Grug Therapy: α-Blockers
α-Blocking agents (prazosin, terazosin) and the combined αβ-blocker labetalol lower blood pressure by blocking postsynaptic vasoconstricting α-adrenergic receptors, thereby decreasing systemic vascular resistance. They cause less tachycardia than hydralazine but more orthostatic hypotension, because they also block venoconstriction and reduce venous return.
These medications sometimes produce a profound hypotensive response to the initial dose (the so-called dose phenomenon). This side effect is less likely if the first dose is given at bedtime and kept small (1 mg of prazosin, 1 mg of terazosin, 100 mg of labetalol). Tolerance, with loss of hypotensive effect, can develop with long-term therapy and limit the usefulness of α-blockers. The exact mechanism of this phenomenon is unknown.
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