148 Section 3/ Drugs Acting on ANSIt is another α 1 -selective blocker used in
hypertension and benign prostatic
hypertrophy.
DOXAZOSIN
It is another potent and selective α 1 -
adrenoceptor antagonist and quinazoline
derivative. It’s antihypertensive effect is
produced by a reduction in smooth muscle
tone of peripheral vascular beds.
Doxazosin is well absorbed after oral
administration and approximately two third
of the dose is bioavailable.
TAMSULOSIN
It is uroselective α1A blocker and has
been found effective in improving BPH
symptoms.
Adverse effects include dizziness and
retrograde ejaculation.
YOHIMBINE
It is an alkaloid obtained from an
African plant Yohimbehe. Chemically it is
an indolealkylamine related to reserpine.
It has selective α 2 blocking property with
short duration of action and also blocks 5-
hydroxytryptamine receptors. It produces
increase in heart rate and blood pressure
due to increase in noradrenaline release.
It does not have any role in clinical
practice.
Therapeutic Uses of Alpha Blockers
- Diagnosis and treatment of
pheochromocytoma:Phenoxybenzamine
is used in the treatment of inoperable
pheochromocytoma. Phentolamine
test is used for the diagnosis of
pheochromocytoma.
2. Hypertension: The selective α 1 blockers
such as prazosin, terazosin, doxazosin
etc. are used in the management of
essential hypertension.
3. Congestive heart failure: The selective
α 1 blocker affords symptomatic relief
in congestive heart failure in short
term.
4. Peripheral vascular disease: In
Raynaud’s phenomenon, the drugs like
prazosin, phenoxybenzamine produce
a symptomatic relief.
5. Alpha blockers can be used in
secondary shock due to any blood/
fluid loss accompanied by reflex
vasoconstriction.
6. Benign prostate hypertrophy: Alpha
receptor blockers increase urinary flow
rate and causing more complete
emptying of urinary bladder in benign
prostate hypertrophy patients.
7. Migraine: Ergotamine is used in the
treatment of migraine.
8. Erectile dysfunction: Injection of papav-
erine with or without phentolamine in
the corpus cavernosum has been found
to be effective in erectile dysfunction of
penis.
Other drugs used for erectile dysfunction
Sildenafil: It is orally active selective in-
hibitor of phosphodiesterase type 5 useful in
treatment of erectile dysfunction. It results
in reduced breakdown of cyclic guanosine
monophosphate (cGMP) which is responsible
for nitric acid (NO) mediated vasodilatation
in corpora cavernosa. Thus inducing an erec-
tile response to sexual stimulation. It has no
direct relaxant effect on smooth muscle of
corpus cavernosa and has no effect in absence
of sexual stimulation.