Acetylcholine is a neurotransmitter that when released, acts on muscarinic and nicotinic receptors (shown in slide). However, all other agonists and antagonists will differentiate between the two receptors. Despite their commonly used name, the “anticholinergic” drugs antagonize only the muscarinic receptors.
There are three principle muscarinic (M) receptors: M1= postganglionic and CNS, M2= postsynaptic in heart nodes and myocard, and M3= postsynaptic in smooth muscle, vascular endothelium and secretory glands. The M4 stimulates molecular mechanisms like the M2 receptors but its function is unclear. The M5 receptor is like the M1 receptor and is also found in the CNS, but its role is elusive.
Ipratropium and tiotropium are the two anticholinergics (or parasympatholytics) currently used. Both are only available for administration via the inhalational route.
Ipratropium (the older drug) has no selectivity for M1, M2, or M3 receptors and lasts for about 6 hours. However, tiotropium is a long-acting anticholinergic which can be used once daily. In addition, since its long-activity can be attributed to its relative slow dissociation from the M3 receptors versus M2 receptors, there is some selectivity for its action.
Anticholinergic drugs also affect mucus secretion.
Since systemic anticholinergic drugs can block all muscarinic receptors, tachycardia, increased contractility, blurred vision, dry mouth, decreased sweating, constipation, and confusion are effects that can be expected in a dose dependent manner.
See also: anticholinergics for mucus.
Short-acting inhaled anticholinergic agents have no selectivity for the muscarinic receptors.
Extra info: The inhaled anticholinergic agents are non-selective, thus affect all muscarinic receptors. However, since these agents are administered directly into the airways, usually only the M3 receptors are effected.
There are 3 muscarinic receptors from CNS to airway muscle.
Extra info: There are 5 known muscarinic receptors. However, the functions and agonist/antagonists are known for only the M1, M2, and M3 receptors.