Question

Salbutamol causes bronchodilation by its action on

• A. \(\alpha\) - 1 adrenergic receptor
• B. \(\alpha\) - 2 adrenergic receptor
• C. \(\beta\) - 1 adrenergic receptor
• D. \(\beta\) - 2 adrenergic receptor

Hint:

Adrenoceptor Targets:
$\alpha_1 \rightarrow$ Vasoconstriction
$\alpha_2 \rightarrow$ Sedation (Presynaptic inhibition)
$\beta_1 \rightarrow$ Heart (Inotropy/Chronotropy)
$\beta_2 \rightarrow$ BRONCHODILATION & Vasodilation (Salbutamol, Clenbuterol).

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Bronchial smooth muscle pharmacology: selective $eta_2$-adrenoceptor agonists activate $G_s$-adenylyl cyclase signaling to induce profound airway bronchodilation.
Key Formula or Approach:
\[ \text{Salbutamol} \xrightarrow{\beta_2\text{-Receptor}} G_s \rightarrow \uparrow \text{cAMP} \rightarrow \text{PKA Activation} \rightarrow \downarrow \text{Intracellular Ca}^{2+} \implies \text{Bronchodilation} \]

Step 2: Detailed Explanation:
Pharmacology of Salbutamol (Albuterol):
- A short-acting, selective $\beta_2$-adrenergic receptor agonist.
- Binds to $\beta_2$-adrenoceptors on bronchial smooth muscle cells, activating the stimulatory G-protein ($G_s$) to stimulate adenylyl cyclase and elevate intracellular cAMP.
- Elevated cAMP activates protein kinase A (PKA), which phosphorylates and inactivates myosin light chain kinase (MLCK) while stimulating $\text{Ca}^{2+}$ extrusion, causing profound relaxation of bronchial smooth muscle (Bronchodilation) in feline asthma, recurrent airway obstruction (RAO/heaves) in horses, and canine bronchospasm.

Step 3: Final Answer:
Hence, Salbutamol causes bronchodilation by acting on \(\beta\) - 2 adrenergic receptor, matching option (D).