Catecholamines Bind to Cell Surface Receptors
Among the listed options, catecholamines (B) are the ligands that bind to cell surface receptors.
Mechanism of Action of Catecholamines
Catecholamines such as epinephrine and norepinephrine bind to adrenergic receptors, which are cell surface glycoproteins 1. These receptors are critical components of the sympathetic nervous system and mediate various physiological responses.
Evidence from Drug Labels and Guidelines:
Epinephrine acts on both alpha (α)- and beta (β)-adrenergic receptors located on the cell surface. Its mechanism involves:
- Direct myocardial stimulation (positive inotropic action)
- Increased heart rate (positive chronotropic action)
- Peripheral vasoconstriction 2
Norepinephrine functions as a peripheral vasoconstrictor through alpha-adrenergic action and as an inotropic stimulator of the heart through beta-adrenergic action 3
Receptor Location and Signaling:
Adrenergic receptors are definitively located on the cell surface and function through transmembrane signaling pathways:
- When catecholamines bind to beta-adrenergic receptors on the cell surface, they activate adenylate cyclase, leading to intracellular formation of cAMP 4
- The signal transduction involves sequential interaction of the beta-adrenergic receptor with guanine nucleotide-binding protein (Gs) and adenylate cyclase catalyst 4
Why Other Options Are Incorrect:
Steroids (A) - Steroids do not bind to cell surface receptors. They are lipophilic molecules that pass through the cell membrane and bind to intracellular receptors.
Retinoids (C) - Similar to steroids, retinoids are lipid-soluble compounds that pass through the cell membrane to bind to intracellular nuclear receptors.
Thyroid hormones (D) - Thyroid hormones primarily act through nuclear receptors located inside the cell, not on the cell surface.
Cell Surface Receptor Characteristics
Cell surface receptors like adrenergic receptors have specific structural and functional features:
- They are embedded in the plasma membrane with domains extending into both extracellular and intracellular spaces
- They transduce external stimuli into intracellular signals 1
- They can engage in both cis interactions (with ligands on the same cell) and trans interactions (with ligands on different cells) 5
Clinical Relevance
Understanding which ligands bind to cell surface receptors is crucial because:
- Activation or blockade of adrenergic receptors is a major therapeutic approach for cardiovascular disorders, including hypertension, angina pectoris, and cardiac arrhythmias 1
- Alpha-1 adrenergic receptors have emerging roles in cognition, cardioprotection, and metabolism 6
- Cell surface receptor targeting is important in drug delivery systems and nanomedicine development 7
The distinction between cell surface and intracellular receptors is fundamental to understanding drug mechanisms of action and developing targeted therapies.