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NBME 24 Answers

nbme24/Block 3/Question#29

Drug X is given to a 25-year-old normal subject. ...

Partial agonist at β-adrenergic receptors

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submitted by tea-cats-biscuits(63),

Partial agonists have weak agonist activity on their own (thus in this case it causes HR to increase, b-adrenergic effect) but when an actual agonist is present (aka when you are exercising, you are producing NE and E that have full b-agonist effects), partial agonist actually have a mild antagonist effect (thus the heart rate decreases).