Answers

1. B. The adrenoceptors that epinephrine acts on to affect heart rate, renin release, bronchiolar tone, and glycogenolysis are p-receptors. Prazosin is an a-antagonist so would not antagonize epinephrine at those receptors. The radial smooth muscle in the iris has a-receptors that when activated, contract the radial muscle which dilates the pupil. This action is antagonized by prazosin.

2. D. Propranolol and metoprolol are selective for preceptors, whereas prazosin and phenoxybenzamine are selective for a-receptors. Labetalol is the only antagonist in this list that has the ability to reduce responses mediated by both a- and p-receptors.

3. D. What is shown is an increase in pressure caused by norepinephrine and the reduction of the effect by drug X. Therefore, drug X is an antagonist of norepinephrine. Two of the choices are adrenocep-tor antagonists, prazosin and propranolol, which are a- and p-receptor antagonists, respectively. The adrenoceptors that mediate vasoconstriction are a-receptors. Therefore, prazosin, the a-blocker, is the correct choice. Cocaine, because it blocks neuronal uptake of norepinephrine, would actually enhance the response to this catecholamine, as would guanethidine, because it also blocks neuronal uptake of norepinephrine. Atropine is a muscarinic receptor antagonist and would not affect responses to norepinephrine.

4. C. The vasodilation produced by drug Y is antagonized by timolol, a p-receptor antagonist. Although bradykinin, histamine, acetylcholine, and isopro-terenol will all cause vasodilation, only isopro-terenol does so by activating p-receptors. Phenylephrine is a sympathomimetic that is selec tive for a-receptors and would be expected to increase, rather than decrease, perfusion pressure.

5. A. Both sets of responses to isoproterenol are mediated by p-adrenoceptors, and all the choices are p-antagonists. However, drug X is more effective in antagonizing cardiac responses to isoproterenol than it is the bronchiolar responses. Drug X is therefore a cardioselective p-blocker, that is, selective for Pj over p2 receptors. Metoprolol is the only pi-selective antagonist among the choices.

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