OXYTOCIN AUGMENTS BAROREFLEX BRADYCARDIA IN CONSCIOUS RATS

Previous studies have demonstrated augmentation of baroreflex-mediated bradycardia by arginine vasopressin (AVP). However, the specific rece ptor subtype responsible for mediating this augmentation has not been determined. In the present study, experiments were performed in consci ous rats to determine the possible involvement of oxytocin receptors i n this response. Infusion of oxytocin at a dose that had no effect on baseline hemodynamic values significantly augmented the bradycardic re sponse to IV bolus doses of methoxamine. Prior treatment with selectiv e antagonists to either oxytocin, V-1 vasopressinergic or V-2 vasopres sinergic receptors reversed this enhancement. In a separate set of exp eriments, baroreflex-mediated bradycardic responses to IV bolus doses of AVP were assessed. Pretreatment with the selective oxytocin recepto r antagonist reversed vasopressinergic augmentation of baroreflex sens itivity. Finally, combined vasopressinergic and oxytocinergic stimulat ion of the baroreflex was assessed. Treatment with both AVP and oxytoc in did not augment baroreflex-mediated bradycardia greater than AVP al one. We conclude from these experiments that AVP and oxytocin both aug ment baroreflex sensitivity, although the receptor type(s) responsible are not clear.