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    pharmacology is offline Member
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    drug x increases BP, drug y with drug x decreases BP

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    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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    stepanida is offline Junior Member
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    ans.B.
    Phentolamine causes epinephrine reversal.

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    pharmacology is offline Member
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    Re: drug x increases BP, drug y with drug x decreases BP

    Quote Originally Posted by pharmacology
    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


    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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