All of the following are features of metoprolol in comparison to propanolol EXCEPT:
**Question:** All of the following are features of metoprolol in comparison to propanolol EXCEPT:
A. Selective β1-receptor blocker
B. Less cardioselective
C. More lipophilic
D. Lower affinity for β2-receptors
**Core Concept:** Metoprolol and propranolol are both non-selective β-blockers used for various cardiovascular conditions. However, they differ in their selectivity for β1 vs β2 receptors, and lipophilicity. Metoprolol is a more selective β1-blocker than propranolol, meaning it binds more strongly to β1 receptors in the heart, reducing cardiac workload. On the other hand, propranolol is less cardioselective, affecting both β1 and β2 receptors, which can lead to more respiratory depression and bronchoconstriction.
**Why the Correct Answer is Right:** The correct answer (D) refers to the lower affinity of metoprolol for β2 receptors. Metoprolol is less cardioselective than propranolol, meaning it binds to both β1 and β2 receptors. Lower affinity for β2 receptors implies that metoprolol will cause less bronchoconstriction and respiratory depression compared to propranolol.
**Why Each Wrong Option is Incorrect:**
A. Selective β1-receptor blocker: This option is correct, as metoprolol is indeed a more selective β1-receptor blocker than propranolol.
B. Less cardioselective: This is incorrect, as metoprolol is less cardioselective than propranolol, not more.
C. More lipophilic: Lipophilicity is a property of drugs, not a feature of β-blockers. Lipophilicity does not explain the difference in their selectivity for β1 receptors.
D. Lower affinity for β2 receptors: This is correct, as metoprolol has lower affinity for β2 receptors compared to propranolol, leading to fewer respiratory and cardiac side effects.
**Clinical Pearl:** Understanding the differences in the selectivity of metoprolol and propranolol for β receptors is crucial when choosing a β-blocker for a patient. Selective β1-blockers like metoprolol may be preferred in patients with respiratory or cardiac issues, as they cause fewer side effects on the lungs and heart.