Hormonal regulation

• Hormones


  Chemicals released by cells or glands in one part of the body, which send messages affecting cells in other parts of the organism
• Hormones


  • Usually transported in the blood
  • Cells respond to hormones by expressing specific receptors, which, when bound, activate signal transduction mechanisms leading to specific cellular responses
• Types of Hormones


  • Protein and Peptides (e.g. insulin, glucagon)
  • Modified Amino Acids (Monoamines) (e.g. epinephrine, norepinephrine)
  • Lipids and Steroids (e.g. cortisol, testosterone, estrogen)
• Types of Hormone Release

  • Endocrine Hormones (secreted directly into the bloodstream to travel to distant target organs)
  • Paracrine Hormones (act locally by diffusing through the interstitial spaces to nearby target tissues)
• Neuronal Signaling


  1. Electrical signals (nerve impulses) travel rapidly from the cell body of a neuron along the axon to the axon tip

  2. Neurotransmitters are released at the axon tip, diffusing to the target cell (another neuron, muscle cell, or secretory cell) located very close to the release site
• Endocrine Signaling


  1. Hormones are secreted into the bloodstream and carried throughout the body to target tissues

  2. The target tissues can be far from the hormone-secreting cells
• Major Endocrine Systems and Target Tissues

  • Central Nervous System to Target Tissues
  • Thymus (involved in immune response)
  • Pineal Gland (secretes melatonin, regulating sleep cycles)
  • Gastrointestinal Tract (contains cells that secrete hormones regulating digestion and metabolism)
• Insulin


  • Produced by β cells in the pancreas
  • Facilitates glucose uptake by cells, promotes glycogen synthesis in the liver and muscles, and stimulates fat synthesis in adipose tissue
• Glucagon


  • Produced by α cells in the pancreas
  • Promotes the breakdown of glycogen to glucose in the liver (glycogenolysis) and stimulates gluconeogenesis (the synthesis of glucose from non-carbohydrate sources)
• Epinephrine (Adrenaline)


  • Secreted by the adrenal medulla
  • Increases blood glucose levels by promoting glycogen breakdown, enhances muscle contraction, and prepares the body for 'fight or flight' responses
• Growth Hormone and Growth Factors

  • Stimulate growth, cell reproduction, and regeneration
  • Increase protein synthesis and promote the use of fat for energy
• Leptin


  Produced by adipose (fat) tissue, regulates energy balance by inhibiting hunger
• Adiponectin


  Enhances sensitivity to insulin and regulates glucose levels and fatty acid breakdown
• Thyroxine (T3 and T4)

  • Secreted by the thyroid gland
  • Regulates the rate of metabolism and energy production
• Cortisol


  • Produced by the adrenal cortex
  • Increases blood sugar through gluconeogenesis, suppresses the immune system, and aids in fat, protein, and carbohydrate metabolism
• Insulin Secretion by Pancreatic β Cells

  1. High blood glucose levels increase intracellular ATP in β cells, closing K+ channels, leading to membrane depolarization

  2. This opens Ca2+ channels, allowing Ca2+ influx, which triggers insulin release through exocytosis
• Regulation in the Well-Fed State

  1. After a meal, glucose, fatty acids, and amino acids enter the liver

  2. Insulin stimulates glucose uptake by tissues, conversion to acetyl-CoA, and synthesis of fatty acids stored as triacylglycerols in adipose tissue
• Regulation in the Fasting State

  1. The liver becomes the primary source of glucose for the brain

  2. Glycogen is broken down, and gluconeogenesis produces glucose from amino acids and glycerol

  3. Fatty acids are used as fuel, and excess acetyl-CoA is converted to ketone bodies
• Insulin Effects


  • Promotes glucose uptake by cells
  • Increases glycogen synthesis in the liver and muscles
  • Enhances lipid synthesis in adipose tissue
• Glucagon Effects


  • Stimulates glycogen breakdown in the liver
  • Promotes gluconeogenesis
• Hypothalamic Regulation


  • The hypothalamus regulates food intake and energy expenditure by interacting with adipose tissue
  • Hormones like leptin and insulin modulate hunger and energy metabolism through neurosecretory cells
• AMP-Activated Protein Kinase (AMPK)


  • Activated by increased levels of AMP
  • Regulates energy by controlling anabolic and catabolic pathways through phosphorylation of key enzymes
• Adiponectin


  • Produced in response to low energy reserves
  • Stimulates energy production processes and inhibits energy consumption processes, acting through AMPK