Effect of amitraz and its metabolites on intestinal motility

1. Amitraz was rapidly metabolised to BTS27271 after intravenous administration to sheep. 2. Amitraz and BTS27271 had significant H1-histamine antagonist activity on isolated guinea-pig ileum. BTS27271 was approximately 3.3 times as potent as amitraz. 3. Intravenous injection of amitraz and its metabolite BTS27271 caused an immediate cessation of caecal motility in sheep, which persisted for 74-245 min. 4. Caecal stasis induced by amitraz was reversed by yohimbine but only partially reversed by 2-pyridylethylamine. 5. The results suggest that despite the significant antihistamine activity of amitraz and BTS27271 in vitro, it is probably the alpha-2-adrenergic agonist activity that is the most important in causing large intestinal stasis in vivo