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It is well known that the bronchial muscles have only parasympathetic innervation. But they still express $\beta_2$ receptors. How does this system still cause bronchial relaxation? What is the exact mechanism?

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    $\begingroup$ Do you have a source for the first statement? I find that the first sentence here: ncbi.nlm.nih.gov/pubmed/11240156 seems to contradict it. Halfway through the abstract might be the answer to your question. $\endgroup$ – kmm Jan 8 at 19:55
  • $\begingroup$ To be frank... I really just referred a single source .... The source is Rang&Dale's Pharmacology Ed-7 Chapter 12( Chemical mediators and the Autonomic nervous system) Pg 141. The exact statement is " Bronchial smooth muscle has only a parasympathetic (constrictor) innervation......." $\endgroup$ – Vaibhav Ghegade Jan 9 at 21:00
  • $\begingroup$ The exact statement is " Bronchial smooth muscle has only a parasympathetic (constrictor) innervation (although its tone is highly sensitive to circulating adrenaline—acting probably to inhibit the constrictor innervation rather than on the smooth muscle directly)."This gave the impression that there is absolutely no sympathetic innervation , when the case is the contrary! Thanks for directing me to the above paper! $\endgroup$ – Vaibhav Ghegade Jan 9 at 21:10
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Good question, but first I'll address a note in the comments.

In bronchial smooth muscle, do β2 receptors receive sympathetic innervation?

Canning and Fischer (2001) claim that bronchial smooth muscle do receive sympathetic innervation. But it would still be correct to say that β2 expression plays little role in regulating smooth muscle tone in the human airway under normal conditions.

How do β2 receptors cause bronchial smooth muscle relaxation?

You already know that this receptor is associated with bronchial relaxation. An article by Billington and Penn (2003) claims it works through Gs coupling. This binds adenylyl cyclase causing it to form cyclic AMP which activates protein kinase A (PKA) which carries out many more functions.

WHY are β2 receptors present on bronchial smooth muscles considering [minimal sympathetic] innervation?

Circulating chatecholamines released due to emotion, exercise and the like. Billington and Penn (2003) note "prophylactic bronchoprotection" as the central role for Gs-coupled receptors in airway smooth muscle.

Why ELSE are β2 receptors present on bronchial smooth muscle?

β2 receptors are important in regulation of airway smooth muscle cytokine/chemokine synthesis, and inhibition of airway smooth muscle growth (DNA synthesis/cell proliferation) i.a. (Billington and Penn 2003).

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  • $\begingroup$ Billington, CK, Penn, RB 2003, 'Signaling and regulation of G protein-coupled receptors in airway smooth muscle', Respiratory Research, vol. 4, no. 1, PMID: 12648290. ncbi.nlm.nih.gov/pubmed/12648290 $\endgroup$ – Roby Vicary Jan 9 at 2:22
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    $\begingroup$ Thanks a ton for the clear explaination! It is a massive help! $\endgroup$ – Vaibhav Ghegade Jan 9 at 21:15

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