pharm6 - guest1

[ArchivalUser]

A medical student is performing experiments on an anesthetized animal for her pharmacology class. An arterial
line is inserted to monitor blood pressure, and the animal is given an intravenous dose of epinephrine. The
injection produces an increase in blood pressure. The student then injects an unknown drug, followed fifteen
minutes later by readministration of epinephrine. The second administration of epinephrine now produces a
decrease in blood pressure. To which of the following classes does the unknown drug belong?


A. Acetylcholinesterase inhibitor

B. Nicotinic ganglionic blocker

C. Nonselective alpha receptor agonist

D. Nonselective alpha receptor antagonist

E. Nonselective beta receptor antagonist

[ArchivalUser]

dd

[ArchivalUser]

d??

[ArchivalUser]

The correct answer is D. This classic drug response, called epinephrine reversal, is a favorite on the USMLE
and in pharmacology classes. Epinephrine, a nonselective alpha and beta adrenergic agonist, increases blood
pressure. The unknown drug is an alpha adrenergic antagonist, such as phentolamine, which blocks
epinephrine's vasoconstrictive action on arterioles. Subsequent administration of epinephrine produces only
beta receptor stimulation, causing vasodilation in skeletal muscle, leading to a decrease in blood pressure.
Epinephrine, for all practical purposes, now acts like the nonspecific beta agonist, isoproterenol. This effect is
called epinephrine reversal because of the fact that epinephrine originally increases BP and then produces the
opposite effect after phentolamine administration.