When activated by b-adrenergic receptors, the G protein:
**Core Concept**
The activation of b-adrenergic receptors by catecholamines like epinephrine and norepinephrine triggers a signaling cascade that ultimately leads to increased intracellular cyclic AMP (cAMP) levels. This is a crucial mechanism in regulating various physiological processes, including cardiac output, glucose metabolism, and lipid mobilization.
**Why the Correct Answer is Right**
When a catecholamine binds to a b-adrenergic receptor, it activates the associated G protein, which is a heterotrimeric complex consisting of alpha, beta, and gamma subunits. The activated G protein then stimulates the enzyme adenylate cyclase, which catalyzes the conversion of ATP to cAMP. This increase in cAMP levels activates protein kinase A (PKA), leading to the phosphorylation and subsequent activation of various downstream targets, including phospholamban and glycogen phosphorylase.
**Why Each Wrong Option is Incorrect**
**Option A:** Activates phospholipase C - This is incorrect because phospholipase C is activated by Gq protein-coupled receptors, not b-adrenergic receptors.
**Option C:** Activates protein kinase C - This is incorrect because protein kinase C is activated by diacylglycerol (DAG) and calcium ions, not directly by b-adrenergic receptors.
**Option D:** Converts guanosine diphosphate (GDP) to guanosine triphosphate (GTP) - This is incorrect because the conversion of GDP to GTP is a step in the activation of G proteins, but it is not the direct effect of b-adrenergic receptor activation.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the activation of b-adrenergic receptors by catecholamines has a profound impact on cardiovascular function, including increased heart rate, contractility, and vascular tone. This is why beta blockers, which inhibit b-adrenergic receptor activity, are often used to manage conditions like hypertension, angina, and heart failure.
**✓ Correct Answer: B. Activates adenylate cyclase**