title: Alpha 2-adrenergic receptor agonist effects on supraventricular and ventricular automaticity in dogs with complete atrioventricular block.
abstract: Complete atrioventricular block was induced in 26 pentobarbital-anesthetized dogs to determine the effects of the alpha 2-adrenergic receptor agonists, xylazine and medetomidine, on supraventricular and ventricular automaticity. Prazosin and atipamezole, alpha-adrenoceptor antagonists, were administered to isolate alpha 1- or alpha 2-adrenoceptor effects. Six dogs served as controls and were given glycopyrrolate (0.1 mg/kg of body weight, IV) and esmolol (50 to 75 micrograms/kg/min, IV) to induce parasympathetic and beta 1-adrenergic blockade, respectively. Eight dogs were given sequentially increasing doses of xylazine (n = 5), 0.000257 mg (10(-9)M) to 25.7 mg (10(-4)M) and medetomidine (n = 3), 0.000237 mg (10(-9)M) to 2.37 mg (10(-5)M) after parasympathetic and beta 1-adrenergic blockade. Twelve dogs were given xylazine (n = 6, 1.1 mg/kg, IV) or medetomidine (n = 6, 0.05 mg/kg, IV) after parasympathetic and beta 1-adrenergic blockade. Three dogs given xylazine and 3 dogs given medetomidine were administered prazosin (0.1 mg/kg, IV) followed by atipamezole (0.3 mg/kg, IV). The order of prazosin and atipamezole was reversed in the remaining 3 dogs given either xylazine or medetomidine. Complete atrioventricular block and administration of glycopyrrolate and esmolol resulted in stable supraventricular and ventricular rates over a 4-hour period. Increasing concentration of xylazine or medetomidine did not cause significant changes in supraventricular or ventricular rate. Xylazine and medetomidine, in the presence of the alpha-adrenoceptor antagonists, prazosin (alpha 1) and atipamezole (alpha 2), did not cause significant changes in supraventricular or ventricular rate.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Department of Veterinary Clinical Sciences, College of Veterinary Medicine, Ohio State University, Columbus 43210-1089.
authors.forename: T K
authors.initials: TK
authors.lastname: Day
authors.forename: W W
authors.initials: WW
authors.lastname: Muir
mesh_terms: D000316:Adrenergic alpha-Agonists / Q000037:antagonists & inhibitors / Q000494:pharmacology*; D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000818:Animals; D004283:Dog Diseases / Q000139:chemically induced / Q000503:physiopathology*; D004285:Dogs; D005557:Formaldehyde; D006024:Glycopyrrolate / Q000494:pharmacology; D006327:Heart Block / Q000139:chemically induced / Q000503:physiopathology / Q000662:veterinary*; D006339:Heart Rate / Q000187:drug effects*; D007093:Imidazoles / Q000494:pharmacology; D020926:Medetomidine; D011224:Prazosin / Q000494:pharmacology; D016276:Ventricular Function / Q000187:drug effects* / Q000502:physiology; D014991:Xylazine / Q000494:pharmacology
chemical_list: D000316:Adrenergic alpha-Agonists; D000317:Adrenergic alpha-Antagonists; D007093:Imidazoles; C050701:atipamezole; D005557:Formaldehyde; D014991:Xylazine; D020926:Medetomidine; D006024:Glycopyrrolate; D011224:Prazosin