alpha 2-Adrenergic receptor agonist effects on supraventricular and ventricular automaticity in dogs with complete atrioventricular block

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 microgram/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. alpha 2-Adrenoceptor agonists do not induce direct alpha 1- or alpha 2-adrenoceptor-mediated depression of supraventricular or ventricular rate in dogs with complete atrioventricular block.