146 Section 3/ Drugs Acting on ANSconvert adrenaline induced stimulation
response to depressor response. Blockage of
vasoconstrictor receptors reduces peripheral
resistance, venous return and cardiac output
leading to fall in blood pressure. The
decrease in blood pressure by alpha blockers
can reduce renal blood flow, which can lead
to reduction of glomerular filtration rate and
more reabsorption of sodium and water in
the tubules and ultimately sodium retention
and increase in blood volume.
Positive inotropic and chronotropic
effects of catecholamines are not blocked by
alpha blockers, but these drugs can block
catecholamine induced cardiac arrhythmias.
Certain stimulant actions of
catecholamines on various smooth muscles
are blocked by alpha blockers such as
uterine contraction of certain species,
contraction of vas deferens and retractor
penis, stimulation of seminal vesicles and
vas deferens. Alpha blockers can inhibit
ejaculation and produce impotence. Salivary
secretion and sweat formation induced by
catecholamines is blocked by alpha blockers.
Alpha blockers also produce certain
metabolic effects such as inhibitory action
of adrenaline on insulin secretion is blocked
and adrenaline induced rise in blood
potassium level is also blocked.
Miosis and nasal stuffiness can occur
with alpha blockers by acting on radial
muscles of iris and nasal blood vessels.
Diarrhoea may occur due to increase in
intestinal motility.
The common adverse effects with alpha
blockers are palpitation, nasal stuffiness,
postural hypotension, retention of fluid,
diarrhoea, inhibition of ejaculation and
impotence.
PHENOXYBENZAMINE
It is a potent alpha-adrenergic blocking
agent and only haloalkylamine used
clinically. It effectively prevents the
responses mediated by alpha receptors and
diastolic blood pressure tends to decrease.
It interferes with the reflex adjustment of
blood pressure and produces postural
hypotension. It increases the cardiac output
and decreases the total peripheral resistance.
It also antagonizes cardiac arrhythmias
provoked by catecholamines. Apart from
these effects, phenoxybenzamine has other
actions also e.g. antagonism of acetylcholine,
histamine, 5-hydroxytryptamine (serotonin).
However, the vasodilatation produced by
phenoxybenzamine is because of alpha
blockage. Adverse reactions are miosis,
dryness of mouth, inhibition of ejaculation,
palpitation, nasal stuffiness and in higher
doses, postural hypotension and reflex
bradycardia.
It is used in the management of
pheochromocytoma and also to treat
peripheral vasospastic conditions e.g.
Raynaud’s disease and shock syndrome.
Phentolamine, another alpha blocker is
exclusively used for the diagnosis of
pheochromocytoma and for the prevention
of abrupt rise in blood pressure during
surgical removal of adrenal medulla tumors.ERGOT ALKALOIDS
Ergot is a parasitic fungus (Claviceps
purpurea). It contains different alkaloids of
complex chemical structure. The amino acid
ergot alkaloids are ergotamine, ergocristine,
ergocornine, ergosine, ergocryptine and their
dehydrogenated derivatives can block the