5.21: Cholinergic Antagonists: Pharmacological Actions
Antimuscarinic drugs block muscarinic receptors in multiple systems, including the gut, eye, smooth muscles, respiratory tract, cardiovascular, and central nervous systems. They produce similar effects with varying selectivity depending on the specific agent and tissue. Here are the key pharmacological actions of antimuscarinics:
Gastrointestinal Effects: Antimuscarinics reduce gut contractions, increase gastric emptying, and slow intestinal transit. They partly inhibit gastric acid secretion and significantly reduce salivary secretions. Mucin and proteolytic enzyme secretions are partially inhibited, while bile production remains unaffected.
Smooth Muscle Relaxation: Antimuscarinics relax smooth muscles in the bronchi, biliary, and urinary tract. They inhibit secretions in the mouth, nose, bronchi, and pharynx, leading to dryness of airway mucous membranes and preventing reflex bronchoconstriction. They also decrease bladder and ureter tone, reducing the urge to urinate.
Ocular Effects: Antimuscarinics cause pupil dilation (mydriasis) and paralysis of accommodation (cycloplegia) in the eye, impairing near vision. They increase intraocular pressure and inhibit lacrimal secretions, which can be problematic for individuals with narrow-angle glaucoma.
Cardiovascular Effects: Antimuscarinics increase heart rate, resulting in tachycardia. Low doses may paradoxically cause bradycardia. However, they have minimal impact on blood pressure, as cholinergic impulses do not regulate vascular tone. Nevertheless, tachycardia can indirectly elevate blood pressure.
Central Nervous System Effects: Antimuscarinics have excitatory effects on the central nervous system. Higher doses can cause hallucinations and agitation. They help prevent motion sickness, suppress rigidity and tremors in Parkinson's disease, and inhibit sweating, leading to dry and warm skin. They also possess mild anesthetic properties. Tertiary agents have more pronounced central effects as they can cross the blood-brain barrier.
Antimuscarinic drugs can have side effects that vary with the dose. These may include dry mouth, difficulty swallowing, flushing, fever, impaired gastric emptying, and heart palpitations. Higher doses can result in excitement, rapid pulse, hallucinations, psychotic behavior, cardiovascular collapse, convulsions, and coma.
