title: Antagonism of the emetic action of xylazine by alpha-adrenoceptor blocking agents.
abstract: The intramuscular injection of xylazine (2 mg/kg) evoked vomiting in 81% of the dogs studied. Adrenoceptor antagonists showing alpha 2-blocking activity, yohimbine, tolazoline and phentolamine, antagonized the xylazine-induced vomiting in a dose-dependent manner. Of these antagonists, yohimbine was the most effective, since the maximal antagonistic effect was seen at 0.5 mg/kg yohimbine, a dose at which the other drugs had less or no effect. The adrenoceptor antagonists showing alpha 1-blocking activity, prazosin and phenoxybenzamine, at the doses studied did not prevent the emesis induced by xylazine. A beta-adrenoceptor antagonists, propranolol, was ineffective in reducing xylazine-induced vomiting. The dopamine receptor antagonists, metoclopramide and domperidone, did not prevent xylazine-induced vomiting nor did yohimbine antagonize apomorphine-induced vomiting. The xylazine-induced vomiting was not prevented by atropine, naloxone or hexamethonium. These results indicate that the xylazine-induced vomiting is mediated by alpha 2-adrenoceptors and does not appear to involve beta-adrenoceptors, cholinoceptors, dopamine or opiate receptors in the emetic pathway.
mesh_terms: D000317:Adrenergic alpha-Antagonists / Q000494:pharmacology*; D000818:Animals; D004285:Dogs; D004639:Emetics; D005260:Female; D005730:Ganglionic Blockers / Q000494:pharmacology; D008297:Male; D009294:Narcotics / Q000494:pharmacology; D010277:Parasympathomimetics / Q000494:pharmacology; D011954:Receptors, Dopamine / Q000187:drug effects; D013843:Thiazines / Q000037:antagonists & inhibitors*; D014991:Xylazine / Q000037:antagonists & inhibitors*; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D000317:Adrenergic alpha-Antagonists; D004639:Emetics; D005730:Ganglionic Blockers; D009294:Narcotics; D010277:Parasympathomimetics; D011954:Receptors, Dopamine; D013843:Thiazines; D014991:Xylazine; D015016:Yohimbine