Effect of clonidine on sympathetic nervous activity in hydralazine-treated hypertensive patients

Summary

Six patients with arterial hypertension were studied in hospital. They were given hydralazine iv in increasing doses, (2 mg, 3 mg, 5 mg, 6 mg, 10 mg) every 15 min, until the blood pressure was normal or side effects were encountered. Clonidine 150–600 µg/day was administered orally. Dose-blood pressure and heart rate-response curves to hydralazine and cold pressor test were performed when the patients were not taking drugs and after one week of clonidine administration; the urine noradrenaline excretion rate was also measured. The maximal iv dose of hydralazine reduced mean blood pressure from 133.7±7.54 (mean ± SEM) to 118.5±5.84 mm Hg (P<0.05) and increased heart rate from 77.3±2.88 to 102.2±4.36 beats/min (P<0.001). Clonidine decreased mean blood pressure from 133.7±7.54 to 116.8±7.75 mm Hg (P<0.001) and heart rate from 77.3±2.88 to 63.8±4.91 beats/min (P<0.001). During administration of maximal intravenous doses of hydralazine to patients treated with oral clonidine, mean blood pressure decreased from 116.8±7.75 to 98.4±3.61 mm Hg (P<0.001) and heart rate increased from 63.8±4.91 to 76.0±5.54 beats/min (P<0.001). The heart rate response to hydralazine was dose-related and the dose-response curve was shifted to the right by clonidine. The increase in mean blood pressure induced by the cold pressor test was significantly attenuated by clonidine (P<0.05). Clonidine decreased significantly the urine noradrenaline excretion rate from 43.78±9.31 to 13.66±3.65 µg/24 hr (P<0.05).