Which of the following actions is not caused by H1 receptor –
**Core Concept**
The H1 receptor is a subtype of histamine receptor, primarily involved in mediating allergic responses and regulating physiological functions such as sleep-wake cycle, appetite, and gastric acid secretion. Histamine released from mast cells binds to H1 receptors on smooth muscle, blood vessels, and other tissues, leading to increased permeability, vasodilation, and smooth muscle contraction.
**Why the Correct Answer is Right**
H1 receptors are responsible for the contraction of smooth muscle in the bronchi and gastrointestinal tract, leading to bronchospasm and increased gastric acid secretion. The activation of H1 receptors also leads to increased permeability of blood vessels, resulting in edema, and vasodilation, contributing to the symptoms of allergic reactions such as flushing and pruritus.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is a known effect of H1 receptor activation. The contraction of smooth muscle in the bronchi is a classic example of an H1-mediated response.
**Option B:** This option is incorrect because it is a known effect of H1 receptor activation. The increased permeability of blood vessels is a key feature of allergic reactions mediated by H1 receptors.
**Option C:** This option is incorrect because it is not a known effect of H1 receptor activation. H1 receptors are not directly involved in the regulation of cardiac function.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that H1 antihistamines are commonly used to treat allergic reactions, including urticaria, angioedema, and anaphylaxis. However, they can also cause sedation and dry mouth due to their action on the central nervous system and muscarinic receptors.
**Correct Answer: C. H1 receptors are not directly involved in the regulation of cardiac function.**