D. ALPHA RECEPTOR BLOCKING DRUGS +lt;tab5-2+gt; 1. Prazosin: Prazosin is an a1-selective b

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D. ALPHA RECEPTOR BLOCKING DRUGS 1. Prazosin: Prazosin is an a1-selective blocking drug. It is much more useful for the treatment of hypertension than the less selective - blockers phentolamine and phenoxybenzamine because it evokes little if any reflex tachycardia. Prazosin has also been used in the treatment of congestive heart failure (see Chapter 4). Terazosin is a newer, somewhat longer-acting a1-selective agent. Its therapeutic and adverse effects are very similar to those of prazosin. Indications: * Hypertension. * Congestive heart failure (unlabeled). * Raynaud's syndrome (unlabeled). Contraindications, Adverse Reactions, and Overdose Toxicity: * Severe postural hypotension after the first dose especially in hypovolemic (eg, diuresed) patients. Syncope may result. The effect is best managed by starting therapy with the minimum dose (1 mg) ad- ministered just after going to bed. The effect usually diminishes rapid- ly after the first dose and the dosage can then be incremented as re- quired. * Dizziness, headache, and sedation occur in 5-10% of patients at the start of therapy, diminishing thereafter. * Fluid retention often occurs in patients not also treated with a diuretic. * Overdose: Hypotension and CNS depression have been reported. Interactions: The drug would be expected to have additive effects with all other antihypertensive agents. 2. Phenoxybenzamine: Phenoxybenzamine is an irreversible -receptor blocker. It produces a powerful but nonselective sympathoplegia; com- pensatory responses include tachycardia and fluid retention. Indications: * Pheochromocytoma * Reduction of urinary bladder sphincter tone in urinary bladder dis- orders (unlabeled). Contraindications and warnings: * Orthostatic hypotension (warning), especially if it would result in risk of stroke or myocardial ischemia (true of all hypotensive agents). Adverse effects and Overdosage Toxicity: * Excessive alpha sympathetic blockade: hypotension, nasal congestion, miosis, and impairment of ejaculation. * Nausea, vomiting, diarrhea. * Sedation. 3. Phentolamine: Phentolamine is a competitive alpha receptor blocking drug. It is not selective for 1 or 2 receptors. Indications: * Treatment of acute hypertensive reactions in patients with pheochromocytoma. It is also approved for use as a provocative test for pheochromocytoma, although this test is hazardous (because of the risk of excessive hypotensive response) and has been superseded by measure- ment of urinary or plasma catecholamines and their metabolites. Phentolamine is also labeled for the treatment of tissue necrosis due to extravasation of norepinephrine or dopamine. * Management of hypertensive crises arising from excessive sympathomimetic action as in MAO inhibitor drug reactions and rebound hypertension following clonidine withdrawal (unlabeled). Contraindications and warnings: Same as for phenoxybenzamine. Adverse effects: * Sympathoplegic adverse effects, similar to those listed for phenoxybenzamine but shorter in duration (15 min to 2 hr as compared to 24 to 48 hr for phenoxybenzamine). * Gastrointestinal stimulation with increased acid secretion, nausea, vomiting, and diarrhea. * Overdose is characterized by transient hypotension that can usually be managed by administration of fluids.


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