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Overview of Receptor Pathways

  • The video focuses on the various receptor pathways important for hormone action.
  • There are numerous types of receptors and hormones, but the video highlights the most significant pathways.

Types of Hormones

  • Hormones can be classified into two primary types:
    • Peptide Hormones
    • Also known as water-soluble hormones.
    • Examples: FSH (Follicle-stimulating hormone), LH (Luteinizing hormone), Growth hormone, Glucagon, Parathyroid hormone, Insulin, Oxytocin, ADH (Antidiuretic hormone).
    • Steroid Hormones
    • Lipid-soluble hormones derived from cholesterol.
    • Examples: Testosterone, Estrogen, Progesterone, Aldosterone, Cortisol, Gonadocorticoids, Vitamin D, Thyroxine.

Hormonal Mechanisms of Action

1. Peptide Hormones - Mechanism of Action

  • Water Solubility: Peptide hormones are hydrophilic and soluble in water, which allows their amino acids to interact with water.
  • Cell Membrane Incompatibility:
    • Peptide hormones cannot penetrate the phospholipid bilayer of the cell membrane because they are too large and water-soluble.
    • They exert effects by binding to specific membrane receptors.
  • Second Messenger Systems:
    • Peptide hormones work through second messenger systems, involving multiple intracellular signaling pathways.
    • Focus on two pathways:
    1. GQ Pathway
    2. G Stimulatory Pathway

2. Steroid Hormones - Mechanism of Action

  • Lipid Solubility: Steroid hormones, being lipid soluble, can easily cross cell membranes.
  • Intracellular Receptors: These hormones typically bind to receptors located inside the cytoplasm or nucleus.
  • Generic Pathway of Steroid Hormones:
    • When a steroid hormone like testosterone diffuses through the membrane, it binds to an intracellular receptor, displacing heat shock proteins (HSP).
    • The steroid-receptor complex binds to hormone response elements (HRE) on DNA, prompting transcriptional activity leading to various cellular responses (e.g., protein synthesis, cell proliferation).

Detailed Breakdown of Peptide Hormonal Pathways

1. G Stimulatory Protein Pathway

  • G Protein-Coupled Receptors (GPCR):
    • GPCRs are seven-pass transmembrane receptors coupled with G proteins.
    • When a hormone (e.g., epinephrine) binds to the receptor, it activates the G stimulatory protein (Gs).
  • Activation of Gs:
    • Initially, Gs is bound to GDP.
    • Hormonal binding causes a conformational change and GDP is replaced by GTP, activating Gs.
  • Effector Enzyme Activation:
    • The activated Gs protein interacts with an effector enzyme, primarily Adenylate Cyclase, triggering the conversion of ATP to cyclic AMP (cAMP).
  • Activation of Protein Kinases:
    • cAMP activates Protein Kinases (PK), leading to phosphorylation of proteins.
    • Phosphorylation can change enzyme activity, alter membrane permeability, or affect gene transcription.
Example Effects of G Stimulatory Pathway:
  • Metabolic Regulation: Altering glycolysis, gluconeogenesis, or glycogen metabolism.
  • Ion Channel Regulation: Phosphorylation changes membrane ion permeability (e.g., calcium influx).
  • Gene Activation: Phosphorylation of transcription factors promotes gene expression and cellular growth.

2. GQ Protein Pathway

  • Activation: GQ proteins are activated similarly by hormones (e.g., oxytocin binding to its receptor).
  • Phospholipase C Activity:
    • Activated GQ binds to and stimulates Phospholipase C (PLC).
    • PLC cleaves phosphatidylinositol 4,5-bisphosphate (PIP2) into inositol trisphosphate (IP3) and diacylglycerol (DAG).
Role of IP3 and DAG:
  • IP3 Function:
    • Binds to receptors in the endoplasmic reticulum (ER), facilitating the release of calcium ions into the cytoplasm.
    • Calcium can bind to calmodulin, activating additional kinases and triggering muscle contractions, among other effects.
  • DAG Role:
    • Activates Protein Kinase C (PKC), similarly leading to downstream phosphorylation of various proteins.
    • The effects can include changes in cellular metabolism, protein synthesis, and ion channel activity, similar to the Gs pathway.

Steroid Hormone Pathways

3. Steroid Hormones: Mechanism in the Nucleus

  • Intracellular Interaction:
    • Once inside the cell, steroid hormones (e.g., testosterone) bind to cytosolic or nuclear receptors, displacing HSPs.
    • The activated receptor-hormone complex translocates to the nucleus and binds to HRE on DNA.
Effects of Steroid Hormones:
  • Gene Regulation: Modulating transcription and subsequent protein synthesis, influencing multiple cellular functions such as metabolism, growth, and ion transport.

Inhibition of Pathways

  • Inhibition Mechanisms:
    • The activity of pathways can be inhibited by enzymes such as Phosphodiesterase (PDE), which degrade cAMP or other signaling molecules to prevent excessive stimulation.
    • PDE serves as a regulatory mechanism to control the duration and intensity of signaling pathways, ensuring cellular homeostasis.

Summary

  • Within the video, the comparative mechanisms of action for peptide vs. steroid hormones are outlined, emphasizing the roles of second messengers in peptide hormone signaling and diverse effects medications on target cells.