Effect of the Centrally Acting Agent Clonidine on Circulating Catecholamines at Rest and During Exercise

Comparison with the Effects of Beta-Blocking Agents

¹ From the Department of Internal Medicine III, Medical University Clinic of Heidelberg, Germany

To determine the amount of sympathetic outflow suppression due to the ₂-adrenergic receptor stimulating agent clonidine, its effect on heart rate, blood pressure, and on circulating plasma catecholamines was assessed in ten healthy subjects at rest and during submaximal ergometric exercise. Similar exercise studies were performed in eight healthy normotensive persons after β-blockade using propanolol. In nine healthy subjects the effect of an acute intravenous (IV) intervention of clonidine on plasma catecholamines was compared with the results obtained after IV administration of the cardioselective β-blocker metoprolol or of both drugs. Clonidine taken orally produced significantly reduced plasma levels of epinephrine and norepinephrine (p<0.01). During submaximal ergometric exercise, the sympatholytic effect of clonidine was relatively less marked than at rest (p<0.05). After β-blockade, either orally with propanolol or IV with metoprolol, plasma catecholamines at rest did not change significantly; their plasma levels during exercise, however, exceeded those obtained after administration of placebo (p<0.05). Giving IV clonidine and metoprolol combined revealed no significant changes of plasma catecholamines at rest and during exercise; heart rate and blood pressure decreased significantly (p<0.001). Clonidine on the one hand and the β-blocking agents on the other exhibit oppositely directed effects on plasma levels of catecholamines.