Interactions of Leucine-enkephalin with -Adrenoceptors in the Conscious Dog

¹ From the Departments of Medicine, Tulane University School of Medicine, and the Veterans Administration Medical Center, New Orleans, Louisiana

Leucine-enkephalin (Leu-ENK) administered systemically to the conscious dog increases heart rate and blood pressure, a response prevented by naloxone and hence mediated by opiate receptors. Clonidine attenuates the heart rate increase without affecting the pressor response. Propranolol also reduces the magnitude of the heart rate increase but to a lesser degree than does clonidine. Clonidine appears to inhibit the Leu-ENK-induced heart rate increase by both a centrally mediated increase in vagal tone and a peripheral presynaptic ₂-agonist effect. The presence of a pressor response to Leu-ENK after prazosin pretreatment suggests that Leu-ENK may also increase blood pressure independently of the ₁-adrenoceptor. Thus, Leu-ENK appears to act via a naloxone-sensitive receptor (perhaps a vagal afferent chemoreceptor), the response involving both sympathetic and parasympathetic mechanisms. The pressor response may also involve a direct effect on peripheral vessels or a non-₁-adrenergic efferent pathway.