19.5: Antiasthma Drugs: Muscarinic Receptor Antagonists

Muscarinic receptor antagonists, also known as antimuscarinic agents, are a class of bronchodilators used to treat asthma, although they are more commonly used to treat COPD. They work by inhibiting the action of acetylcholine (ACh), a neurotransmitter, on muscarinic receptors found in the airways.

Antimuscarinic agents compete with ACh for the same binding site on the muscarinic receptors. By binding to these receptors, they inhibit the downstream effects of ACh and block the parasympathetic nerve impulses that cause bronchoconstriction and mucus production. Their action helps open the airways and improve breathing in people with asthma.

Among the antimuscarinic agents used to treat asthma are atropine and its quaternary derivative, ipratropium bromide. These drugs block all muscarinic receptor subtypes, relaxing the smooth muscles in the airways and reducing mucus secretion. The net effect is relief from wheezing, chest tightness, and shortness of breath.

These medications are usually administered through inhalation, ensuring direct delivery to the respiratory tract and minimizing systemic side effects. Ipratropium bromide, in particular, is a polar compound with poor systemic absorption. This allows it to be delivered in high doses directly to the airways, where it exerts its therapeutic effects.

While generally safe and well-tolerated, antimuscarinic agents can cause some systemic effects due to their action on muscarinic receptors in other body parts. These may include dry mouth, confusion, blurred vision, urinary retention, and constipation. Patients using these medications should be made aware of these potential side effects and advised to seek medical attention if they occur. Despite these possible side effects, antimuscarinic agents remain valuable in treating both COPD and asthma.

Muscarinic receptor antagonists, or antimuscarinic agents, are a class of bronchodilators used for asthma treatment. They inhibit the action of acetylcholine, a neurotransmitter that influences the airways through muscarinic receptors.

These agents compete with acetylcholine for the same binding site on the muscarinic receptor to inhibit acetylcholine's downstream effects and block parasympathetic functions like narrowing the pulmonary airways.

Antimuscarinics such as ipratropium bromide are key agents used in asthma therapy.

They block all the muscarinic receptor subtypes, leading to the relaxation of the smooth airway muscles and reduced mucus secretion.

These agents are typically administered via inhalation route to deliver high doses. This limits the CNS side effects of polar agents, such as ipratropium bromide, which are poorly absorbed into the systemic circulation.

Antimuscarinics are generally safe and well-tolerated for asthma management. They may trigger minor systemic effects, including dry mouth, confusion, blurred vision, urinary retention, and constipation.