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Humans have receptors for two kinds of beta adrenergic compounds such as catecholamines to control cardiac muscle contractions. Some are beta 1 receptors that promote increased heart rate. Other drugs, called beta blockers, slow heart rate. Smooth muscle cells, however, have beta 2 receptors, which mediate muscle relaxation. Blockers of these effects are sometimes used to treat asthma.
-The description above illustrates which of the following?
A)The fact that a drug acts on one type of receptor does not mean that it will act on another type.
B)Beta blockers can be used effectively on any type of muscle.
C)Beta adrenergic receptors must be in the cytosol if they are going to influence contraction and relaxation.
D)The chemical structures of the beta 1 and beta 2 receptors must have the same active sites.

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Quizplus · Accepted Answer

The passage clearly states that while beta 1 receptors promote increased heart rate, beta 2 receptors mediate muscle relaxation. Therefore, a drug that affects one type of receptor (such as a beta blocker that slows heart rate by blocking beta 1 receptors) may not have the same effect on another type of receptor (such as beta 2 receptors that mediate muscle relaxation). This demonstrates that the fact that a drug acts on one type of receptor does not necessarily mean that it will act on another type, making choice A the best answer. Choices B, C, and D are not supported by the information provided in the passage.

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