Antagonistic effects of alpha-adrenoceptor blocking agents on reticuloruminal hypomotility induced by xylazine in cattle.

The intravenous injection of a standard dose (0.05 mg/kg) of xylazine inhibited reticuloruminal motility in cattle. Pretreatment with adrenoceptor antagonists showing alpha 2-blocking activity, tolazoline (0.5 mg/kg) and yohimbine (0.2 mg/kg), antagonized the xylazine-induced reticuloruminal amotility. Tolazoline was more effective than yohimbine, since an antagonistic effect was not seen at 0.5 mg/kg yohimbine, and yohimbine at 0.2 mg/kg was less effective than tolazoline at 0.5 mg/kg. An adrenoceptor antagonist showing alpha 1-blocking activity, prazosin, did not prevent the inhibition of reticuloruminal motility by xylazine. The xylazine-induced reticuloruminal amotility was also not prevented by either a dopamine receptor antagonist, domperidone, or an opiate receptor antagonist, naloxone. These results suggest that xylazine inhibits bovine reticuloruminal motility through its activation of alpha 2-adrenoceptors, and show that tolazoline can be used as a specific antagonist of xylazine in studies of the alpha-adrenergic influence on reticuloruminal motility in cattle.