N Muscarinic receptor blockers

• Mechanism: Antagonists of the cardiac form of the muscarinic receptor (M2) are useful in the abolition of an inappropriate bradycardia or excessively slow AV node conduction due to excessive activity in the cardiac parasympathetic nerves.

• Effectiveness: The nonselective muscarinic receptor blocker, atropine (Figure 13), is sometimes used in acute situations (e.g., bradycardia in acute myocardial infarction), in other emergency settings, and during anesthesia when excessive bradycardia is present. The cardiac form of the muscarinic receptor is the M2 subtype. The only compounds with some selectivity for this subtype are gallamine and AF-DX 116 (Figure 13). Gallamine was originally introduced as a blocker at the skeletal neuromuscular junction (neuromuscular blocker), but it has some selectivity for M2 receptors. AF-DX 116 is more specific for the M2 receptor, but despite this, the usual muscarinic blocker used is typically atropine.

• Toxicity: Blockade of muscarinic receptors in other tissues account for side effects such as dry mouth, visual disturbances, and difficulties with micturition. Adenosine

• Mechanism: The effects of adenosine (Figure 14) on atria and nodes are almost identical to those of ACh. Activation of the A1 subtype of adenosine receptors (P1) is linked to the potassium channels activated by acetylcholine (IKACh). Activation slows the rate of rise of pacemaking potentials and reduces nodal calcium currents, thereby reducing AV nodal conduction.

• Effectiveness: Adenosine's success rate in terminating paroxysmal supraventricular nodal tachycardia is as high as 95%. Thus it has replaced the less efficacious verapamil. Many episodes of paroxysmal supraventricular nodal tachycardia can be terminated by vagotonic maneuvers. These include the Valsalva maneuver (forced expiration against a closed glottis and the nostrils pinched shut), activation of the diving reflex (face in cold water), and carotid massage to activate the baroreceptor reflex.

• Toxicity: Although adenosine is a potent vasodilator, the half-life of adenosine is very short (~ 10 s) making its side effects tolerable, e.g., short-lived hypotension and occasional bronchoconstriction episode.

34 Atropine

Figure 13 The prototypical muscarinic receptor blocker atropine and two other blockers with selectivity for M2 receptor (cardiac form).

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