Overview
This lecture explains how hormones communicate with target cells by binding to specific receptors and describes the differences between intracellular and plasma membrane hormone receptors.
Hormone Action and Receptors
- Hormones affect only target cells that have specific receptors for that hormone.
- The number of hormone receptors on a cell determines its sensitivity and response to the hormone.
- Up-regulation increases receptor numbers in response to low hormone levels, making cells more sensitive.
- Down-regulation decreases receptor numbers in response to high hormone levels, making cells less sensitive.
- Receptor binding changes cellular activity, increasing or decreasing normal body processes.
Intracellular Hormone Receptors
- Lipid-derived hormones (e.g., steroids) diffuse across cell membranes and bind to intracellular receptors.
- Steroid hormones and their receptors regulate gene expression by altering transcription and mRNA synthesis.
- The hormone-receptor complex acts as a transcription regulator in the nucleus, producing proteins that change cell function.
- Examples: steroid hormones, vitamin D, and thyroxine use intracellular or nuclear receptors.
Plasma Membrane Hormone Receptors
- Amino acid-derived and polypeptide hormones cannot cross the cell membrane; they bind to plasma membrane receptors.
- Hormone binding activates a signaling pathway inside the cell without the hormone crossing the membrane.
- The hormone is the "first messenger" and activates a "second messenger" (e.g., cAMP) inside the cytoplasm.
- G-proteins associated with the receptor are activated and trigger adenylyl cyclase to convert ATP to cAMP.
- cAMP activates protein kinases, which phosphorylate other proteins, leading to cellular responses.
- Signal amplification occurs as one hormone triggers many downstream molecules and actions.
- Phosphodiesterase (PDE) breaks down cAMP to stop hormone signaling.
Hormone Responses
- Cellular responses depend on receptor types and available substrate molecules inside the cell.
- Effects include changes in membrane permeability, metabolic pathways, protein/enzyme synthesis, and hormone release.
Key Terms & Definitions
- Hormone — Chemical messenger that regulates specific functions in target cells.
- Receptor — Protein on or in a cell that binds a specific hormone.
- Up-regulation — Increase in receptor numbers in response to low hormone levels.
- Down-regulation — Decrease in receptor numbers due to high hormone levels.
- Intracellular Receptor — Receptor located inside the cell, usually for lipid-soluble hormones.
- Plasma Membrane Receptor — Receptor located on the cell surface for lipid-insoluble hormones.
- G-protein — Membrane protein involved in transmitting signals from hormone-receptor binding to intracellular pathways.
- Second Messenger — Molecule (like cAMP) that carries the signal from the cell membrane to inside the cell.
- Transcription Regulation — Process of controlling gene expression at the DNA level.
Action Items / Next Steps
- Review the mechanisms of hormone signaling for both intracellular and plasma membrane receptors.
- Study Figures 37.5 and 37.6 for visual understanding of hormone pathways.