Endomorphin 1 and 2, the endogenous μ-opioid agonists, produce biphasic changes in systemic arterial pressure in the cat

The endogenous peptides endomorphin I and 2 are newly isolated, potent, selective μopioid receptor agonists. In the present study, responses to the endomorphin peptides were investigated in the systemic vascular bed of the cat, Endomorphin 1 and 2 induced dose-related biphasic changes in systemic arterial pressure when injected in doses of 1-30 nmol/kg i.v. The biphasic responses to endomorphin 1 and 2 were characterized by an initial increase followed by a decrease in systemic arterial pressure. In terms of relative vasodepressor activity, endomorphin 1 and 2 were similar in potency and approximately 10-fold less potent than the ORL, ligand nociceptin (orphanin FQ) in decreasing systemic arterial pressure. The biphasic arterial pressure changes in response to endomorphin 1 and 2 were inhibited by the opioid receptor antagonist naloxone in a dose of 2 mg/kg i.v. These results demonstrate that endomorphin 1 and 2 produce significant, naloxone-sensitive changes in systemic arterial pressure that are characterized by an initial increase followed by a secondary decrease in arterial pressure in the cat.