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BASIC PHARMACOLOGY OF ANTIHYPERTENSIVE AGENTS
All antihypertensive agents act at one or more of the four ana-tomic control sites depicted in Figure 11–1 and produce their effects by interfering with normal mechanisms of blood pressure regulation. A useful classification of these agents categorizes them according to the principal regulatory site or mechanism on which they act (Figure 11–3). Because of their common mechanisms of action, drugs within each category tend to produce a similar spec-trum of toxicities. The categories include the following:
1. Diuretics, which lower blood pressure by depleting the bodyof sodium and reducing blood volume and perhaps by other mechanisms.
2. Sympathoplegic agents, which lower blood pressure by reduc-ing peripheral vascular resistance, inhibiting cardiac function, and increasing venous pooling in capacitance vessels. (The lat-ter two effects reduce cardiac output.) These agents are further subdivided according to their putative sites of action in the sympathetic reflex arc .
3. Direct vasodilators, which reduce pressure by relaxing vascu-lar smooth muscle, thus dilating resistance vessels and—to varying degrees—increasing capacitance as well.
4. Agents that block production or action of angiotensin andthereby reduce peripheral vascular resistance and (potentially) blood volume.
The fact that these drug groups act by different mechanisms permits the combination of drugs from two or more groups with increased efficacy and, in some cases, decreased toxicity. (See Box: Resistant Hypertension & Polypharmacy.)
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