Metabolic

Hormones act as key regulators of numerous metabolic pathways in the body. Understanding how hormones influence these pathways is crucial for understanding endocrine disorders

Hormonal Regulation of Metabolism

Hormones: Chemical messengers produced by endocrine glands that travel through the bloodstream to target cells, where they elicit specific physiological effects
Metabolic Pathways: Biochemical reactions that occur in cells and tissues to regulate energy production, storage, and utilization
Key Hormones and Their Metabolic Effects
- Insulin
- Glucagon
- Epinephrine (Adrenaline)
- Cortisol
- Growth Hormone
- Thyroid Hormones (T3 and T4)
General Principles of Hormonal Regulation
- Hormones exert their effects by binding to specific receptors on target cells
- Hormone-receptor binding triggers intracellular signaling cascades that alter enzyme activity and gene expression
- Hormonal regulation can be rapid (e.g., changes in enzyme activity) or slow (e.g., changes in gene expression)
- Hormones often work in opposition to maintain metabolic homeostasis

Insulin

Source: Pancreatic beta cells
Stimulus for Release: High blood glucose levels
Target Tissues: Liver, muscle, adipose tissue
Metabolic Effects
- Glucose Uptake
  - Increases glucose uptake into muscle and adipose tissue by promoting the translocation of GLUT4 glucose transporters to the cell membrane
  - Facilitates glucose entry into cells, lowering blood glucose levels
- Glycogenesis
  - Stimulates glycogen synthesis (glycogenesis) in the liver and muscle
  - Promotes the conversion of glucose to glycogen for storage
- Glycolysis
  - Enhances glycolysis in the liver and muscle
  - Increases the breakdown of glucose to pyruvate and ATP
- Gluconeogenesis
  - Inhibits gluconeogenesis in the liver
  - Reduces the production of glucose from non-carbohydrate precursors
- Lipogenesis
  - Stimulates fatty acid synthesis (lipogenesis) in the liver and adipose tissue
  - Promotes the conversion of excess glucose to triglycerides for storage
- Lipolysis
  - Inhibits lipolysis in adipose tissue
  - Reduces the breakdown of triglycerides to fatty acids and glycerol
- Protein Synthesis
  - Stimulates protein synthesis in muscle and other tissues
  - Promotes the uptake of amino acids and the formation of new proteins
- Protein Degradation
  - Inhibits protein degradation
  - Reduces the breakdown of proteins to amino acids
Overall Effect: Lowers blood glucose levels and promotes energy storage

Glucagon

Source: Pancreatic alpha cells
Stimulus for Release: Low blood glucose levels
Target Tissue: Liver
Metabolic Effects
- Glycogenolysis
  - Stimulates glycogen breakdown (glycogenolysis) in the liver
  - Promotes the release of glucose from glycogen stores
- Gluconeogenesis
  - Enhances gluconeogenesis in the liver
  - Increases the production of glucose from non-carbohydrate precursors
- Lipolysis
  - Stimulates lipolysis in adipose tissue (to a lesser extent than epinephrine)
  - Promotes the release of fatty acids and glycerol
- Ketogenesis
  - Stimulates ketogenesis in the liver
  - Promotes the production of ketone bodies from fatty acids
- Glycolysis
  - Inhibits Glycolysis in the liver
- Glycogenesis
  - Inhibits Glycogenesis in the liver
Overall Effect: Raises blood glucose levels and provides alternative energy sources (fatty acids and ketone bodies)

Epinephrine (Adrenaline)

Source: Adrenal medulla
Stimulus for Release: Stress, exercise, low blood glucose levels
Target Tissues: Liver, muscle, adipose tissue
Metabolic Effects
- Glycogenolysis
  - Stimulates glycogen breakdown in the liver and muscle
  - Provides glucose for energy during stress or exercise
- Gluconeogenesis
  - Enhances gluconeogenesis in the liver
  - Increases glucose production
- Lipolysis
  - Stimulates lipolysis in adipose tissue
  - Promotes the release of fatty acids for energy
- Insulin Secretion
  - Inhibits insulin secretion
  - Reduces glucose uptake into cells
Overall Effect: Raises blood glucose levels and mobilizes energy stores

Cortisol

Source: Adrenal cortex
Stimulus for Release: Stress, low blood glucose levels
Target Tissues: Liver, muscle, adipose tissue
Metabolic Effects
- Gluconeogenesis
  - Stimulates gluconeogenesis in the liver
  - Increases glucose production from amino acids and glycerol
- Protein Catabolism
  - Promotes protein breakdown in muscle
  - Releases amino acids for gluconeogenesis
- Lipolysis
  - Stimulates lipolysis in adipose tissue
  - Releases fatty acids for energy
- Insulin Resistance
  - Induces insulin resistance in peripheral tissues
  - Reduces glucose uptake into cells
Overall Effect: Raises blood glucose levels and provides energy during stress, but prolonged elevation can lead to metabolic dysfunction

Growth Hormone

Source: Anterior pituitary gland
Stimulus for Release: Growth, exercise, low blood glucose levels
Target Tissues: Liver, muscle, adipose tissue
Metabolic Effects
- Protein Synthesis
  - Stimulates protein synthesis in muscle and other tissues
  - Promotes growth and development
- Lipolysis
  - Stimulates lipolysis in adipose tissue
  - Releases fatty acids for energy
- Insulin Resistance
  - Induces insulin resistance in peripheral tissues
  - Reduces glucose uptake into cells
- Gluconeogenesis
  - Stimulates Gluconeogenesis in the liver
Overall Effect: Promotes growth, protein synthesis, and energy mobilization, but can also impair glucose metabolism

Thyroid Hormones (T3 and T4)

Source: Thyroid gland
Stimulus for Release: Thyroid-stimulating hormone (TSH) from the pituitary gland
Target Tissues: Most tissues in the body
Metabolic Effects
- Basal Metabolic Rate (BMR)
  - Increases BMR
  - Elevates energy expenditure and oxygen consumption
- Carbohydrate Metabolism
  - Enhances glucose absorption from the intestine
  - Increases glycolysis and gluconeogenesis
- Lipid Metabolism
  - Stimulates lipolysis
  - Reduces cholesterol levels
- Protein Metabolism
  - Promotes protein synthesis and degradation
Overall Effect: Increases metabolic rate, enhances energy utilization, and affects carbohydrate, lipid, and protein metabolism

Summary Table of Hormonal Effects on Metabolic Pathways

Hormone	Glycogenesis	Glycogenolysis	Gluconeogenesis	Glycolysis	Lipogenesis	Lipolysis	Protein Synthesis	Protein Degradation
Insulin	Increase	Decrease	Decrease	Increase	Increase	Decrease	Increase	Decrease
Glucagon	Decrease	Increase	Increase	Decrease	Decrease	Increase	Decrease	Increase
Epinephrine	Decrease	Increase	Increase	Increase	Decrease	Increase	Decrease	Decrease
Cortisol	Decrease	Increase	Increase	Decrease	Decrease	Increase	Decrease	Increase
Growth Hormone	Decrease	Increase	Increase	Decrease	Decrease	Increase	Increase	Decrease
Thyroid Hormone	Increase	Increase	Increase	Increase	Decrease	Increase	Increase	Increase

Clinical Significance

Diabetes Mellitus: Imbalance of insulin and glucagon leads to hyperglycemia and metabolic dysfunction
Cushing’s Syndrome: Excess cortisol causes hyperglycemia, muscle wasting, and lipolysis
Acromegaly: Excess growth hormone causes insulin resistance and glucose intolerance
Hyperthyroidism: Excess thyroid hormones cause increased metabolic rate, weight loss, and glucose intolerance
Hypothyroidism: Deficiency of thyroid hormones causes decreased metabolic rate, weight gain, and glucose intolerance

Key Terms

Hormone: A chemical messenger produced by endocrine glands
Receptor: A protein on a target cell that binds to a hormone
Signal Transduction: The process by which a hormone-receptor interaction triggers intracellular events
Glycogenesis: The synthesis of glycogen from glucose
Glycogenolysis: The breakdown of glycogen to glucose
Gluconeogenesis: The synthesis of glucose from non-carbohydrate precursors
Glycolysis: The breakdown of glucose to pyruvate
Lipogenesis: The synthesis of fatty acids
Lipolysis: The breakdown of triglycerides to fatty acids and glycerol
Protein Synthesis: The formation of new proteins
Protein Degradation: The breakdown of proteins

Biochemicals

Physiology