Figure 723

Peripheral ai-adrenergic antagonists. Prazosin is a lipophilic highly selective ai-adrenergic antagonist. It is absorbed well (approximately 90%) but undergoes variable first-pass hepatic metabolism. Peak plasma concentrations occur in 2 to 3 hours. It is extensively metabolized by the liver and predominantly excreted in the feces. The plasma half-life of prazosin (2 to 4 hours) is not prolonged in patients with renal insufficiency.

Terazosin is a water-soluble quinazoline analogue of prazosin with about one third of its potency. It is completely absorbed and undergoes minimal first-pass hepatic metabolism. Peak plasma concentrations occur in 1 to 2 hours. It is extensively

Postganglionic sympathetic neuron -

Varicosities

Synaptic cleft

Varicosity

Nerve impulse induces exocytotic NA release

Varicosities

Synaptic cleft

Postsynaptic a- receptors

Target organ

Nerve impulse induces exocytotic NA release

Vesicle containing NA

Sympathetic C-fiber

-Presynaptic a-receptor

Postsynaptic a-receptor

Response

Postsynaptic a-receptor

Postsynaptic a- receptors

Target organ

Vesicle containing NA

Sympathetic C-fiber

-Presynaptic a-receptor

Synaptic cleft

Response metabolized by the liver and predominantly excreted in the feces. The plasma half-life of terazosin (approximately 12 hours) is not prolonged in patients with renal insufficiency.

Doxazosin is also a water-soluble quinazoline analogue of prazosin, with about half its potency. It is absorbed well but undergoes significant first-pass hepatic metabolism; bioavailability is approximately 65%. Peak concentrations occur in 2 to 3 hours. It is extensively metabolized by the liver and primarily eliminated in the feces. The plasma half-life of doxa-zosin (approximately 22 hours) is not prolonged in patients with renal insufficiency [6,9].

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