Glucagon is secreted by:
**Core Concept**
Glucagon is a hormone secreted by the pancreas, specifically by the alpha cells of the islets of Langerhans. It plays a crucial role in regulating blood glucose levels by stimulating glycogenolysis and gluconeogenesis.
**Why the Correct Answer is Right**
Glucagon acts by binding to glucagon receptors on the surface of hepatocytes, which triggers a cascade of intracellular signaling pathways. This ultimately leads to the activation of key enzymes involved in glycogenolysis, such as phosphorylase, and gluconeogenesis, such as pyruvate carboxylase. The stimulation of these pathways results in the increased production of glucose, which is then released into the bloodstream.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because glucagon is not secreted by the beta cells of the pancreas, which are responsible for secreting insulin.
**Option B:** This option is incorrect because glucagon is not secreted by the delta cells of the pancreas, which are responsible for secreting somatostatin.
**Option C:** This option is incorrect because glucagon is not secreted by the PP cells of the pancreas, which are responsible for secreting pancreatic polypeptide.
**Clinical Pearl / High-Yield Fact**
Glucagon is often used clinically in the treatment of hypoglycemia, particularly in cases where glucose administration is not feasible. It is also used to stimulate glucose production in patients with liver dysfunction.
**Correct Answer: A. Alpha cells of the islets of Langerhans.**