ABSTRACT
The endothelium plays an important role in maintaining vascular homeostasis by synthesizing and releasing several endothelium-derived relaxing factors (EDRFs) , such as prostacyclin, nitric oxide (NO), and yet unidentified endothelium-derived hyper polarizing factor ( E D H F ) . This study was designed to examine a hypothesis that H 2 0 2 derived from endothelial N O synthase (eNOS) is an E D H F . EDHF-mediated relaxa tions and hyperpolarizations to acetylcholine were markedly attenuated in small mesenteric arteries from eNOS-knockout (KO) mice. In the e N O S - K O mice, vasodilating and hyperpolarizing responses of vascular smooth muscle per se were preserved and the increase in intracellular calcium in endothelial cells in responses to acetylcholine also was preserved. Antihypertensive treatment with hydralazine failed to improve the EDHF-mediated relaxations. Catalase, which dismutates H 2 0 2 to form oxygen and water, inhibited EDHF-mediated relaxations as well as hyperpolarizations while it did not affect endothelium-independent relaxations to levcromakalim, a K + channel opener. Exogenous H 2 0 2 elicited similar relaxations and hyperpolarizations in endothelium-stripped arteries. Finally, laser confocal microscopic examination with per oxide-sensitive fluorescence dye demonstrated that the endothelium produced H 2 0 2 during stimulation with acetylcholine and that the H 2 0 2 production was markedly reduced in e N O S - K O mice. These results indicate that H 2 0 2 is an E D H F in small mesenteric arteries of the mouse and that eNOS is a major source of the reactive oxygen species.
