Overview
This lecture covers how hormones are transported in the blood, the nature of hormone receptors, and the cellular mechanisms of hormone action, focusing on the differences between water-soluble and lipid-soluble hormones.
Hormone Transport in the Blood
- Hormones are transported either freely in blood or bound to carrier proteins.
- Free hormones are used first, while protein-bound hormones serve as a reserve.
- Kidney disease can cause loss of protein-bound hormones, affecting hormone levels (e.g., T3, T4).
Hormone Binding and Receptors
- A ligand is any molecule (like a hormone) that binds to a receptor.
- Binding is highly specific—like a lock and key (e.g., growth hormone binds only its receptor).
- Upregulation: cells increase receptor numbers when hormone levels are low.
- Downregulation: cells decrease receptor numbers when hormone levels are high.
Water-Soluble vs. Lipid-Soluble Hormones
- Water-soluble or large hormones (catecholamines, peptides) cannot enter cells; their receptors are on cell membranes.
- Lipid-soluble and small molecules (steroids, thyroid hormones) cross membranes and bind receptors inside the cell or nucleus.
Membrane-Bound Receptor Mechanisms
- Membrane-bound receptors work for hormones that cannot enter the cell (water-soluble).
- Involve a "second messenger" system, often using G-proteins.
- Hormone binds to membrane receptor → activates G-protein → G-protein activates adenylyl cyclase → produces cAMP (second messenger) → activates protein kinase → cellular response.
- Amplification: One hormone activates multiple pathways quickly.
Alternative Second Messengers
- Other second messengers include IP3 and DAG.
- Know the names but not the detailed mechanisms for this course.
Lipid-Soluble Hormone Action
- Lipid-soluble hormones cross cell and nuclear membranes to bind to intracellular receptors.
- Hormone-receptor complex binds DNA, alters transcription and translation, leading to new protein synthesis.
- This process is slower and does not amplify like membrane, G-protein systems.
Hormone Examples by Mechanism
- Water-soluble: catecholamines (epinephrine, norepinephrine), peptide and protein hormones.
- Lipid-soluble: steroid hormones (cortisol, aldosterone, sex steroids), thyroid hormones.
Key Terms & Definitions
- Hormone — a chemical messenger secreted into the blood.
- Ligand — any molecule that binds to a receptor.
- Receptor — protein that specifically binds a ligand.
- Upregulation — increase in receptor numbers on a cell.
- Downregulation — decrease in receptor numbers on a cell.
- Second Messenger — intracellular molecule (e.g., cAMP) that transmits signals from membrane receptors.
- G-protein — membrane protein that relays signals from receptors to enzymes.
- Adenylyl Cyclase — enzyme that converts ATP to cAMP.
Action Items / Next Steps
- Review mechanisms of cAMP as a second messenger.
- Memorize which hormones use G-protein/second messenger systems and which use intracellular receptors.
- Prepare for exam questions on hormone transport, receptor types, and signal transduction pathways.