Question

Upon being secreted from the pancreas, glucagon binds to the receptors on liver cells, which initiates a signaling cascade that regulates glycogen metabolism, glycolysis/gluconeogenesis, and fatty acid metabolism. Explain the following:

1. The metabolic conditions in which glucagon is released from the pancreas

2. How it affects metabolism of glycogen, glucose (glycolysis/gluconeogenesis), and fatty acids

3. Why it makes metabolic sense for glucagon to regulate these pathways in this manner in the liver cells.

Answer 1

Answer1= whenever their is hypoglycemia(decrease blood glucose in blood) glucagon is secreted from beta cells of pancrease to regulate blood glucose levels in blood.

Answer2=the main action of glucagon is to promote hypergylcemia in response to hypoglcemia.

It acts on liver where it stimulates glycogenolysis( breakdown of glycogen to glucose) and gluconeogensis( formation of glucose from fatty acids).

Glucagon acts on hepatocytes via receptor due to which adenylate cyclase is activated which increases cAMP and phosphorylase kinases whoch ultimately leads to activation of an enzyme called glycogen phosphorylase (rate limiting step in glcogenolysis) and helps in increasing the blood glucose levels.

It also inhibits GLYCOLYSIS(use of glucose)

All the mechanism leads to INCREASE in blood glucose

Glucagon stimulates lipolysis( increase in fatty acids from triglycerides from adipose tissue)

Answer3=insulin has action on both liver and muscles(glycogenesis)

Epinephrine causes glycogenolysis both in muscles and liver

But GYLCOGEN acts only on liver not skeletal mucles because as explained it acts through receptors which are present in liver , intestinal smooth muscles, brain tissue but not in skeletal muscles...and moreever the effect of glucagon induced glycogenolysis in liver is most prominent .(in controlling blood glucose) therefore often only liver gets mentioned.