Minoxidil is a very efficacious orally active vasodilator. The effect results from the opening of potassium channels in smooth muscle membranes by minoxidil sulfate, the active metabolite. Increased potassium permeability stabilizes the membrane at its resting potential and makes contraction less likely. Like hydrala-zine, minoxidil dilates arterioles but not veins. Because of its greater potential antihypertensive effect, minoxidil should replace hydralazine when maximal doses of the latter are not effective or in patients with renal failure and severe hyperten-sion, who do not respond well to hydralazine.
Pharmacokinetic parameters of minoxidil are listed in Table 11–2. Even more than with hydralazine, the use of minoxidil is associ-ated with reflex sympathetic stimulation and sodium and fluid retention. Minoxidil must be used in combination with a β blocker and a loop diuretic.
Tachycardia, palpitations, angina, and edema are observed when doses of β blockers and diuretics are inadequate. Headache, sweat-ing, and hypertrichosis, which is particularly bothersome in women, are relatively common. Minoxidil illustrates how one person’s toxicity may become another person’s therapy. Topical minoxidil (as Rogaine) is used as a stimulant to hair growth for correction of baldness.