Pharmacology for Dentistry

Parasympathomimetics (Cholinergic Agents) 157

bladder and increase in the frequency of
ureteral peristaltic waves.

Acetylcholine causes bronchoconstriction
and increase in bronchial secretion and can
precipitate the bronchial asthma.

Effect on secretions: Acetylcholine
increases the salivary, sweat, lacrimal,
nasopharyngeal, gastric and bronchial
secretions. All these secretory effects
are blocked by atropine and enhanced
by cholinesterase inhibitors e.g. phys-
ostigmine.

Effect on eye: After intraconjunctival
instillation, acetylcholine causes miosis, due
to contraction of circular muscles of the iris
and fall in intraocular tension.

Nicotinic Actions

Effect on autonomic ganglia: It can
cause stimulation of sympathetic and
parasympathetic ganglia and stimulation of
adrenal medulla, which leads to rise in
arterial blood pressure due to peripheral
vasoconstriction.

Effect on myoneural junction:
Acetylcholine cause stimulation of skeletal
muscles and in larger concentration at the
myoneural junction can produce paralysis
of skeletal muscles.

Effect on CNS: Acetylcholine when
injected parenterally does not cross blood
brain barrier being a quarternary
ammonium compound and does not have
any central action.

Therapeutic Uses

Because of nonselective actions,
acetylcholine can not be used for any
therapeutic purpose.

CHOLINOMIMETIC DRUGS

METHACHOLINE

It is effective orally and resistant to

pseudo-cholinesterase and possesses longer

duration of action. Its nicotinic action is less

than acetylcholine and actions are more

marked on CVS as compared to GIT and

urinary system. Earlier it was used for CVS

disorders such as peripheral vascular

disease and paroxysmal supraventricular

tachycardia. But now, it is rarely used in

therapeutics.

CARBACHOL

It is resistant to both true and pseudo-

cholinesterase and is more potent than

methacholine and action is more prolonged.

It is used topically for ophthalmic purpose.

BETHANECHOL

It is resistant to hydrolysis by both true

and pseudocholinesterase and has mainly

muscarinic actions. It has been used in

postoperative and postpartum non-

obstructive urinary retention and

gastroesophageal reflux.

PILOCARPINE

It is a natural alkaloid obtained from leaves

of Pilocarpus microphyllus and Pilocarpus

jaborandi. Pilocarpine is direct acting

muscarinic agonist. It acts on M 3 receptor. It

produces contraction of iris to produce miosis.

It also stimulates ciliary muscle resulting in

increased accommodation and improved

outflow of aqueous humor. As a result of

miosis the pressure on canal of Schlemm is

reduced and hence improves drainage and