Oxytocin augments baroreflex bradycardia in conscious rats

Previous studies have demonstrated augmentation of baroreflex-mediated bradycardia by arginine vasopressin (AVP). However, the specific receptor subtype responsible for mediating this augmentation has not been determined. In the present study, experiments were performed in conscious rats to determine the possible involvement of oxytocin receptors in this response. Infusion of oxytocin at a dose that had no effect on baseline hemodynamic values significantly augmented the bradycardic response to IV bolus doses of methoxamine. Prior treatment with selective antagonists to either oxytocin, V1 vasopressinergic or V2 vasopressinergic receptors reversed this enhancement. In a separate set of experiments, baroreflex-mediated bradycardic responses to IV bolus doses of AVP were assessed. Pretreatment with the selective oxytocin receptor antagonist reversed vasopressinergic augmentation of baroreflex sensitivity. Finally, combined vasopressinergic and oxytocinergic stimulation of the baroreflex was assessed. Treatment with both AVP and oxytocin did not augment baroreflex-mediated bradycardia greater than AVP alone. We conclude from these experiments that AVP and oxytocin both augment baroreflex sensitivity, although the receptor type(s) responsible are not clear.