drpkaur
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guys ...i know this has been posted before but still i am posting it again just to review....
IV administration of drug X to an anesthetized animal produces an
increase in blood pressure. After administration of drug Y,
readministration of drug X produces a decrease in blood pressure.
Which of the following pairs of drugs could produce this sequence of events?
Drug X Drug Y
A. Acetylcholine Neostigmine
B. Epinephrine Phentolamine
C. Isoproterenol Atropine
D. Norepinephrine Propranolol
E. Phenylephrine Hexamethonium
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| suv
Forum Elite
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B
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| drpkaur
Forum Guru
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Explanation plz..
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| Dragonfly
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it is called "Epinephrine reversal".
phentolamine blocks alpha-R which Epi used to stim and increase the BP. no alpha-R means in the periphery only Beta2-R are left to stim. so you can guess the result.
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| drpkaur
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Topics: 196
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u guys are absolutely correct....the correct answer is B....
The correct answer is B. First, eliminate all answers in which Drug X does not produce an increase in blood pressure (BP). Choice A should be eliminated because acetylcholine stimulates the noninnervated muscarinic (M3) receptors that are located on endothelial cells of the vasculature. Stimulation of these receptors releases endothelial-derived relaxing factor (EDRF; nitric oxide), which produces a relaxation of the neighboring smooth muscle cells, leading to a decrease in BP. Choice C should be eliminated because isoproterenol (a nonspecific beta agonist) decreases BP by stimulating beta-2 receptors in the vasculature.
Epinephrine, norepinephrine, and phenylephrine all increase BP, so the remaining answers must be eliminated by examining the effects of Drug Y on Drug X. Start with choice B: Epinephrine is an agonist at alpha-1, alpha-2, beta-1, and beta-2 receptors; phentolamine is an antagonist at alpha-1 and alpha-2 receptors. Therefore, after the administration of phentolamine, epinephrine can stimulate only beta receptors, which would produce a decrease in BP. Epinephrine is now acting like isoproterenol. This is called epinephrine reversal (the name stems from the fact that epinephrine originally increases BP and then produces the opposite effect after phentolamine administration). Therefore, choice B is correct.
Choice D: Norepinephrine is an agonist at alpha-1, alpha-2, and beta-1 receptors; propranolol is a nonselective beta antagonist. After administration of propranolol, norepinephrine can stimulate only alpha receptors, which will still cause vasoconstriction (primarily via alpha-1 stimulation in the vasculature) and therefore increase BP.
Choice E: Phenylephrine is an alpha-1 agonist; hexamethonium is a nicotinic ganglionic blocker. Hexamethonium administration would be predicted to eliminate the baroreceptor response after the second phenylephrine administration by blocking the peripheral ganglia. However, phenylephrine will still reach the alpha-1 receptors on the vasculature to produce an increase in blood pressure.
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