Failure of naloxone to reduce the clonidine induced reduction of blood pressure and plasma noradrenaline in patients with essential hypertension.

Studies in animals and man indicate a functional interaction between the adrenergic and the opiate systems. In the present study the effect of the opiate receptor blocker naloxone on the reduction of blood pressure and plasma noradrenaline induced by the alpha 2-agonist clonidine was investigated in nine patients with essential hypertension. In a randomised manner the patients received a bolus dose of naloxone (10 micrograms/kg) or physiological saline followed by a slow infusion of naloxone (5 micrograms/kg/h) or saline, respectively. Fifteen minutes after the respective bolus dose, clonidine (3 micrograms/kg) was infused over 10 minutes. Naloxone had no effect on the clonidine induced hypotension and reduction of plasma noradrenaline. Accordingly, there is no evidence that the clonidine induced reduction of blood pressure and plasma noradrenaline involves opiate receptors that can be blocked by naloxone. Plasma adrenaline increased significantly during the early phase of naloxone infusion.