Ethylketocyclazocine decreases noradrenaline release and blood pressure in the rabbit at a peripheral opioid receptor

Abstract

Summary Rabbits were pithed and their sympathetic outflow was stimulated electrically via the pithing rod. Arterial blood pressure, heart rate, the endogenous plasma noradrenaline level, the plasma ³H-noradrenaline clearance and the noradrenaline release rate (the rate of entry of endogenous noradrenaline into the plasma) were determined. Ethylketocyclazocine 0.1 mg kg⁻¹+0.02 mg kg⁻¹ h⁻¹ and 1 mg kg⁻¹ + 0.2 mg kg⁻¹ h⁻¹ but not 0.01 mg kg⁻¹+ 0.002 mg kg⁻¹ h⁻¹ decreased blood pressure, the endogenous plasma noradrenaline level and the noradrenaline release rate. The effects of ethylketocyclazocine 1 mg kg⁻¹+ 0.2 mg kg⁻¹ h⁻¹ were antagonized by naloxone 1 mg kg⁻¹ + 0.5 mg kg⁻¹ h⁻¹. Given alone, naloxone caused no change. It is concluded that ethylketocyclazocine inhibits action potential-evoked release of noradrenaline from postganglionic sympathetic neurones, and hence can lower blood pressure, by a peripheral effect, possibly mediated by opioid receptors at the terminal axons.