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Cholinergic Antagonists: Pharmacological Actions
Antimuscarinic drugs block muscarinic receptors in multiple systems, including the gut, eye, smooth muscles, respiratory tract, cardiovascular, and ce…
Cholinergic antagonists, such as antimuscarinics, block muscarinic receptors to inhibit muscarinic functions in the body.
Tertiary antimuscarinics can cross the BBB and have greater excitatory central effects than quaternary agents.
They also inhibit perspiration and regulate the hypothalamus, increasing the body temperature.
In the eye, antimuscarinics relax the ocular ciliary muscles, inducing mydriasis and cycloplegia. Furthermore, they decrease aqueous humor drainage, leading to increased intraocular pressure.
In the respiratory system, antimuscarinics relax the smooth muscles and reduce secretions, to prevent bronchoconstriction.
Following administration, antimuscarinics often cause tachycardia, leading to an increase in blood pressure.
Within the gut, inhibition of the parasympathetic stimulation by antimuscarinics reduces gut motility and secretions.
In the urinary system, by limiting parasympathetic inputs, antimuscarinics decrease bladder contraction to effectively inhibit micturition.
Antimuscarinics have dose-dependent side effects, and severe cases of poisoning are associated with convulsions and coma.
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Q1: How do antimuscarinics affect the eye and vision?
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.
Q2: What respiratory effects do antimuscarinics produce?
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.
Q3: How do antimuscarinics affect heart rate and blood pressure?
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.
Q4: What gastrointestinal effects result from antimuscarinic use?
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.
Q5: How do tertiary antimuscarinics differ from quaternary agents in their central effects?
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.
Q6: What effects do antimuscarinics have on the urinary system?
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.
Q7: What are the dose-dependent side effects of antimuscarinics?
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.