Question

Why is the M3 subtype receptor found on smooth muscle fibers of the bronchioles? A detailed explanation wold be helpful. thank you

Answer 1

The Muscarinic receptors or the Muscarinic acetylcholine receptors are a class of G protein-coupled receptors with Acetylcholine acting as their legand. The neural pathway for the bronchoconstriction in the airways (lungs) is mediated by these muscarinic receptors. Thus these receptors maintain the tone of the smooth muscle when they are activated by the release of the acetylcholine by the parasympathetic nervous system via the vagus nerve. Several experimental data have conclusively found that the M3 subtype of the muscarinic receptor which is dominated amongst the other muscarinic receptor in the constriction function of these receptors on the smooth muscles. Now the question comes as to why only there is M3 mediated bronchoconstriction in the airways of the lungs via smooth muscles. In order to understand that we will have to see where are the other subtypes of the muscarinic receptors located. So there are M1, M2, M3, M4, and M5 muscarinic receptor subtypes in total and have been widely distributed in the body. The Muscarinic receptor subtype which is present in the central nervous system is the M1 subtype whereas the M2 muscarinic receptors are largely present in the heart and the lungs. M3 muscarinic receptors are present in the airway smooth muscle cells (smooth muscle of the bronchioles) and some exocrine glands. Apart from that the M4 and M5 subtypes are again found to be expressed in the brain. Thus first of all because of the expression location in the body i.e. smooth muscles and secondly, the function of contraction of the smooth muscles makes it important for the M3 subtype to be present in the airways and thus bronchioles. Furthermore, the M3 receptor activation leads to the Ca2+ signaling pathway and contraction of the smooth muscle fibers a property that makes this receptor to be present in the airways when the vagus nerve has to relay a signal for bronchoconstriction in the bronchioles.