The drugs in this group antagonize the effects of acetylcholine. Most of these drugs are antagonists directly at the nicotinic or muscarinic receptor. Some act on the ion channel associated with the nicotinic receptor, and still others block acetylcholine release.
The prototypic muscarinic antagonist is ATROPINE.
In this group of compounds, it is useful to consider a prototype drug and then compare the other drugs with it. The prototype drug for the muscarinic antagonists is atropine.
All of the muscarinic antagonists are competitive antagonists for the binding of acetylcholine to the muscarinic receptor.
These drugs compete with acetylcholine for binding to the muscarinic receptor. They have no intrinsic activity. In other words, in the absence of acetylcholine, they would have no effect.
The effects and side effects of these drugs are opposite of the drugs considered in the previous chapter (the cholinomimetics)
Central nervous system (CNS)
Mydriasis, cycloplegia (blurred vision)
Reduced sweating, flushing
Reduced motility and secretions
Increased heart rate (high doses)
Bronchial dilation and decreased secretion
Drowsiness, hallucinations, coma
Compare these effects to those listed in the corresponding box in Chapter 7. The important ones to remember are the common side effects of drugs that have anticholinergic properties (many of the CNS drugs); that is, dry eyes, dry mouth, blurred vision, constipation, and urinary retention. If you master the anticholinergic effects now, it will save you considerable effort later.
Many muscarinic antagonists are currently available and their names do not sound all alike. Some name recognition exercises may be useful here.
Some of these drugs have particular uses. Learn the names of these drugs first and add the others later.
Muscarinic antagonists are used preoperatively to reduce secretions.
SCOPOLAMINE is used to prevent motion sickness.
Scopolamine has an effect on the CNS to reduce motion sickness. It is usually administered using a transdermal patch.
Long-acting anticholinergics are used in the treatment of chronic obstructive pulmonary disease (COPD) to produce bronchodilation.
As you know from Chapter 6, activation of β2 receptors will result in relaxation of the smooth muscle in the bronchial tree. Thus, β2 agonists will produce bronchodilation. So will muscarinic antagonists, such as glycopyrrolate and tiotropium. Whether to use a β2 agonist or a muscarinic antagonist in a particular patient has to do with the underlying pathophysiology of the pulmonary disease ...