Review Question - QID 212768

The decrease in heart rate that occurs several minutes after administration of a sympathomimetic drug is most likely due to a vagal reflex response, which could be prevented by pretreatment with the ganglionic blocker hexamethonium.

Ganglionic blockers are drugs that block nicotinic receptors at sympathetic and parasympathetic autonomic ganglia. They were originally developed for the treatment of hypertension; however, they have low selectivity and a wide array of side effects that limit their use clinically. These side effects include severe orthostatic hypertension, cycloplegia, and constipation. Despite this, ganglionic blockers are often used in experimental models to prevent vagal reflex responses to changes in blood pressure. For example, they can be used to prevent reflex bradycardia after norepinephrine administration.

Incorrect Answers:
Answer 1: Dobutamine is a sympathomimetic that has a positive inotropic effect. Though this drug would increase heart rate, it would not be used prior to adrenergic administration to prevent reflex vagal responses.

Answer 3: Physostigmine is a cholinesterase inhibitor that is used for glaucoma or to reverse atropine overdose. It would increase vagal signaling rather than decreased vagal signaling.

Answer 4: Metoprolol is a beta-adrenergic blocker that is commonly used in treating hypertension. It would decrease the heart rate, which is the opposite of the intended effect.

Answer 5: Scopolamine is a muscarinic antagonist; however, it primarily has activity in the central nervous system where it is used to treat motion sickness.

Bullet Summary:
Ganglionic blockers are used experimentally to prevent reflex vagal responses to changes in blood pressure.