasoconstrictors. Because EETs also increase vascular resistance in isolated perfused rabbit lungs (33), the predominant activity of EETs in