Glucagon
Glucagon is a polypeptide hormone secreted by alpha cells in pancreatic islets of Langerhans. Together with epinephrine, cortisol, and growth hormone, glucagon oppoeses the activity of insulin.
Glucagon is composed of 29 amino acids that results from the cleavage of a large precursor.
Glucagon binds to its receptor on hepatocytes, activating adenylyl cyclase and the cAMP second messenger system. This results in phosphorylation-mediated changes in key regulatory enzymes.
Glucagon Secretion
Glucagon secretion from alpha cells is stimulated by a variety of stimuli:
- Low blood glucose is the primary stimulus for glucagon release, acting to prevent hypoglycemia. Glucose and insulin both inhibit glucagon release.
- Amino acids stimulate both insulin and glucagon release. This prevents insulin-induced hypoglycemia after a protein meal.
- Epinephrine from the adrenal medulla, or norepinephrine from the pancreas, stimulate glucagon release in anticipation of increased glucose use during times of stress, trauma, or extreme exercise.
Metabolic Effects of Glucagon
Glucoagon acts primarily on the liver.
Effects on Carbohydrates
Glucagon causes an immediate rise in glucose levels via breakdown of liver (not muscle) glycogen and an increase in gluconeogenesis.
Effects on Lipid Metabolism
- glucagon has minimal effects on adipose tissue.
- promotes hepatic oxidation of fatty acids
- promotes formation of ketone bodies from acetyl CoA.
- inhibits cholesterol synthesis by acting on HMG CoA reductase levels.
Effects on Proteins
Glucagon increases amino acid uptake by the liver in order top use the carbon skeleton for gluconeogenesis. This decreases plasma amino acid levels.