3. Types and Mechanisms of Hormonal Action

3.3. Receptor Types

1. Membrane Receptor Mechanism (for Water-Soluble Hormones)

  • Receptor location: Cell surface.
  • Receptor types:
    • Direct effects on membrane ion channels (surface receptors)
    • Direct activation of intracellular enzymes (surface receptors)
    • Activation of G proteins/second messenger system (surface receptors)
    • Direct activation of specific genes (intracellular receptors).

Note: A certain receptor type may be present in 100 – 1,000,000 copies on a single cell.

  • Mechanism:
    • Hormone binds to receptor → activates second messenger system (e.g., cAMP, IP₃/DAG).
    • Second messengers amplify the signal inside the cell.
  • Examples: Insulin, adrenaline.

2. Intracellular Receptor Mechanism (for Lipid-Soluble Hormones)

  • Receptor location: Cytoplasm or nucleus.
  • Mechanism:
    • Hormone enters cell → binds intracellular receptor → hormone-receptor complex binds DNA → regulates gene expression (transcription).
  • Examples: Cortisol, thyroid hormones