Cardiovascular effects of intravenously administered etorphine were investigated in mechanically ventilated normotensive rats under pentobarbitone anaesthesia. 2. Etorphine (0.1-2 micrograms kg-1) induced a dose-related bradycardia and hypotension which was prevented by pretreatment with naloxone (0.1 mg kg-1). 3. After bilateral vagotomy etorphine (1 microgram kg-1) produced a pressor effect which was prevented by prazosin (0.5 mg kg-1), but unaltered by adrenalectomy. 4. The bradycardia due to etorphine was abolished by bilateral vagotomy, but only partially reduced by atropine (1 mg kg-1) and still evident after propranolol (1.5 mg kg-1). 5. Etorphine was without effect on blood pressure in the pithed rat, although there was a small bradycardia which was not seen after naloxone. 6. The data presented indicate that etorphine produces an opioid receptor-mediated stimulation of both vagal (partially cholinergic) and sympathetic outflow and a direct cardiodepressant effect.
