[#3] PÂNCREAS ENDÓCRINO: GLUCAGON | MK Fisiologia

Introduction and Overview

In this section, Mirian Kurauti introduces the topic of glucagon, a hormone synthesized and secreted by the endocrine pancreas. The discussion also references insulin, another hormone produced by the endocrine pancreas.

Glucagon Synthesis and Secretion

• Glucagon is synthesized by alpha cells in the pancreatic islets.

• The synthesis process involves transcription of the glucagon gene into messenger RNA, translation into amino acids forming pre-pro-glucagon, cleavage to form pro-glucagon, and further modifications to produce active glucagon.

• Glucagon and its derived peptides are stored in secretory vesicles within alpha cells and released through exocytosis upon stimulation.

Regulation of Glucagon Secretion

• Decreased blood glucose levels stimulate glucagon secretion from alpha cells.

• Mechanisms involving glucose transporters (GLUT1), ATP production, potassium channels, calcium influx, and exocytosis contribute to glucagon release.

Regulation of Glucagon Secretion

This section delves into the regulation of glucagon secretion in response to changes in blood glucose levels. It explores how factors like insulin, somatostatin, and neurotransmitters influence glucagon release.

Influence of Insulin and Somatostatin

• Insulin acts paracrinely on alpha cells via specific receptors to inhibit glucagon secretion.

• Somatostatin secretion is also stimulated by increased blood glucose levels and inhibits glucagon release through receptor activation.

Glucagon Function and Target Organs

Here, the focus shifts to the function of glucagon once it enters circulation. The liver emerges as a primary target organ for glucagon action due to its specific receptors.

Action of Glucagon in Target Organs

• Glucagon circulates briefly with a short half-life before reaching the liver where it binds to specific receptors on hepatocytes.

How Glucagon Acts on Hepatic Cells

In this section, the mechanism of action of glucagon on hepatic cells is discussed, focusing on its signaling pathway and effects on liver metabolism.

Glucagon Signaling Pathway

• Glucagon acts on hepatic cells by binding to a G-protein-coupled receptor different from the insulin receptor.

• Upon binding, glucagon activates adenylate cyclase, leading to the production of cAMP as a second messenger.

Effects of Glucagon on Liver Metabolism

• Glucagon signaling stimulates glycogen breakdown (glycogenolysis) instead of synthesis, releasing stored glucose.

• It promotes gluconeogenesis, generating glucose from substrates like oxaloacetate and certain amino acids.

• Simultaneously, glucagon inhibits glycolysis while increasing intracellular glucose levels in hepatic cells.

Glucagon's Impact Beyond the Liver

This section explores how glucagon influences tissues beyond the liver, particularly adipose tissue and its role in lipid metabolism.

Glucagon's Effects on Adipose Tissue

• If glucagon reaches adipose tissue, it activates receptors that stimulate lipid degradation (lipolysis).

• Lipolysis releases glycerol and fatty acids into circulation for utilization by other tissues or conversion into glucose and ketone bodies in the liver.

Summary: Key Points about Glucagon

A concise summary highlighting essential aspects of glucagon's function and regulation in the body.

Key Takeaways

• Glucagon is a protein hormone secreted by pancreatic alpha cells primarily during low blood sugar levels.

• Its main target is the liver where it promotes glycogen breakdown, gluconeogenesis, and ketone body production.