Decreased vascular resistance after intra-arterial injection of [met]enkephalin in the hindquarters of conscious rabbits.

The hemodynamic effects of i.a. [met]enkephalin were studied in the hindquarter vasculature of chronically prepared, conscious rabbits. A new method allowed i.a. injection while simultaneously measuring blood pressure and blood flow to this vascular bed. [Met]enkephalin produced a dose-dependent (3-300 micrograms/kg) decrease in hindquarter vascular resistance (18-42% change from base line). The duration of the response ranged from 28 sec to over 2 min. Heart rate decreased 16 to 45% over the same dose range but returned to preinjection levels in 5 sec. Only the bradycardia was abolished by pretreatment with atropine methyl nitrate. All hemodynamic changes were eliminated or significantly reduced after pretreatment with the ganglionic blocking agent, chlorisondamine hydrochloride. The opioid antagonist, naloxone hydrochloride, abolished the hemodynamic effects of [met]enkephalin. Resistance decreases in the mesenteric vasculature were coincident with those in the hindquarters. The time to onset of the response was delayed when [met]enkephalin was injected i.v. These data indicate activation of a reflex originating in the hindquarters that resulted in opioid-dependent increased efferent parasympathetic activity to the heart and decreased sympathetic tone to at least two vascular beds.