To evaluate the role of the sympathetic nervous system in essential hypertension and the influence of clonidine, 28 male subjects with mild to moderate hypertension were either treated with low-dose clonidine (n = 14, mean age: 42.4 ± 2.1 years) or were randomized to a nontreated control group (n = 14, mean age: 40.2 ± 2.4 years). Clinical blood pressure and heart rate were assessed, and after 4 weeks of treatment plasma catecholamines and the hemodynamic response to mental arithmetic, cold stimulation, and ergometric exercise were compared between both groups. Under therapy with 0.075 mg of clonidine per day casual blood pressure dropped and diastolic BP was significantly lower than in the control group. Heart rate did not change. After 4 weeks the plasma levels of epinephrine and norepinephrine were the same in both groups. During mental arithmetic total peripheral resistance decreased in the treated group while it did not change in the control group, but both groups did not differ in their percentage changes in the hemodynamic parameters in response to the three stress tests. It has been concluded that clonidine reduces sympathoadrenergic activity, but in a low dosage this effect is obviously restricted to a reduction of arteriolar tone.
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