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Section IIICardiovascular drugs
Urinary – it relaxes the bladder trigone and sphincter muscle thereby improving
urine flow in those with benign prostatic hypertrophy. Impotence and priapism
have been reported.
Central nervous system – fatigue, headache, vertigo and nausea all decrease with
continued use.
Miscellaneous – it may produce a false-positive when screening urine for metabo-
lites of noradrenaline (VMA and MHPG seen in phaeochromocytoma).Kinetics
Plasma levels peak about 90 minutes following an oral dose with a variable oral
bioavailability of 50–80%. It is highly protein bound, mainly to albumin, and is
extensively metabolized in the liver by demethylation and conjugation. Some of
the metabolites are active. It has a plasma half-life of 3 hours. It may be used safely
in patients with renal impairment as it is largely excreted in the bile.Selectiveα 2 -blockade
Yohimbine
The principal alkaloid of the bark of the yohimbe tree is formulated as the hydrochlo-
ride and has been used in the treatment of impotence. It has a variable effect on the
cardiovascular system resulting in a raised heart rate and blood pressure but may
precipitate orthostatic hypotension. In vitro it blocks the hypotensive responses of
clonidine. It has an antidiuretic effect and can cause anxiety and manic reactions. It
is contraindicated in renal or hepatic disease.β-Adrenoceptor antagonists
β-Adrenoceptor antagonists (β-blockers) are widely used in the treatment of hyper-
tension, angina and peri-myocardial infarction.
They are also used in patients with phaeochromocytoma (preventing the reflex
tachycardia associated withα-blockade), hyperthyroidism (propranolol), hyper-
trophic obstructive cardiomyopathy (to control infundibular spasm), anxiety associ-
ated with high levels of catecholamines, topically in glaucoma, in the prophylaxis of
migraine and to suppress the response to laryngoscopy and at extubation (esmolol).
They are all competitive antagonists with varying degrees of receptor selectivity.
Inaddition some have intrinsic sympathomimetic activity (i.e. are partial agonists),
whereas others demonstrate membrane stabilizing activity. These three features
form the basis of their differing pharmacological profiles. Prolonged administration
may result in an increase in the number ofβ-adrenoceptors.Receptor selectivity
Insuitable patients, the useful effects ofβ-blockers are mediated via antagonism
ofβ 1 -adrenoceptors, while antagonism ofβ 2 -adrenoceptors results in unwanted