- Anticholinergics are those antagonize the effect of neurotransmitter (Ach) on autonomic effectors and in the CNS.
- Natural alkaloid
e.g. Atropine, Hyoscine (Scopolamine)
- Semisynthetic derivatives
e.g. Homatropine, Atropine Methonitrate, Hyoscine butyl bromide, Ipratropium bromide, Tiotropium bromide
- Synthetic derivatives
A. Mydriatics: e.g. Cyclopentolate, Tropicamide
B. Quaternary Compound: e.g. Propantheline, Oxyphenonium, Clidinum, Pipenzolate, Methylbromide, Isopropamide, Glycopyrolate.
C. Tertiary amine: e.g. Dicyclomine, Valethamate, Pirenzepine.
D. Vasicoselective: e.g. Oxybutynin, Flavoxate, Tolterodine.
E. Anti-parkinsonian: e.g. Bipiridine, Procyclidine, Trihexyphenidyl (Benzhexol)
Mechanism of action
- Anticholinergics are the class of drugs that blocks the neurotransmitter (Ach) in CNS and PNS.
- It combine reversibly with muscarinic cholinergic receptor thus preventing access of neurotransmitter (Ach) in these sites.
- Atropine sulphate is a tertiary amine.
- Administered i.v./i.m. in a range of 0.01-0.02mg/kg up to 0.4-0.6mg (Adult dose)
- Cardiac vagal nerve is completely blocked with larger dose (severe bradycardia).
- It shows CNS stimulant action, but not appropriate at low dose.
- Stimulate many medullary centres, vagal respiratory, vasomotor (controlling the internal diameter of blood volume).
- In high dose causes: Cortical excitation, Restlessness, Disorientation (A state of mental confusion), Hallucination and delirium (mental confusion).
- Tachycardia (Most prominent), due to blockade of M2 receptor at SA node.
- Topical ingestion of Atropine causes Mydriasis (Paralysis of sphincter pupillae).
- Paralysis of Accommodation (Cycloplegia)
- Increase in IOP.
- Smooth muscle
- GIT relaxation: Mediated by M3 blockade.
- Contraction of stomach and intestine is reduced leads to constipation.
- Bronchodilation: Especially COPD and Asthma patient.
- Also antagonizes histamine, prostaglandin, leukotrienes, mediated vagal over activity.
- Relaxation of ureter and bladder.
- Atropine markedly decrease sweat, salivary tracheobronchial and lacrimal secretion by M3 blockade.
- Skin and eyes becomes dry, talking and swallowing may be difficult.
- Body temperature
- Increase temperature, due to inhibition of sweating as well as stimulation of temperature regulating centre in the hypothalamus.
- Local anesthetic
- Mild anesthetic action on the cornea.
- Sensitive to different organs and tissue: saliva, sweat, bronchial smooth muscle, heart smooth muscle, intestine, GIT.
- Rapidly absorbed from g.i.t.
- Freely penetrate cornea.
- Crosses BBB is somewhat restricted.
- About 50% metabolized in liver.
- Excreted unchanged in urine.
- t ½ 3-4 hours.
- Hyoscine is more completely metabolized and has better blood-brain barrier penetration.