title: Endothelium-derived hyperpolarizing factor determines resting and stimulated forearm vasodilator tone in health and in disease.
abstract: BACKGROUND
  We assessed the contribution of endothelium-derived hyperpolarizing factors to resting and agonist-stimulated vasodilator tone in health and disease. Tetraethylammonium chloride (TEA) was used to inhibit K(+)(Ca) channel activation and fluconazole was used to inhibit cytochrome P450 2C9-mediated epoxyeicosatrienoic acid synthesis. We hypothesized that endothelium-derived hyperpolarizing factors contribute to resting vascular tone by K(+)(Ca) channel activation and epoxyeicosatrienoic acid release and that endothelium-derived hyperpolarizing factors compensate for reduced nitric oxide bioavailability at rest and with endothelium-dependent vasodilators.
  
  
  RESULTS
  In 103 healthy subjects and 71 nonhypertensive subjects with multiple risk factors, we measured resting forearm blood flow (FBF) using venous occlusion plethysmography before and after intra-arterial infusions of N(G)-monomethyl-l-arginine (L-NMMA), TEA, fluconazole, and their combination. The effects of these antagonists on resting FBF and on bradykinin- and acetylcholine-mediated vasodilation were studied. Resting FBF decreased with TEA and L-NMMA in all subjects (P<0.001); however, the vasoconstrictor response to L-NMMA was greater (P=0.04) and to TEA was lower (P=0.04) in healthy subjects compared with those with risk factors. Fluconazole decreased resting FBF in all subjects, and the addition of TEA further reduced FBF after fluconazole, suggesting that cytochrome P450 metabolites and other hyperpolarizing factor(s) activate K(+)(Ca) channels. Both L-NMMA and TEA attenuated bradykinin-mediated vasodilation in healthy and hypercholesterolemic subjects (P<0.001). In contrast, acetylcholine-mediated vasodilation remained unchanged with TEA in healthy subjects but was significantly attenuated in hypercholesterolemia (P<0.04).
  
  
  CONCLUSIONS
  First, by activating TEA-inhibitable K(+)(Ca) channels, endothelium-derived hyperpolarizing factors, together with nitric oxide, contribute to resting microvascular dilator tone. The contribution of K(+)(Ca) channel activation compared with nitric oxide is greater in those with multiple risk factors compared with healthy subjects. Second, activation of K(+)(Ca) channels is only partly through epoxyeicosatrienoic acid release, indicating the presence of other hyperpolarizing mechanisms. Third, bradykinin, but not acetylcholine, stimulates K(+)(Ca) channel-mediated vasodilation in healthy subjects, whereas in hypercholesterolemia, K(+)(Ca) channel-mediated vasodilation compensates for the reduced nitric oxide activity. Thus, enhanced endothelium-derived hyperpolarizing factor activity in conditions of nitric oxide deficiency contributes to maintenance of resting and agonist-stimulated vasodilation. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT00166166.
authors.affiliation: Emory Clinical Cardiovascular Research Institute, Division of Cardiology, Department of Medicine, Emory University, Atlanta, GA, USA.
authors.forename: Muhiddin A
authors.initials: MA
authors.lastname: Ozkor
authors.forename: Jonathan R
authors.initials: JR
authors.lastname: Murrow
authors.forename: Ayaz M
authors.initials: AM
authors.lastname: Rahman
authors.forename: Nino
authors.initials: N
authors.lastname: Kavtaradze
authors.forename: Ji
authors.initials: J
authors.lastname: Lin
authors.forename: Amita
authors.initials: A
authors.lastname: Manatunga
authors.forename: Arshed A
authors.initials: AA
authors.lastname: Quyyumi
mesh_terms: D000109:Acetylcholine / Q000008:administration & dosage; D000328:Adult; D001685:Biological Factors / Q000502:physiology*; D001920:Bradykinin / Q000008:administration & dosage; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology; D004730:Endothelium, Vascular / Q000187:drug effects / Q000502:physiology; D004791:Enzyme Inhibitors / Q000008:administration & dosage; D005260:Female; D005542:Forearm / Q000098:blood supply; D006801:Humans; D006937:Hypercholesterolemia / Q000453:epidemiology / Q000503:physiopathology*; D008297:Male; D008875:Middle Aged; D009569:Nitric Oxide / Q000502:physiology; D010991:Plethysmography; D026902:Potassium Channel Blockers / Q000008:administration & dosage; D015221:Potassium Channels / Q000502:physiology; D012039:Regional Blood Flow / Q000187:drug effects / Q000502:physiology*; D012307:Risk Factors; D019789:Tetraethylammonium / Q000008:administration & dosage; D014664:Vasodilation / Q000187:drug effects / Q000502:physiology*; D055815:Young Adult; D019323:omega-N-Methylarginine / Q000008:administration & dosage
chemical_list: D001685:Biological Factors; D004791:Enzyme Inhibitors; D026902:Potassium Channel Blockers; D015221:Potassium Channels; C067410:endothelium-dependent hyperpolarization factor; D019323:omega-N-Methylarginine; D009569:Nitric Oxide; D019789:Tetraethylammonium; D003577:Cytochrome P-450 Enzyme System; D000109:Acetylcholine; D001920:Bradykinin