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Abstract-Whether NO is involved or not in sustained conduit artery flow-mediated dilatation in humans remains unclear.Moreover, the role of endothelium-derived hyperpolarizing factor (EDHF), synthesized by cytochrome epoxygenases and acting through calcium-activated potassium channels, and its relationship with NO during flow-mediated dilatation have never been investigated previously. In 12 healthy subjects we measured radial artery diameter (echotracking) and blood flow (Doppler) during flow-mediated dilatation induced by gradual distal hand skin heating (34 to 44°C), during the local infusion of saline and inhibitors of NO synthase (N G -monomethyl-L-arginine [L-NMMA]: 8 to 20 mol/min per liter), calcium-activated potassium channels (tetraethylammonium chloride: 9 mol/min per liter), and cytochrome epoxygenases (fluconazole: 0.4 to 1.6 mol/min per liter), alone and in combination. Mean wall shear stress, the flow-mediated dilatation stimulus, was calculated at each level of flow, and the diameter-wall shear stress relationship was constructed. During heating, compared with saline, the diameter-shear stress relationship was shifted downward by L-NMMA, tetraethylammonium, fluconazole, and, in a more pronounced manner, by the combinations of L-NMMA with tetraethylammonium or with fluconazole. Therefore, maximal radial artery flow-mediated dilatation, compared with saline (0.62Ϯ0.03 mm), was decreased under our experimental conditions by L-NMMA (Ϫ39Ϯ4%), tetraethylammonium chloride (Ϫ14Ϯ4%), fluconazole (Ϫ18Ϯ6%), and to a greater extent, by the combinations of L-NMMA with tetraethylammonium (Ϫ64Ϯ4%) or with fluconazole (Ϫ71Ϯ3%). This study demonstrates that NO and a cytochrome-related EDHF are involved in peripheral conduit artery flow-mediated dilatation in humans during sustained flow conditions. Moreover, the synergistic effects of the inhibitors strongly suggest a functional interaction between NO and EDHF pathways. Key Words: conduit arteries Ⅲ endothelium Ⅲ flow-mediated dilatation Ⅲ nitric oxide Ⅲ endothelium-derived hyperpolarizing factor Ⅲ cytochrome P450 C onduit artery flow-mediated dilatation (FMD) is a fundamental mechanism that regulates vascular conductance at rest and during exercise, as well as maintaining wall shear stress within physiological values. Vasodilatation is related to the integrity of the endothelium that releases vasodilatating factors in response to the increase in shear stress. 1 In humans, we 2 and others 3,4 have demonstrated previously the major contribution of NO to peripheral conduit artery FMD in response to reactive hyperemia after a brief period of distal ischemia. This test is currently used in clinical experiments as an index of endothelial function and NO availability. 5,6 However, recent data suggest that, during more sustained hyperemia, conduit artery FMD may occur independently of NO release. [7][8][9] In fact, the local inhibition of NO synthesis by N G -monomethyl-L-arginine (L-NMMA) in healthy subjects did not affect radial artery FMD induced by hand skin heating...

Cytochrome-derived epoxyeicosatrienoic acids may be important endothelium-derived hyperpolarizing factors, opening calcium-activated potassium channels, but their involvement in the regulation of arterial stiffness during changes in blood flow in humans is unknown. In healthy volunteers, we measured arterial pressure, radial artery diameter, wall thickness, and flow (NIUS02) during hand skin heating in the presence of saline or inhibitors of NO synthase (N(G)-monomethyl-L-arginine), calcium-activated potassium channels (tetraethylammonium), and cytochrome epoxygenases (fluconazole). Arterial compliance and elastic modulus were calculated and fitted as functions of midwall stress to suppress the confounding influence of geometric changes. Under saline infusion, heating induced an upward shift of the compliance-midwall stress curve and a downward shift of the modulus-midwall stress curve demonstrating a decrease in arterial tone and stiffness when blood flow increases. These shifts were reduced by N(G)-monomethyl-L-arginine and abolished by the combinations of N(G)-monomethyl-L-arginine+tetraethylammonium and N(G)-monomethyl arginine+fluconazole. In parallel, in isolated mice coronary arteries, fluconazole and tetraethylammonium reduced the relaxations to acetylcholine. However, fluconazole did not affect the relaxations to the openers of calcium-activated potassium channels of small- and intermediate-conductance NS309 and of large-conductance NS1619 excluding a direct effect on these channels. Moreover, tetraethylammonium reduced the relaxations to NS1619 but not to NS309, suggesting that the endothelium-derived hyperpolarizing factor involved mainly acts on large-conductance calcium-activated potassium channels. These results show in humans that, during flow variations, arterial stiffness is regulated by the endothelium through the release of both NO and cytochrome-related endothelium-derived hyperpolarizing factor.