On which receptors do Lopressor (metoprolol) and Lanetalol work?

Receptor Selectivity of Labetalol and Lopressor (Metoprolol)

Lopressor (Metoprolol) - Beta-1 Selective Blocker

Metoprolol is a beta-1 selective (cardioselective) adrenergic receptor blocker that preferentially blocks beta-1 receptors, though this selectivity is not absolute at higher plasma concentrations where it also inhibits beta-2 adrenoreceptors. 1

Receptor Specificity

• Metoprolol selectively blocks beta-1 adrenergic receptors, which are primarily located in cardiac tissue 2, 1
• At higher plasma concentrations, metoprolol loses its selectivity and also inhibits beta-2 adrenoreceptors located in bronchial and vascular musculature 1
• The drug has no intrinsic sympathomimetic activity and membrane-stabilizing activity is only detectable at doses much greater than required for beta-blockade 1

Clinical Evidence of Beta-1 Selectivity

• In healthy subjects, metoprolol cannot reverse the beta-2-mediated vasodilating effects of epinephrine, unlike non-selective beta-blockers which completely reverse these effects 1
• In asthmatic patients, metoprolol reduces FEV1 and FVC significantly less than propranolol (a non-selective beta-blocker) at equivalent beta-1 receptor blocking doses 1
• The beta-1 blocking activity (cardioselectivity) resides specifically in the S-isomer of metoprolol, while the R-isomer exhibits beta-2 blocking activity 3

Labetalol - Non-Selective Alpha and Beta Blocker

Labetalol blocks alpha-1, beta-1, and beta-2 adrenergic receptors, making it a non-selective blocker with combined alpha and beta blocking properties. 2

Receptor Specificity

• Labetalol blocks alpha-1 adrenergic receptors in addition to beta receptors 2
• Labetalol blocks both beta-1 and beta-2 adrenergic receptors, similar to non-selective beta-blockers 2
• This combined alpha and beta blockade makes labetalol particularly useful in acute hypertensive situations, such as aortic dissection 2

Clinical Context

• The European Heart Journal guidelines specifically mention labetalol as an option for patients with aortic dissection who require blood pressure control 2
• The combined receptor blockade provides both reduction in cardiac contractility (beta-blockade) and peripheral vasodilation (alpha-blockade) 2

Key Clinical Distinction

The fundamental difference is that metoprolol is cardioselective (primarily beta-1), while labetalol is non-selective, blocking alpha-1, beta-1, and beta-2 receptors. 2, 1

Practical Implications

• Metoprolol's beta-1 selectivity may facilitate administration to patients with asthma or COPD where non-selective beta-blockade is relatively contraindicated 4
• Labetalol's combined alpha and beta blockade makes it useful when both heart rate reduction and vasodilation are desired simultaneously 2
• Both drugs lose their selectivity characteristics at higher doses—metoprolol begins blocking beta-2 receptors, and the clinical significance of receptor selectivity diminishes 1