Minoxidil compensatory responses

Drugs that dilate blood vessels by acting directly on smooth muscle cells through nonautonomic mechanisms are useful in treating many hypertensive patients. Three major mechanisms are utilized by vasodilators release of nitric oxide, opening of potassium channels (which leads to hyperpolarization), and blockade of calcium channels (Table 11-3). Compensatory responses are marked for some vasodilators (especially hydralazine and minoxidil) and include salt retention and tachycardia (Table 11-2). 

A. Hydralazine and Minoxidil These older vasodilators have more effect on arterioles than on veins. They are orally active and suitable for chronic therapy. Hydralazine apparently acts through the release of nitric oxide. However, it is rarely used at high dosage because of its toxicity therefore, its efficacy is limited, fts toxicities include compensatory responses (tachycardia, salt and water retention Table 11-2) and drug-induced lupus erythematosus, which is reversible upon stopping the drug. However, this effect is uncommon at dosages below 200 mg/d. 

Minoxidil is extremely efficacious and is thus reserved for severe hypertension. Minoxidil is a prodrug its metabolite, minoxidil sulfate, is a potassium channel opener that hyperpolarizes and relaxes vascular smooth muscle. The toxicity of minoxidil consists of severe compensatory responses (Table 11-2, Figure 11-2), hirsutism, and pericardial abnormalities. 

Marked tachycardia and fluid retention are compensatory responses usually seen with strong vasodilators. The fact that the unknown drug also increases hair growth points strongly at minoxidil. The answer is (C). 

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