Effects mediated by H1 histamine receptor include :
**Core Concept**
The H1 histamine receptor is a G-protein coupled receptor that plays a crucial role in mediating allergic responses, including vasodilation, increased vascular permeability, and smooth muscle contraction. Activation of H1 receptors is responsible for the symptoms of allergic rhinitis, urticaria, and anaphylaxis.
**Why the Correct Answer is Right**
The correct answer is mediated by the H1 receptor, which is coupled to Gq protein, leading to the activation of phospholipase C (PLC). This results in the formation of inositol trisphosphate (IP3) and diacylglycerol (DAG), causing a increase in intracellular calcium and the release of histamine from mast cells. The increase in intracellular calcium leads to the contraction of smooth muscle cells, including those in the airways, and the increase in vascular permeability, resulting in edema.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because this option does not relate to the H1 histamine receptor. The H2 receptor is involved in gastric acid secretion.
**Option B:** Incorrect because this option does not accurately describe the effects mediated by the H1 histamine receptor. While vasodilation is a consequence of H1 receptor activation, it is not the primary effect.
**Option C:** Incorrect because this option is too broad and does not specifically relate to the H1 histamine receptor.
**Clinical Pearl / High-Yield Fact**
The H1 antihistamines, such as diphenhydramine, are effective in treating allergic symptoms by competitively inhibiting the H1 receptor, thereby reducing the symptoms of allergic reactions.
**Correct Answer:** D. Effects include smooth muscle contraction, increased vascular permeability, and vasodilation.