Peptide hormones represent one of the most important classes of signaling molecules in human physiology, regulating metabolism, reproduction, growth, and homeostasis.
Characteristics of Peptide Hormones
Chemical Properties - **Hydrophilic** — Cannot cross cell membranes - **Water-soluble** — Travel freely in blood plasma - **Short half-lives** — Rapidly degraded (minutes) - **Synthesized as precursors** — Require proteolytic activation
Mechanism of Action 1. Bind **cell surface receptors** (GPCRs or RTKs) 2. Activate **second messenger systems** (cAMP, Ca²⁺, IP₃) 3. Trigger **phosphorylation cascades** 4. Produce **rapid, transient effects**
Major Peptide Hormones
Metabolic Regulation | Hormone | Source | Function | |---------|--------|----------| | **Insulin** | Pancreatic β-cells | Lowers blood glucose | | **Glucagon** | Pancreatic α-cells | Raises blood glucose | | **GLP-1** | Intestinal L-cells | Stimulates insulin, reduces appetite | | **Ghrelin** | Stomach | Stimulates hunger |
Reproduction and Development | Hormone | Source | Function | |---------|--------|----------| | **Oxytocin** | Hypothalamus | Labor, bonding, milk ejection | | **Vasopressin (ADH)** | Hypothalamus | Water retention, vasoconstriction | | **GnRH** | Hypothalamus | Triggers LH/FSH release | | **Growth Hormone (GH)** | Anterior pituitary | Promotes growth |
Stress and Homeostasis | Hormone | Source | Function | |---------|--------|----------| | **ACTH** | Anterior pituitary | Stimulates cortisol release | | **Calcitonin** | Thyroid C-cells | Lowers blood calcium | | **PTH** | Parathyroid | Raises blood calcium | | **ANP/BNP** | Heart | Reduce blood pressure |
Peptide vs. Steroid Hormones
| Feature | Peptide Hormones | Steroid Hormones |
|---|---|---|
| Solubility | Water-soluble | Lipid-soluble |
| Receptor location | Cell surface | Intracellular |
| Speed of action | Fast (seconds-minutes) | Slow (hours-days) |
| Duration | Short | Long |
| Storage | Stored in vesicles | Synthesized on demand |
| Transport | Free in plasma | Bound to carriers |