SBA — Single Best Answer · Review
Question 438
Pharmacology → General
Classification
Question
SSBAQuestion Header
A patient with supraventricular tachycardia receives a drug that slows the heart but can also produce bronchoconstriction. Which receptor profile best accounts for this combination of effects?
Question Stem
Select the single best answer.
Options
A
Selective beta-1 agonism
B
Selective beta-2 antagonism
C
Combined alpha-1 and beta-1 agonism
D
Non-selective beta-1 and beta-2 antagonism
E
Selective alpha-2 agonism
Explanation
Propranolol is a non-selective beta-adrenoceptor antagonist acting at both beta-1 and beta-2 receptors. Beta-1 antagonism produces negative chronotropic cardiac effects, while antagonism of bronchial beta receptors can produce bronchoconstriction.
Option Validity
A) Beta-1 agonism would not explain the negative chronotropic action of propranolol.
B) Isolated beta-2 antagonism could affect the bronchi but would not account for the characteristic cardiac beta-1 blockade.
C) Propranolol is an antagonist and does not act through combined alpha-1 and beta-1 agonism.
E) Selective alpha-2 agonism describes the principal action of clonidine, not propranolol.
Further Reading
Saxena R. Textbook for MRCOG-1. Chapter 12: Pharmacology. Beta Blockers, p.345.