Effects on the Cardiovascular System and Vascular Tone
The primary cardiac target of muscarinic antagonists is the $M_2$ muscarinic receptor. By blocking these receptors, the drugs abolish the inhibitory vagal tone. In the sinoatrial (SA) node, this leads to increased automaticity and tachycardia, while in the atrioventricular (AV) node, it improves conduction velocity.
Interestingly, low doses may cause a paradoxical transient bradycardia. This is attributed to central nervous system (CNS) vagal stimulation or the blockade of presynaptic autoreceptors, which initially enhances acetylcholine release.
Under resting conditions, muscarinic antagonists have little direct effect on vascular tone because blood vessels lack parasympathetic innervation (receptors are located on the endothelium). However, they can prevent vasodilation induced by administered muscarinic agonists.
Effects on Smooth Muscle and Glands
By acting on $M_3$ receptors, these agents counteract stimulatory parasympathetic input, producing a spasmodic and relaxant effect:
- Gastrointestinal tract motility is decreased.
- The detrusor muscle of the urinary bladder relaxes.
- Bronchial smooth muscle tone is reduced.
- Key feature: Conversely, sphincter tone is increased.
Blockade of these same $M_3$ receptors leads to generalized suppression of exocrine gland secretions (bronchial, salivary, lacrimal, and sweat glands). Blockade of $M_1$ receptors in the stomach decreases histamine release, ultimately reducing hydrochloric acid secretion.
Clinical Applications
Due to their diverse pharmacodynamic profile, muscarinic antagonists (especially atropine) are used across multiple medical specialties:
- Ophthalmology: Used for funduscopic examination (inducing prolonged mydriasis) and determining true refractive error (inducing cycloplegia). They are also used in inflammatory conditions such as iridocyclitis to provide functional rest and prevent synechiae.
- Cardiology: Management of vagally mediated AV block and symptomatic sinus bradycardia.
- Pulmonology: Relief of bronchospasm. Inhaled quaternary ammonium compounds are preferred because, unlike atropine, they are poorly absorbed across mucosal surfaces and lack systemic side effects.
- Anesthesiology: Used in pre-anesthetic medication to prevent reflex bradycardia, reduce glandular secretions, and minimize the risk of laryngospasm.
- Toxicology: Used as an antidote in poisonings caused by anticholinesterase agents (including organophosphates) and muscarinic agonists.
- Neurology: Central nervous system-penetrating agents are used in Parkinson's disease to reduce tremor and rigidity.