Q&A: Cellular and Tissue Receptor Function

In biochemistry and pharmacology, receptors are chemical structures, composed of protein, that receive and transduce signals that may be integrated into biological systems. These signals are typically chemical messengers which bind to a receptor and cause some form of cellular and/or tissue response, e.g. a change in the electrical activity of a cell.

This article is for Medical Students & Professionals

This is a Question & Answer revision article designed for medical students and professionals preparing for the PLAB, MRCP or USMLE examinations. They are based on actual questions from these examinations. You may find more useful one of our many articles on Diseases & Conditions, Medical Syndromes, Health & Wellness or Home Remedies.

In this article:
Receptor function
MCQ exam: clinical scenario
MCQ exam: answer
MCQ exam: explanation

Receptor function

There are three main ways the action of the receptor can be classified: relay of signal, amplification, or integration. Relaying sends the signal onward, amplification increases the effect of a single ligand, and integration allows the signal to be incorporated into another biochemical pathway. In this sense, a receptor is a protein-molecule that recognizes and responds to endogenous chemical signals. For example, an acetylcholine receptor recognizes and responds to its endogenous ligand, acetylcholine. However, sometimes in pharmacology, the term is also used to include other proteins that are drug targets, such as enzymes, transporters, and ion channels.

Receptor proteins can be classified by their location. Transmembrane receptors include ion channel-linked (ionotropic) receptors, G protein-linked (metabotropic) hormone receptors, and enzyme-linked hormone receptors. Intracellular receptors are those found inside the cell, and include cytoplasmic receptors and nuclear receptors. A molecule that binds to a receptor is called a ligand, and can be a protein or peptide (short protein), or another small molecule such as a neurotransmitter, hormone, pharmaceutical drug, toxin, calcium ion or parts of the outside of a virus or microbe. The endogenously designated -molecule for a particular receptor is referred to as its endogenous ligand. E.g. the endogenous ligand for the nicotinic acetylcholine receptor is acetylcholine but the receptor can also be activated by nicotine and blocked by curare.

Receptors of a particular type are linked to specific cellular biochemical pathways that correspond to the signal. While numerous receptors are found in most cells, each receptor will only bind with ligands of a particular structure. This has been analogously compared to how locks will only accept specifically shaped keys. When a ligand binds to a corresponding receptor, it activates or inhibits the receptor's associated biochemical pathway.

MCQ exam: clinical scenario

A comparison of norepinephrine and phenylephrine shows that, when given at therapeutic doses both can:

a) Decrease skin blood flow
b) Stimulate liver gluconeogenesis
c) Cause reflex tachycardia
d) Relax pregnant uterus
e) Increase A-V conduction

MCQ exam: answer

The correct answer is A.
Decrease skin blood flow is the correct choice.

MCQ exam: explanation

Norepinephrine works on alpha-1 and 2 and beta-1 receptors. Phenylephrine works on alpha-1 only. Stimulation of liver gluconeogenesis is activated by Beta-2. Reflex tachycardia is elicited by blocking alpha-1. Relaxation of pregnant uterus is achieved by activation of Beta-2. Increase of A-V conduction is accomplished by activation of Beta-1. As we notice these are functions that are not in common to both drugs.

On the other hand decrease in skin blood flow is an alpha-1 function. This is an action that is done by both drugs. This makes choice A the correct answer.

Reference(s)
1). Hall, JE (2016). Guyton and Hall Textbook of Medical Physiology. Philadelphia, PA: Elsevier Saunders. pp. 930–937. ISBN 978-1-4557-7005-2.
2). Alberts B, Bray D, Hopkin K, Johnson A, Lewis J, Raff M, Roberts K, Walter P (2014). Essential Cell Biology (Fourth ed.). New York, NY, USA: Garland Science. p. 534. ISBN 978-0-8153-4454-4.
3). Malenka RC, Nestler EJ, Hyman SE (2009). "Chapter 9: Autonomic Nervous System". In Sydor A, Brown RY (eds.). Molecular Neuropharmacology: A Foundation for Clinical Neuroscience (2nd ed.). New York: McGraw-Hill Medical. p. 234. ISBN 9780071481274.
4). Drugs.com: Curare. Available online: https://www.drugs.com/mmx/curare.html