One of the following activities is not mediated through b2 adrenergic receptors –
**Question:** One of the following activities is not mediated through b2 adrenergic receptors -
A. Increase heart rate
B. Inhibit gastric secretion
C. Induce bronchoconstriction
D. Increase intraocular pressure
**Core Concept:**
b2 adrenergic receptors are a type of adrenergic receptor that are predominantly found in the heart, smooth muscle, and the lung airways. They are involved in various physiological processes, particularly in response to adrenaline and noradrenaline. b2 agonists, such as salbutamol, are commonly used in clinical practice for their bronchodilator effect.
**Why the Correct Answer is Right:**
Option D (Increase intraocular pressure) is not mediated through b2 adrenergic receptors. b2 receptors are primarily involved in bronchodilation and vasodilation, not affecting intraocular pressure directly. In contrast, b1 receptors are responsible for regulating intraocular pressure, particularly through the activation of the renin-angiotensin-aldosterone system (RAAS).
**Why Each Wrong Option is Incorrect:**
Option A (Increase heart rate) is incorrect because b2 adrenergic receptors do contribute to heart rate increase, particularly through the sympathetic nervous system. However, the main mediator of heart rate increase is the a1 adrenergic receptors.
Option B (Inhibit gastric secretion) is incorrect since b2 receptors do not directly regulate gastric secretion. The primary receptors responsible for gastric secretion inhibition are the H2 receptors.
Option C (Induce bronchoconstriction) is incorrect because b2 receptors, as mentioned earlier, are involved in bronchodilation. Activation of b2 receptors leads to relaxation of bronchial smooth muscle, resulting in bronchodilation, not bronchoconstriction.
**Core Concept:**
b2 adrenergic receptors are a subtype of G protein-coupled receptors (GPCRs) that play a significant role in mediating the physiological responses to adrenaline and noradrenaline. These receptors are involved in various physiological processes, including bronchodilation, vasodilation, and increased heart rate.
**Why Each Wrong Option is Incorrect:**
Option A (Increase heart rate) is incorrect because b2 adrenergic receptors primarily contribute to increased heart rate through the activation of the sympathetic nervous system. The primary receptors responsible for cardiac contractility are the a1 adrenergic receptors.
Option B (Inhibit gastric secretion) is incorrect because b2 receptors do not directly regulate gastric secretion. The primary receptors involved in inhibiting gastric secretion are the H2 receptors, which are responsible for inhibiting gastric acid secretion via the inhibition of histamine release.
Option C (Induce bronchoconstriction) is incorrect because, as mentioned earlier, b2 receptors are involved in bronchodilation. Activation of b2 receptors leads to relaxation of bronchial smooth muscle, resulting in bronchodilation, not bronchoconstriction.
**Clinical Pearls:**
1. Understanding the activation of b2 receptors is crucial for understanding the physiological effects of adrenaline and noradrenaline on the cardiovascular and respiratory systems. This knowledge is