8-4. ALPHA ADRENERGIC BLOCKING AGENTS
Effects produced by these agents occur because the alpha-receptors are blocked
while beta-receptors are still capable of producing their effects.
(a) Phentolamine causes blockage of the alpha1 receptors. This
causes vasodilatation that results in decreased blood pressure.
(b) Phentolamine also causes blockage of alpha2 receptors. This
causes a release of norepinephrine. Since the normal effect of norepinephrine is
blocked at the alpha2 receptor, the effect of epinephrine on the cardiac beta-receptors
(2) Indication/use. Phentolamine is used to prevent or treat dermal necrosis
and sloughing caused by the extravasation (administration outside the vein) of
(3) Side effects. Phentolamine can cause side effects such as tachycardia,
flushing, cardiac arrhythmias, and orthostatic hypotension.
(1) Pharmacological actions. Prazosin is an antihypertensive agent that
selectively blocks alpha1 receptors. This drug produces vasodilation and reduces
peripheral resistance, but it produces little effect upon cardiac output.
Indications/uses. Prazosin is an antihypertensive agent.
(3) Cautions and warnings. This agent should be used caution with patients
who have severe cardiac disease or a history of mental depression.
(4) Side effects. Side effects associated with the use of prazosin include
dizziness, sudden fainting, drowsiness, and lack of energy.
c. Other alpha blockers include terazosin (Hytrin) and doxazosin (Cardura).
They are used for hypertension.