Parasympatholytics (Anticholinergic Agents) 161

Anticholinergic or cholinergic blocking
agents are the agents which block the action
of acetylcholine at the postganglionic
parasympathetic nerve endings. They are
also termed as antimuscarinic or muscarinic
blockers and atropine is the classical
antagonist which blocks the effect of
acetylcholine on muscarinic receptors. The
nicotinic antagonists also block certain
actions of acetylcholine and are termed as
ganglion blocking agents.

The anticholinergics can be classified as
in table 3.5.1.

The cholinergic blocking agents which
mainly include atropine and related al-
kaloid are obtained from the plant Atropa
belladonna (deadly nightshade), Atropa
acuminata, Hyoscyamus niger (black hen-
bane) and Datura stramonium (datura) and
semisynthetic atropine/hyoscine ana-
logues and synthetic compounds.

The two important alkaloids of
belladonna are atropine and hyoscine
(scopolamine).

ATROPINE

The alkaloids namely atropine,

hyoscyamine and scopolamine are obtained

from Atropa belladonna. Atropine is dl-

hyoscyamine, and, l-isomer is more potent

than d-form both peripherally and centrally.

Atropine blocks the muscarinic effects of

acetylcholine, the antagonism between

acetylcholine and atropine is of competitive

type.

Pharmacological Actions

Pharmacological actions of atropine are

due to equal blockade of M 1 , M 2 and M 3

muscarinic receptors.

Effect on cardiovascular system:

Atropine initially decrease the heart rate due

to stimulation of vagal centre followed by

tachycardia due to peripheral vagal block

on SA node. It also shortens effective

refractory period of AV node and facilitates

AV conduction. In therapeutic doses,

atropine completely blocks the peripheral

vasodilatation and decrease in blood

pressure produced by cholinergic agents.

(Mode of Action of Drugs)

PharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamicsPharmacodynamics

Chapter

1.4

Chapter

3.5

ParasympatholyticsParasympatholytics

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