Cardiovascular Hemodynamic Interactions between Clonidine and Minoxidil in Hypertensive Patients

¹ From the Clinical Pharmacology Section, Department of Pharmacology and Division of Cardiology, Vargas Medical School, Central University of Venezuela, Caracas, Venezuela

The systemic, cardiovascular hemodynamic and biochemical interactions between clonidine and minoxidil were studied in ten patients with refractory and/or accelerated hypertension. Clonidine in oral doses of 150 to 900 µg/day decreased mean blood pressure (MAP) 18.6 mm Hg (p <0.01), average heart rate (HR) 16.4 bpm (p <0.01), limb blood flow 1.63 ml/100 g min (p <0.05), plasma renin activity (PRA) 1.13 ng/ml/hr (p <0.025), and urinary noradrenaline excretion rate 16.45 µg/24hr (p <0.05). Clonidine increased the preejection period index (PEPI) 12.4 msec (p <0.001), but did not alter cardiac index (CI), total peripheral resistance index (TPRI), limb vascular resistance nor dopamine β-hydroxylase activity. When minoxidil in oral doses of 5 to 22.5 mg was added, a further decrease in MAP of 24.2 mm Hg (p <0.01) was observed; PEPI decreased 20.6 msec (p <0.01), limb blood flow decreased 13.2 mm Hg/min 100

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g/ml (p <0.05), and total peripheral resistance index decreased 13.3 mm Hg/min m²/L (p <0.05). Minoxidil increased average heart rate 8.2 bpm (p <0.05), PRA 1.68 ng/ml/hr (p <0.05) and urinary noradrenaline excretion rate 5.0 µg/24 hr (p <0.01). Limb blood flow, cardiac index and dopamine β hydroxylase activity were not significantly altered by minoxidil. Neither clonidine nor minoxidil affected cardiovascular responses to treadmill exercise. We concluded that clonidine is a useful alternative agent to block a minoxidil-induced increase in sympathetic nervous activity.