Types of Receptor Pathways in Hormones

Introduction

• Focus on the most important receptor pathways for hormones.
• Hormones are categorized into:
  • Peptide Hormones: Water-soluble.
  • Steroid Hormones: Lipid-soluble.

Peptide Hormones

• Water Soluble: Can interact with water due to hydrophilic amino acids.
• Issues with Cell Membrane:
  • Too large and charged to pass through lipid bilayer of cell membrane.
  • Require membrane receptors to exert effects.
• Second Messenger Systems:
  • Predominantly use second messenger systems for intracellular signaling.
  • Focus on GQ pathway and G-stimulatory pathway.
• Examples:
  • FSH, LH, Growth hormone, Glucagon, Parathyroid hormone, Insulin, Oxytocin, ADH.

Steroid Hormones

• Lipid Soluble: Derived from cholesterol, can pass through lipid bilayer.
• Intracellular Receptors:
  • Do not need membrane receptors.
  • Bind to receptors inside the cell.
• Examples:
  • Testosterone, Estrogen, Progesterone, Aldosterone, Cortisol, Vitamin D.

G-Stimulatory Pathway

• G Protein Coupled Receptors (GPCR):
  • Activated by hormone like Epinephrine.
  • Binds to GDP and converts to GTP upon activation.
  • Activates adenylate cyclase enzyme.
• Adenylate Cyclase Function:
  • Converts ATP to cyclic AMP (cAMP).
  • cAMP activates protein kinases.
• Outcome of Pathway:
  • Protein kinases phosphorylate proteins.
  • Alters membrane permeability, regulates metabolic pathways, protein synthesis, and cell growth.

GQ Pathway

• GQ Protein: Activated by hormones such as Oxytocin.
• Phospholipase C Enzyme:
  • Converts PIP2 into DAG and IP3.
  • DAG activates protein kinase C.
  • IP3 releases calcium from smooth endoplasmic reticulum.
• Calcium Function:
  • Binds to calmodulin, activating further kinases.
  • Important for muscle contraction, membrane changes, etc.

Steroid Hormone Pathway

• Intracellular Receptors:
  • Bound to heat shock proteins when inactive.
  • Hormones like Testosterone can diffuse across the membrane.
• Gene Regulation:
  • Hormone binds to receptor, displacing heat shock proteins.
  • Binds to hormone response elements in DNA, influencing gene expression.

Inhibition of Cyclic AMP Pathway

• Phosphodiesterase Enzymes (PDE):
  • Break down cAMP to prevent overactivation of pathways.

Conclusion

• Reviewed G-stimulatory and GQ pathways, and the mechanism for steroid hormones.
• Highlighted the need for different receptors for peptide and steroid hormones.