L

T ABLE H.l Comparative Information About the Three Classes of «-Adrenoceptor Antagonists

Haloalkylamines

Imidazolines

Quinazolines

Prototype

Others

Antagonism

Selectivity Hemodynamic effects

Actions other than a-blockade

Routes

Adverse reactions

Therapeutic uses

Phenoxybenzamine (Dibenzyline)

Irreversible (non-equilibrium) competitive

Somewhat selective for a1 Decreased peripheral vascular resistance and blood pressure Venodilation is prominent Cardiac stimulation occurs because of cardiovascular reflexes and enhanced release of norepinephrine

Some antagonism of responses to

ACh, 5-HT, and histamine Blockade of neuronal and extraneuronal uptake

Intravenous and oral Oral absorption incomplete and erratic

Postural hypotension, tachycardia, miosis, nasal stuffiness, failure of ejaculation

Conditions of catecholamine excess such as pheochromocytoma Peripheral vascular disease Benign prostatic hypertrophy

Phentolamine (Regitine) Tolazoline (Priscoline)

Equilibrium competitive

Nonselective

Similar to phenoxybenzamine

Cholinomimetic, adrenomimetic, and hista-minelike actions Antagonism of responses to 5-HT

Similar to phenoxybenzamine

Same as phenoxybenzamine and in addition gastrointestinal disturbances

Same as phenoxybenzamine

Prazosin (Minipress)

Terazosin (Hytrin) Doxazosin (Cardura) Trimazosin (Cardovar) Equilibrium competitive

Selective for a1

Decreased peripheral vascular resistance and blood pressure Veins seem to be less susceptible to antagonism than arteries; thus, postural hypotension is less of a problem Cardiac stimulation is less because release of norepinephrine is not enhanced At high doses some direct vasodilator action, probably due to phosphodiesterase inhibition

Oral

Some postural hypotension, especially with the first dose;less of a problem overall than with phenoxybenzamine or phento-lamine Primary hypertension Benign prostatic hypertrophy

ACh, acetylcholine;5-HT, 5-hydroxytryptamine.

or the action of adrenomimetics. Hemodynamically, the effects of prazosin differ from those of phenoxybenza-mine and phentolamine in that venous smooth muscle is not as much affected by prazosin. Postural hypotension during chronic treatment is also less of a problem. Also, increases in heart rate, contractile force, and plasma renin activity, which normally occur after the use of vasodilators and a-blockers, are much less prominent following chronic treatment with prazosin.

Phenoxybenzamine and phentolamine, in addition to blocking postsynaptic a-receptors, also block a2-receptors on nerves and therefore can enhance the release of norepinephrine. When norepinephrine exerts a postsynaptic action by means of (3-adrenoceptors (e.g., cardiac stimulation, renin release), blockade of presy-naptic a2-receptors by phenoxybenzamine and phentol-amine may actually potentiate the responses. Prazosin blocks responses mediated by postsynaptic a1-receptors but has no effect on the presynaptic a2-receptors. Thus, stimulation of the heart and renin release is less prominent with this drug.

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