title: alpha2-Adrenoceptor subsensitivity in mesenteric vascular bed of cholestatic rats: the role of nitric oxide and endogenous opioids.
abstract: Cholestasis is associated with vascular changes and in previous studies decreased response of visceral vessels of cholestatic animals to phenylephrine and acetylcholine has been shown. In the present study, the response of mesenteric vascular bed of cholestatic rats to clonidine (an alpha2-adrenoceptor agonist) was investigated and we also examined the role of endogenous opioids and nitric oxide (NO). Seven-day ligation of bile duct was used as the model to study cholestasis. Six groups of rats, each of which divided into two subgroups (bile duct-ligated and sham-operated), were examined. Three groups of animals were chronically treated with either normal saline, naltrexone (an opioid receptor antagonist, 20 mg/kg/day, s.c.) or aminoguanidine (a selective inducible nitric oxide synthase inhibitor, 150 mg/kg/day, s.c.) for 7 days. After 7 days the response of the mesenteric vascular bed to subsequent doses of clonidine was studied. In other two groups, 7 days after the operation, the response of the mesenteric vascular bed to clonidine in the presence of either yuhimbine, an alpha2-adrenoceptor antagonist, or N(omega)-nitro-L-arginine methyl ester (L-NAME), a non-selective nitric oxide synthase inhibitor, was studied. In the last group, vasodilation response to sodium nitroprusside (an endothelium-independent vasorelaxant) was evaluated. Clonidine caused vasodilation in a dose-dependent manner by acting on endothelial alpha2-adrenoceptors since its effect was antagonized by yohimbine, and this vasodilation was through the L-arginine pathway since there was no response in the presence of L-NAME in the perfusate. Compared to sham-operated rats, there was a significant right shift in the clonidine concentration curves of cholestatic animals. Maximum response in cholestatic rats was significantly lower comparing to the sham group (P<0.01) and the dose of clonidine that causes 50% of maximum response (ED50) was significantly higher in cholestatic rats (P<0.05). Vasodilation response to sodium nitroprusside was the same in cholestatic and sham-operated rats. Seven-day treatment with aminoguanidine recovered the effect of cholestasis. Seven-day treatment with naltrexone caused an increase in maximum response (P<0.01) and a decrease in ED50 (P<0.05) in cholestatic rats, while this treatment in sham-operated rats caused a decrease in the maximum response (P<0.01) and an increase in ED50 (P<0.05). This study showed that cholestasis is associated with decreased responsiveness of mesenteric vascular bed to clonidine and the cholestasis-associated NO overproduction and increased level of endogenous opioids may contribute to this process.
authors.affiliation: Department of Pharmacology, School of Medicine, Tehran University of Medical Sciences, P.O. Box 13145-784, Tehran, Iran.
authors.forename: Amir Ali
authors.initials: AA
authors.lastname: Borhani
authors.forename: Golbahar
authors.initials: G
authors.lastname: Houshmand
authors.forename: Morteza
authors.initials: M
authors.lastname: Samini
authors.forename: Khodadad
authors.initials: K
authors.lastname: Namiranian
authors.forename: Amir Reza
authors.initials: AR
authors.lastname: Hajrasouliha
authors.forename: Sina
authors.initials: S
authors.lastname: Tavakoli
authors.forename: Farzad
authors.initials: F
authors.lastname: Ebrahimi
authors.forename: Ahmad Raza
authors.initials: AR
authors.lastname: Dehpour
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists / Q000494:pharmacology; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D001663:Bilirubin / Q000097:blood; D002779:Cholestasis / Q000503:physiopathology*; D003000:Clonidine / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006146:Guanidines / Q000494:pharmacology; D008297:Male; D008638:Mesenteric Arteries / Q000503:physiopathology*; D008643:Mesentery / Q000098:blood supply; D019331:NG-Nitroarginine Methyl Ester / Q000494:pharmacology; D009271:Naltrexone / Q000494:pharmacology; D009292:Narcotic Antagonists / Q000494:pharmacology; D009569:Nitric Oxide / Q000502:physiology; D019001:Nitric Oxide Synthase / Q000037:antagonists & inhibitors; D009599:Nitroprusside / Q000494:pharmacology; D018847:Opioid Peptides / Q000502:physiology; D051381:Rats; D017207:Rats, Sprague-Dawley; D018341:Receptors, Adrenergic, alpha-2 / Q000502:physiology*; D014664:Vasodilation / Q000187:drug effects; D014665:Vasodilator Agents / Q000494:pharmacology; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D058669:Adrenergic alpha-2 Receptor Antagonists; D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D004791:Enzyme Inhibitors; D006146:Guanidines; D009292:Narcotic Antagonists; D018847:Opioid Peptides; D018341:Receptors, Adrenergic, alpha-2; D014665:Vasodilator Agents; D009599:Nitroprusside; D015016:Yohimbine; D009569:Nitric Oxide; D009271:Naltrexone; D019001:Nitric Oxide Synthase; D003000:Clonidine; D001663:Bilirubin; C004479:pimagedine; D019331:NG-Nitroarginine Methyl Ester