Cholinergic Antagonists: Pharmacological Actions

Antimuscarinics relax the ocular ciliary muscles, causing mydriasis (pupil dilation) and cycloplegia (paralysis of accommodation), which impairs near vision. They also decrease aqueous humor drainage, increasing intraocular pressure, which can be problematic for individuals with narrow-angle glaucoma. Additionally, they inhibit lacrimal secretions, reducing tear production.

Antimuscarinics relax smooth muscles in the bronchi and inhibit secretions in the mouth, nose, bronchi, and pharynx. This leads to dryness of airway mucous membranes and prevents reflex bronchoconstriction. By reducing parasympathetic stimulation, they effectively prevent airway constriction and reduce respiratory secretions.

Antimuscarinics increase heart rate, resulting in tachycardia. Low doses may paradoxically cause bradycardia. While they have minimal direct impact on blood pressure since cholinergic impulses do not regulate vascular tone, tachycardia can indirectly elevate blood pressure through increased cardiac output.

Antimuscarinics reduce gut contractions and slow intestinal transit by inhibiting parasympathetic stimulation. They partly inhibit gastric acid secretion and significantly reduce salivary secretions. Mucin and proteolytic enzyme secretions are partially inhibited, while bile production remains unaffected, leading to reduced gut motility overall.

Tertiary antimuscarinics can cross the blood-brain barrier and have greater excitatory central nervous system effects than quaternary agents. They inhibit sweating, regulate the hypothalamus to increase body temperature, and possess mild anesthetic properties. Higher doses can cause hallucinations, agitation, and psychotic behavior.

Antimuscarinics decrease bladder contraction by limiting parasympathetic inputs, effectively inhibiting micturition. They also decrease bladder and ureter tone, reducing the urge to urinate. This antimuscarinic action on the urinary tract makes these drugs useful for managing urinary incontinence and overactive bladder conditions.

Antimuscarinics produce dose-dependent side effects ranging from mild to severe. Low doses cause dry mouth, difficulty swallowing, and flushing. Higher doses result in excitement, rapid pulse, hallucinations, and psychotic behavior. Severe poisoning is associated with cardiovascular collapse, convulsions, and coma, requiring immediate medical intervention.