Evidence for the involvement of alpha 2-adrenoceptors in the emetic action of xylazine in cats.
Intramuscular injection of xylazine induced dose-dependent vomiting in cats (ED50 = 0.277 mg/kg); administration of standard dose of xylazine (2 mg/kg, 2 times the 100% emetic dose) induced vomiting in 100% of the cats studied. The xylazine-induced vomiting was antagonized by adrenoceptor antagonists possessing alpha 2-blocking activity, which were yohimbine, tolazoline, and phentolamine. Of these antagonists, yohimbine was the most effective; the maximal antagonistic effect was seen at 1 mg of yohimbine/kg, a dose at which the other drugs had little or no effect. At the doses studied, prazosin and phenoxybenzamine, adrenoceptor antagonists with alpha 1-blocking activity, did not prevent vomiting induced by xylazine. Beta-Adrenoceptor (propranolol), dopamine receptor (domperidone and chlorpromazine), a cholinoceptor (atropine), an opiate receptor (naloxone), and a histamine-receptor (diphenhydramine) antagonists, at the doses studied, did not prevent xylazine-induced vomiting. Pretreatment with 6-hydroxydopamine failed to prevent xylazine-induced vomiting. These results indicated that xylazine-induced vomiting in cats is mediated by alpha 2-adrenoceptors and suggested that the alpha 2-adrenoceptors mediating the vomiting attributable to xylazine may not be presynaptic alpha 2-receptors located on noradrenergic nerve terminals.
Hikasa
Y
Y
Department of Veterinary Surgery, School of Veterinary Medicine and Animal Sciences, Kitasato University, Aomori, Japan.
Takase
K
K
Ogasawara
S
S
D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology; D000319:Adrenergic beta-Antagonists / Q000494:pharmacology; D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002371:Cat Diseases / Q000139:chemically induced*; D002415:Cats; D002746:Chlorpromazine / Q000494:pharmacology; D004155:Diphenhydramine / Q000494:pharmacology; D004294:Domperidone / Q000494:pharmacology; D005260:Female; D006892:Hydroxydopamines / Q000494:pharmacology; D008297:Male; D009270:Naloxone / Q000494:pharmacology; D016627:Oxidopamine; D011433:Propranolol / Q000494:pharmacology; D011897:Random Allocation; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D013843:Thiazines / Q000494:pharmacology*; D014839:Vomiting / Q000139:chemically induced / Q000662:veterinary*; D014991:Xylazine / Q000494:pharmacology*
D000317:Adrenergic alpha-Antagonists; D000319:Adrenergic beta-Antagonists; D006892:Hydroxydopamines; D011942:Receptors, Adrenergic, alpha; D013843:Thiazines; D014991:Xylazine; D009270:Naloxone; D004294:Domperidone; D001285:Atropine; D004155:Diphenhydramine; D016627:Oxidopamine; D011433:Propranolol; D002746:Chlorpromazine