Effect of the centrally acting agent clonidine on circulating catecholamines at rest and during exercise. Comparison...

To determine the amount of sympathetic outflow suppression due to the alpha 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 beta-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 beta-blocker metoprolol or of both drugs. Clonidine taken orally produced significantly reduced plasma levels of epinephrine and norepinephrine (p less than 0.01). During submaximal ergometric exercise, the sympatholytic effect of clonidine was relatively less marked than at rest (p less than 0.05). After beta-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 less than 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 less than 0.001). Clonidine on the one hand and the beta-blocking agents on the other exhibit oppositely directed effects on plasma levels of catecholamines.