136 Section 3/ Drugs Acting on ANSdominated by a reflex response to
blood pressure changes.
c. Blood pressure: The enhanced arterial
constriction may lead to a marked rise
in blood pressure. In the presence of
normal cardiovascular reflexes, the rise
in BP elicits a baroreceptor mediated
increase in vagal tone with slowing of
the heart rate.
Eye: The radial pupillary dilator muscle
of the iris contains alpha-receptors; activation
by drugs causes mydriasis.
Respiratory tract: The blood vessels of
the upper respiratory tract mucosa contain
α 1 -receptors; the decongestant action of
alpha stimulants is clinically useful.
Gastrointestinal tract: Relaxation of GIT
smooth muscle can be brought about by both
α and β stimulant agents. Alpha stimulants
especially α 2 selective agonists, decrease
muscle activity indirectly by presynaptically
reducing the release of catecholamines. α 2 -
receptors may also decrease salt and water
flux into the lumen of the intestine.
Effect on endocrine function: Catechola-
mines are important endogenous regulators
of hormone secretion from a number of
glands. Insulin secretion is stimulated by
beta receptors and inhibited by α 2 -receptors.
Similarly, renin secretion is stimulated by
β 1 and inhibited by α 2 receptors; indeed,
beta-receptor antagonists may lower plasma
renin at least part by this mechanism.
Adverse effects of dopamine include
nausea, vomiting, ectopic beats, anginal pain,
tachycardia, palpitation and widened QRS.
Contraindications are atrial or ventricu-
lar tachyarrhythmias, hyperthyroidism and
pheochromocytoma.
It is indicated in shock syndrome due
to MI, trauma, septicaemia, heart surgery,
renal failure and chronic cardiac failure.DOBUTAMINE
It is a derivative of dopamine and has
relatively β 1 -selective action and it also
activates α 1 receptors and do not have D 1
or D 2 receptor agonistic property. It
increases the force of myocardial
contraction and cardiac output without
significant change in heart rate, blood
pressure and peripheral resistance. It is
used as inotropic agent and for short term
management of CHF and also in patients
who are unresponsive to digitalis.EPHEDRINE
Ephedrine is an alkaloid obtained from
‘Ephedra vulgaris’ plant. It act indirectly and
directly on α and β receptors. It increases
blood pressure both by peripheral
vasoconstriction and by increasing the
cardiac output. Ephedrine also relaxes the
bronchial smooth muscles.
Ephedrine stimulates CNS and produces
restlessness, insomnia, anxiety and tremors.
Ephedrine produces mydriasis on local as
well as systemic administration.
Ephedrine is useful for the treatment of
chronic and moderate type of bronchial
asthma, used as nasal decongestant and as
a mydriatic without cycloplegia. It is also
useful in preventing ventricular asystole in
Stokes Adams syndrome. It is also used in
narcolepsy, however amphetamines are the
drug of choice.PSEUDOEPHEDRINE
Pseudoephedrine appears to have less
pressor activity and weaker central nervous