Anticholinergic Drugs and their Pharmacological actions

Anticholinergic Drugs and their Pharmacological actions

‘Anticholinergic drugs’ is restricted the drugs which block actions of Ach on CNS muscarinic receptors and autonomic effectors. This nicotinic receptor antagonists also block some actions of ACh, they are generally known as ‘neuromuscular blockers’ and ‘ganglion blockers’.

Ex- Atropine- It is highly work for muscarinic receptors, but it also synthetic substitutes do possess significant nicotinic blocking property in addition.

CLASSIFICATION

1. Natural alkaloids - Hyoscine, Atropine

2. Semisynthetic derivatives - Atropine methonitrate, Homatropine, Atropine methonitrate, Hyoscine butyl bromide, Ipratropium bromide, Tiotropium bromide.

3. Synthetic compounds-

(a) Mydriatics: Cyclopentolate, Tropicamide.

(b) Antisecretory-antispasmodics:

(i) Quaternary compounds: Propantheline, Oxyphenonium, Clidinium, Pipenzolate methyl bromide, Isopropamide, Glycopyrrolate.

(ii) Tertiary amines: Dicyclomine, Valethamate, Pirenzepine.

(c)Vasicoselective: Oxybutynin, Flavoxate,

(d)Antiparkinsonian: Trihexyphenidyl (Benzhexol), Procyclidine, Biperiden.

PHARMACOLOGICAL ACTIONS

Central nervous system

Atropine has an overall CNS stimulant action. However, these effects are not appreciable at low doses which produce only peripheral effects because of restricted entry into the brain Atropine stimulates many medullary centers—vagal, respiratory, vasomotor. On excessive use it causes cortical excitation, disorientation, hallucinations, restlessness, delirium followed by respiratory depression and coma.

Cardiovascular system

Heart the most prominent effect of atropine is tachycardia. It blocks the M2 receptors on the SA node through which vagal tone decreases HR. Higher the existing vagal tone— more marked is the tachycardia (maximum in young adults, less in children and elderly). On i.m./s.c. injection transient initial bradycardia often occurs. Earlier believed to be due to stimulation of vagal center, it is now thought to be caused by blockade of muscarinic auto receptors (M1) on vagal nerve endings, thereby 

augmenting Ach release.

Fig- Cholinergic-Blocking Drugs acts on cardiac muscle and smooth muscle.

Blood pressure

Since cholinergic impulses are not involved in the maintenance of vascular tone, atropine does not have any consistent or marked effect on BP.

Eye

The autonomic control of iris muscles and the action of mydriatics as well as miotics is illustrated. If they are use topical it makes mydriasis. Atropine shows cycloplegia and abolition in light reflex, lasting 6–10 days. This results in photophobia and blurring of vision.