Week 6: Hormones

Introduction to Hormones

  • Hormones are chemical messengers in the body.
  • They send signals through the bloodstream and affect various processes such as:
    • Growth and development
    • Metabolism
    • Energy utilization from food
    • Sexual function and reproduction
    • Mood regulation
  • Hormones act like keys that fit specific locks (receptors) on target cells.

Types of Hormones on Chemical Basis

  • Hormones are produced in one body region and affect another.
  • Only certain cells can respond to specific hormones based on their receptors.
  • Hormones are categorized into four structural groups:
    • Peptides and proteins: Composed of chains of amino acids.
    • Steroids: Derived from cholesterol, including sex hormones.
    • Amino acid derivatives: Generally derived from amino acids, like tyrosine and tryptophan.
    • Fatty acid derivatives (eicosanoids): Derived from fatty acids involved in signaling processes.

Amino Acid-Derived Hormones

  • Derived from amino acids, retaining their amino group.
  • Properties include hydrophilicity:
    • Thyroid hormones: Formed from tyrosine with iodine.
    • Catecholamines: Include epinephrine and norepinephrine.
    • Other hormones:
    • Serotonin and melatonin from tryptophan.
    • Histamine from histidine.

Peptide and Protein Hormones

  • Composed of 2-200 amino acids;
  • Hydrophilic and soluble in water.
  • Synthesized through the endomembrane system (transcription, translation, modification).
  • Stored in secretory vesicles until secretion is needed.

Steroid Hormones

  • Derived from cholesterol, hydrophobic, easily pass through the plasma membrane.
  • Actions involve binding to intracellular receptors leading to changes in gene expression.
  • Include sex hormones and corticosteroids produced in gonads and adrenal cortex.
  • Not stored; synthesized and released immediately after production.
  • Require transport proteins to circulate in blood due to hydrophobicity.

Fatty Acid Derivatives: Eicosanoids

  • Derived from polyunsaturated fatty acids (arachidonic acid is key).
  • Classes include prostaglandins, prostacyclins, leukotrienes, thromboxanes.
  • Rapidly metabolized and usually have short-lived effects.

Pathways of Hormone Action

  • Hormones bind to specific receptors on target cells.
  • Causes activation of signaling pathways leading to varied cellular responses:
    • Protein synthesis stimulation.
    • Enzyme activation/deactivation.
    • Changes in cell membrane permeability.
    • Changes in cell growth rate and mitosis.

Binding of Hormones

Water-Soluble Hormones
  • Peptide hormones (hydrophilic) bind to receptors on the plasma membrane.
  • Signal transduction often involves G proteins and cAMP as secondary messengers.
Lipid-Soluble Hormones
  • Steroid hormones (hydrophobic) bind to intracellular receptors, target specific DNA sequences.
  • This affects protein synthesis but has a delayed action compared to water-soluble hormones.

Factors Affecting Target Cell Response

  • Hormones interact with specific receptors on target cells:
    • Sensitivity to hormones varies based on receptor number (up-regulation increases sensitivity).
    • Down-regulation occurs when receptors decrease, reducing sensitivity.

Hormonal Interaction

  • Permissiveness: Action of one hormone requires presence of another (e.g., thyroid hormone needs epinephrine).
  • Synergism: Combined effects of hormones are magnified (e.g., FSH and estrogen in egg maturation).
  • Antagonism: One hormone opposes another’s effects (e.g., calcitonin vs parathyroid hormone).

Hormonal Triggering Factors

  • Hormones are regulated by:
    • Neural stimuli: Direct nerve fiber influence on endocrine glands (e.g., hypothalamus-pituitary connection).
    • Hormonal stimuli: Hormones from one gland affecting others (e.g., tropic hormones from the hypothalamus).
    • Humoral stimuli: Non-hormonal substances trigger hormone release (e.g., insulin release due to high blood glucose).

Regulation of Hormonal Secretion

  • Hormones maintain homeostasis via negative feedback loops:
    • Deviation from a set point prompts a response that negates the change (e.g., insulin regulation of blood sugar).
  • Positive feedback loops are rare and enhance changes (e.g., oxytocin during labor).

Summary of Hormonal Mechanisms

  • cAMP serves as a secondary messenger for hydrophilic hormones.
  • G proteins switch cellular responses based on hormone binding.
  • Negative feedback helps regulate blood hormone levels, while positive feedback amplifies responses in certain situations.
  • Hormones that trigger other hormones are tropic hormones, with varied effects depending on receptor distribution.