Effect of amitraz on heart rate and aortic blood pressure in conscious dogs: influence of atropine, prazosin, tolazoline, and yohimbine.

The effect of amitraz on heart rate (HR) and mean aortic blood pressure (MAP) were studied in five conscious male dogs. An iv injection of amitraz (1 mg/kg) caused a decrease in HR, which was accompanied by sinus arrhythmia for at least 60 min. Administration of amitraz also caused an increase in MAP for 20 min. Atropine sulfate (0.045 mg/kg, iv) increased HR and prevented amitraz-induced bradycardia. In addition, atropine potentiated amitraz-induced hypertension for 45 min. Yohimbine, an alpha 2-adrenoreceptor antagonist, given iv at 0.1 mg/kg, prevented hypertension, bradycardia, and sinus arrhythmia induced by amitraz. Tolazoline, a nonselective alpha-adrenoreceptor antagonist, given iv at 5 mg/kg, reduced the bradycardia and sinus arrhythmia caused by amitraz administration but did not change amitraz-induced hypertension. Tolazoline alone also increased both HR and MAP. Prazosin, an alpha 1-adrenoreceptor antagonist, given iv at 1 mg/kg, did not affect the cardiovascular actions of amitraz. The results suggest that (1) alpha 2-adrenoreceptors mediate amitraz-induced bradycardia and hypertension, and (2) yohimbine, but not atropine, can be used to control the untoward reactions of amitraz.