Effect of xylazine on heart rate and arterial blood pressure in conscious dogs, as influenced by atropine, 4-aminopyridine, doxapram, and yohimbine

Abstract

SUMMARY The effects of xylazine on heart rate ( hr ) and mean arterial blood pressure ( abp ) were studied in 5 conscious male dogs. An iv injection of xylazine (1 mg/kg) caused a decrease in hr , which was accompanied by sinus arrhythmia. Xylazine administration also caused an initial increase in abp , which was followed by a decrease. Atropine sulfate (0.045 mg/kg, im ) increased both the abp and hr , but prevented xylazine-induced bradycardia only in 3 of 5 dogs. The other 2 dogs had to be given a supplemental dose of atropine sulfate (0.01 mg/kg, iv ) before xylazine-induced bradycardia was antagonized. In addition, atropine sulfate potentiated xylazine-induced hypertension for 60 minutes. Yohimbine, an α 2 -adrenoreceptor blocking agent, given iv at a dosage of 0.1 mg/kg, antagonized hypertension, hypotension, and bradycardia induced by xylazine. In addition, doxapram HCl, given iv at a dosage of 5.5 mg/kg, antagonized bradycardia but potentiated xylazine-induced hypertension, and an iv injection of 4-aminopyridine at a dosage of 0.5 mg/kg did not affect the cardiovascular actions of xylazine. It was concluded that (1) atropine sulfate at the im dosage of 0.045 mg/kg may be insufficient to antagonize xylazine-induced bradycardia but may potentiate xylazine-induced hypertension, and (2) yohimbine may be useful in antagonizing these untoward reactions associated with xylazine administration. Doxapram and 4-aminopyridine were not found to be beneficial.