Naloxone does not affect the cardiovascular, sedative or neurohormonal effects of clonidine in normal and hypertensive man.

The possibility that some of the cardiovascular, sedative or neurohormonal effects of clonidine are mediated by opiate receptors was investigated in normotensive and hypertensive subjects. In normal subjects intravenous (i.v.) clonidine lowered blood pressure, increased sedation and raised levels of plasma renin activity and growth hormone. Levels of other anterior pituitary hormones (prolactin, luteinizing hormone and follicle stimulating hormone) and of arginine vasopressin were unchanged. The effects of clonidine were similar after the administration of naloxone. In patients with essential hypertension clonidine lowered blood pressure, increased sedation and reduced plasma noradrenaline levels. There was an insignificant fall in levels of plasma renin activity. Prior administration of naloxone did not influence the effects of clonidine. It is concluded that the cardiovascular, sedative and neurohormonal effects of acutely administered clonidine are not dependent on opiate receptor activation in either normal or hypertensive man.