Hormonal Regulation of Carbohydrates

Hormonal Regulation of Carbohydrates

• Glucose availability is crucial for tissues.
• Glycogenolysis converts glycogen to glucose/G6P.
• Gluconeogenesis converts FFAs, AAs, lactate to glucose.
• Insulin lowers blood glucose: opposes hyperglycemia, facilitates glucose transport into cells, enhances glycogen, protein, and fat synthesis, and inhibits gluconeogenesis.
• Glucagon raises blood glucose: counters hypoglycemia, opposes insulin, promotes glycogenolysis and gluconeogenesis.

Pancreatic Hormones

• Pancreas has acini and islets of Langerhans.
• Islets contain 20% alpha cells (glucagon) and 75% beta cells (insulin).
• Both glucagon and insulin are peptide hormones.
• Acini secrete digestive enzymes.

Glucagon

• Alpha cells in the islets of Langerhans secrete glucagon.
• Glucagon is an antagonist to insulin.
• Glucagon primarily stimulates glycogenolysis and gluconeogenesis in the liver.
• Glucagon increases lipid catabolism.
• Plasma glucose concentration controls glucagon output.

Glucagon & cAMP Pathway

• Glucagon binds to receptor, stimulating adenylate cyclase.
• ATP is converted to cAMP.
• cAMP activates protein kinase A (PKA).
• PKA phosphorylates glycogen phosphorylase.
• Phosphorylase initiates glycogenolysis and releases glucose-6-phosphate (G6P).
• G6P is converted back to glucose (only in liver).
• Glucose released to restore blood glucose.

Insulin Decreases Glucose

• Insulin regulates glucose entry into muscle and adipose tissue, not brain.
• Insulin reduces blood glucose concentration.
• 2nd messengers (PI-3 kinase pathway) stimulate GLUT transporters (GLUT4 in muscle) to translocate to the plasma membrane, allowing glucose uptake.
• Glucose enters cells via facilitated diffusion.

Insulin's Anabolic Effects

• Insulin stimulates protein synthesis.
• Importance of post-exercise CHO consumption for glycogen replenishment.

Insulin-Mediated Glucose Uptake

• PI-3 Kinase Pathway regulates insulin-stimulated glucose uptake.
• Insulin, IRS-1, PI3K, PDK, etc. are involved.
• GLUT4 translocation to sarcolemma occurs.

Impaired Glucose Homeostasis

• Diabetes can result from defects in glucose uptake pathway.
• Possible causes include: B-cell destruction (Type 1), insulin resistance (Type 2), altered insulin receptors, or defective processing of the insulin message.
• Insulin synthesis or release may be abnormal.

Regulation of Carbohydrate Metabolism During Exercise

• Adequate glucose is needed during exercise.
• Liver and muscle regulate glucose levels based on exercise intensity and duration.
• Hormones like Glucagon, Epinephrine, Norepinephrine, and Cortisol increase blood glucose.
• Exercise impacts circulating glucose via FFA mobilization and glucose uptake/catabolism.

Regulation of Carbohydrate Metabolism During Exercise (Intensity)

• Catecholamines increase glycogenolysis in liver and muscles as exercise intensity increases.
• Muscle glycogen used before liver glycogen.

Regulation of Carbohydrate Metabolism During Exercise (Duration)

• As exercise duration increases, more liver glycogen is used.
• More muscle glucose uptake and liver glucose release occur.
• Glycogen stores decrease, glucagon level increases.

Hormonal & Glucose Changes (Graph)

• Shows changes in Epinephrine, Norepinephrine, Glucagon, Cortisol, and Glucose over time.

Changes in Glucose & Insulin (Graph)

• Shows changes in glucose and insulin levels over time.

Regulation of Carbohydrate Metabolism During Exercise (Insulin's role)

• Insulin's role in glucose uptake at rest.
• Insulin decreases during exercise; insulin sensitivity increases.
• AMPK pathway facilitates glucose uptake without insulin.
• AMPK is not reliant on insulin to move GLUT4

Exercise-Mediated Glucose Uptake

• Insulin, IRS-1, PI3K, PDK, PKC, Akt, AS160 promote GLUT4 translocation.
• AMPK also facilitates glucose uptake independent of insulin, potentially via AS160 or other means.
• Exercise has a significant impact on blood glucose regulation.

Glycemic Control During Exercise

• Skeletal muscle is the major consumer of glucose in the body during exercise.
• Exercise abruptly decreases plasma glucose levels, an effect that persists for many days.
• Exercise increases insulin sensitivity in working muscles.
• This improved insulin sensitivity has a long term, glycemic control benefit.

Glycemic Control During Exercise (Improved Insulin Sensitivity)

• GLUT-4 translocation from endoplasmic reticulum to the cell surface increases.
• Increased total quantity of GLUT-4 facilitates transport
• Glycogen synthase activity and storage increase.
• Combination of resistance and endurance exercise is beneficial for insulin resistance.

Control of Food Intake (Hunger Hormones)

• Hormones like leptin and ghrelin regulate appetite.
• Leptin is secreted by adipose tissue; it signals satiety, decreasing appetite.
• Ghrelin is secreted by the stomach and increases appetite.

Regulating Caloric Intake

• Hypothalamus is the brain's appetite control center.
• Satiety center in ventromedial nucleus, hunger center in lateral hypothalamus.
• Leptin and ghrelin act in opposing ways in energy balance.

Leptin & Satiety

• Leptin is a peptide hormone secreted by adipose tissue that signals satiety to the hypothalamus.
• Leptin levels are proportional to body fat.
• Obese individuals often have leptin resistance, potentially contributing to overeating.

GI Tract Hormones

• GI tract releases hormones affecting hunger signals.
• Ghrelin increases appetite.
• Cholecystokinin (CCK) decreases appetite when full.
• Glucagon-like peptide 1 (GLP-1) and Peptide YY (PYY) decreases appetite in the small intestine.
• Exercise increases PYY and GLP-1, reducing hunger.

Review Questions

• Explain glucagon's role in regulating blood glucose, including a negative feedback diagram & cAMP signaling steps.
• Explain insulin's role using negative feedback and the PI3-kinase pathway
• Discuss exercise benefits for diabetics via the AMPK pathway.
• Compare and contrast the roles of leptin and ghrelin in regulating hunger.

Figure & Notes References

• Provide references for figures and notes, including authors, titles, editions, and publishers.

This quiz explores the hormonal regulation of carbohydrates, focusing on the roles of insulin and glucagon in glucose homeostasis. It covers key processes such as glycogenolysis and gluconeogenesis, along with the functions of pancreatic hormones. Test your understanding of how these hormones interact to maintain blood glucose levels.