Cardiovascular responses to amitraz in the presence of autonomic antagonists and agonists

Blood pressure and heart rate were recorded continuously in methoxyflurane anaesthetized dogs. Amitraz (i.v.) caused a dose-dependent rise in blood pressure with a fall in heart rate at lower doses. Pressor responses to amitraz were reduced by phentolamine and slightly enhanced by atropine and hexamethonium, while bradycardia was reduced by phentolamine, atropine and hexamethonium. Amitraz reduced the responses to acetylcholine and enhanced pressor responses to tyramine, while reducing bradycardia. Pressor responses to DMPP were reduced, but there was little effect on responses to noradrenaline, isoprenaline or histamine. It is possible that amitraz exerts its cardiovascular effects by stimulation of alpha 2-adrenoceptors and has some similarities to clonidine and xylazine in action.