Endocrine System and Hormone Signaling

Cellular Communication: Chemical Signals

Introduction

  • The nervous system uses both electrical and chemical signaling.
  • Electrical: Action potentials.
  • Chemical: Release of molecules (hormones, neurohormones, neurotransmitters) into extracellular fluid.

Cellular Signaling Principles

  • General template for cellular communication:
    • Stimulus: Various forms.
    • Signaling Cell: Stimulated by the stimulus.
    • Signaling Molecule: Released by the signaling cell (e.g., neurotransmitters).
    • Receptor: Molecule binds to a receptor, stimulating an effector molecule.
    • Response: Initiated by the effector molecule.

Types of Cellular Signaling (Endocrine System)

Local Signaling
  • Paracrine: Signaling to neighboring cells.
  • Autocrine: Self-regulating; cells signal to themselves.
  • Other types (not discussed in detail):
    • Communication via gap junctions (cells physically attached).
    • Contact-dependent signaling.
Paracrine Signaling
  • Para means "next to" or "nearby."
  • Signaling cell releases a molecule that interacts with neighboring cells.
  • Example: White blood cells release histamines, causing dilation of nearby blood vessels.
Autocrine Signaling
  • Auto means "self."
  • Cells release molecules that act on themselves (same type of cell).
  • Example: White blood cells stimulating their own replication during an infection.
Long Distance Signaling
  • Endocrine: Signaling over long distances.
  • Hormones are released into the blood, travel to distant target organs/tissues to elicit action.
  • Classification based on distance between signaling cell and stimulated cell.
  • Endocrine glands produce hormones, secreted into the blood, not ducts.

Endocrine Structures and Hormones

Primary Endocrine Organs
  • Main function is to secrete hormones.
  • Examples: Pineal gland, hypothalamus, pituitary, thyroid, parathyroid, thymus, adrenal gland, pancreas, testes/ovaries (gonads), placenta (during pregnancy).
Secondary Endocrine Organs
  • Main function is something else, but also secrete hormones.
  • Examples: Skin, heart, liver, stomach, kidney, small intestine.
Examples of Hormone-Releasing Organs, Hormones, and Functions:
  • Testes: Testosterone (male characteristics).
  • Ovaries (in females): This would replace the testes as stated above.

Exocrine vs. Endocrine Glands

Endocrine Glands
  • No ducts
  • Secrete hormones into the blood, travel to distant tissues.
Exocrine Glands
  • Have ducts
  • Secrete substances (mucus, enzymes) onto external or internal surfaces.
  • Examples:
    • Sweat glands (external skin).
    • Pancreas releasing pancreatic enzymes into the digestive system (internal surface).
Organs with Both Endocrine and Exocrine Functions
  • Stomach: Gastric glands with endocrine and exocrine secretions.
  • Pancreas: Endocrine (hormones into blood) and exocrine (digestive enzymes into the digestive tract) portions.

Hormone Types: Biochemical Properties & Modes of Action

  • Hormones: Molecules secreted by glands, transported via blood to regulate bodily functions (homeostasis).
  • Hormones grouped into three categories based on structure/chemical composition:
    • Peptide/Protein Hormones
    • Steroid Hormones
    • Amine Hormones
Peptide/Protein Hormones
  • Made of chains of amino acids (polypeptide chains/proteins).
  • Water-soluble, can move through the blood easily.
Steroid Hormones
  • Derived from cholesterol (fatty substance).
  • Lipid-soluble (likes lipids).
Amine Hormones
  • Most derived from the amino acid tyrosine (exception exists).
  • Can be water-soluble or lipid-soluble based on chemical structure.
Solubility & Hormone Interaction
  • Solubility dictates:
    • How hormones are synthesized.
    • How hormones are stored.
    • How hormones are transported.
    • How hormones elicit their actions.
    • How long hormones stay active in the bloodstream.
    • How hormones produce effects in target cells.

Hormone Synthesis and Storage

Peptide Hormones Synthesis
  • Synthesized as preprohormones in the rough endoplasmic reticulum (RER).
  • Modified in the Golgi apparatus.
  • Stored in secretory vesicles, waiting for stimulation and release.
Steroid Hormones Synthesis
  • Synthesized in the smooth endoplasmic reticulum (SER).
  • Examples: Hormones made in gonads (reproductive hormones), adrenal glands (aldosterone, cortisol), placenta (estradiol/estrogen).
Amine Hormones Synthesis
  • Most derived from tyrosine. Two types:
    • Catecholamines.
    • Thyroid hormones.
Cell Membrane Terminology

  • Phospholipid bilayer: Hydrophilic head and hydrophobic tail.
    *Heads on the outside: because of the extracellular and intracellular space which are primarily composed of water.
    *Tails: inwards because it’s trying to get away from the water.

  • Prefixes and suffixes:

    • Hydro: Water.
    • Lipo: Fats/lipids.
    • -philic: Love.
    • -phobic: Fear.

  • Hydrophilic: Water-loving, lipophobic (fat-fearing).

  • Lipophilic: Fat-loving, hydrophobic (water-fearing).

  • Water soluble hormones can't cross cell membranes because of water soluble loving tails inside the cell membranes

  • vesicles: are essentially little cells that also have a membrane

  • lipid Soluble hormones can be store in a vesicle because it diffuses straight out

  • Water-soluble hormones: are stored in secretory vesicles because they can’t diffuse out

  • steroid (fat-loving hormones): Are released immediately because they’re made as need, and then they diffuse straight out of the cell

Hormone Activity

  • Variables: half-life, speed of response, duration of response, clearance.
  • Half-life: Time for hormone concentration to decrease by half.
  • Speed of Response: Varies from seconds to hours depending on activated pathways.
  • Duration of Response: Related to hormone structure and degradation.
  • Clearance: Hormone cleared by the liver, excreted from the body.
Transport and Receptor Binding
  • Blood: Mostly water.
  • Water-soluble hormones: Diffuse and move around the body easily in the blood.
  • Lipid Solubility: will directly affect the hormone because of the receptor
  • Receptor Location: Influenced by hormone solubility.
    • Water-soluble hormones: receptors on the cell surface (can't enter cell).