Answer:
A. epinephrine
Step-by-step explanation:
Bronchial constriction usually involves the tightening of the smooth muscles which surrounds the tube-like bronchial airways. This occurs via extra mucus production, spasming of smooth muscles, or swelling/inflammation of layers within bronchi or bronchioles. Bronchial constriction typically results in symptoms of wheezing, coughing and shortness of breath.
This can be treated via the use of the hormone epinephrine, a bronchiodilator. For some cases like asthma, epinephrine in small aerosolized (inhaled) doses can lead to the temporary relief of symptoms. Epinephrine acts by making bronchial tubes wider to allow the passage of air.
In the body, smooth muscle cells in the veins, arteries and heart are under the control of epinephrine receptors called β2 receptors. Epinephrine binding to the receptor triggers a cell signalling cascade that activates β2 receptors to dilate smooth muscle in the lungs.
Here's how-
- Calcium is essential to mediating muscle cell signalling and thus contraction
- in the lungs, epinepherine-β2 receptor binding leads to enzyme activation (adenylyl cyclase)
- conversion of the energy molecule ATP to the secondary messenger molecule cyclic AMP.
- cAMP inhibits the activity of the enzyme myosin light chain kinase (MLCK) in smooth muscle
- ∴ the enzyme remains inactive and the muscles relax
Summary: epinephrine → muscle relaxation → bronchial dilation (reduced diameter) → decreased respiratory distress → easier breathing.