Glucose Metabolism and Regulation

Glucose Metabolism

• Glucose is the major source of energy in all living organisms.
• Too little glucose can lead to brain problems, while too much glucose can cause osmotic water loss and damage blood vessels.

Sources of Glucose

• Food is a direct source of glucose.
• Glucose can be stored as glycogen in the liver and as fat in adipose tissue.
• Glucose can be synthesized from fat and protein through gluconeogenesis.

Pancreas Functions

• The pancreas produces digestive enzymes for digestion.
• The pancreas produces primary messengers (hormones) such as insulin and glucagon.

Insulin

• Insulin is produced by beta cells in the pancreas.
• Insulin structure consists of 51 amino acids, 2 chains linked by disulfide bonds, and a molecular weight of 5800 dalton.
• Insulin increases glucose uptake in nearly all cells (80%), primarily affecting the liver and muscle, but not the brain.
• Insulin receptor is a tyrosine kinase receptor.
• Insulin secretion is stimulated by increased blood glucose, free fatty acids, amino acids, and certain hormones.
• Insulin secretion is inhibited by fasting, somatostatin, and sympathetic stimulation.

Action of Insulin

• In muscle cells, insulin promotes glucose uptake and storage as glycogen, and inhibits glucose synthesis.
• In liver cells, insulin promotes glucose uptake and storage as glycogen, inhibits glycogen phosphorylase, activates glycogen synthase, inhibits glucose synthesis, and promotes excess glucose conversion to fatty acids.
• In adipose tissue, insulin promotes glucose uptake and conversion to glycerol for fat production.

Glucose Disorders

• Type I diabetes: lack of insulin production, characterized by increased fasting blood glucose, increased blood fatty acids and cholesterol, and protein depletion.
• Type II diabetes: insulin receptor insensitivity, characterized by elevated insulin levels and associated with obesity.
• Hyperglycemia: excessive glucose in the blood, leading to polyphagia, polydipsia, polyuria, and slowly damaging blood vessels.

Glucagon

• Glucagon is produced by alpha cells in the pancreas.
• Glucagon structure consists of 29 amino acids and a molecular weight of 3485 dalton.
• Glucagon prevents hypoglycemia by stimulating glycogenolysis and glucose synthesis from amino acids and fat.
• Glucagon exerts its effects through cAMP.
• Glucagon secretion is stimulated by decreased blood glucose, sympathetic stimulation, and stress exercise.

Other Hormones

• Glucocorticoids (adrenal cortex): stimulate gluconeogenesis, inhibit glucose uptake in muscle and adipose tissue, and stimulate fat breakdown in adipose tissue.
• Sympathetic activity and epinephrine: stimulate glucagon secretion and lipolysis.