Human growth hormone (HGH) works primarily by stimulating growth directly and indirectly through the release of insulin-like growth factor-I (IGF-I).
Here's a breakdown of the mechanisms:
1. Direct Effects:
- Stimulates Bone Growth: HGH's most notable direct effect is on the epiphyseal growth plates, which are located at the ends of long bones. HGH stimulates these plates, leading to bone elongation.
- Metabolic Effects: HGH has several metabolic effects:
- Increases Protein Synthesis: It boosts protein production in various tissues, crucial for muscle growth and repair.
- Mobilizes Fat Stores: HGH promotes the breakdown of triglycerides (fats) in adipose tissue, providing energy.
- Increases Blood Glucose Levels: By reducing insulin sensitivity, HGH can increase blood glucose levels.
2. Indirect Effects (IGF-I Mediated):
- IGF-I Release: HGH stimulates the liver and other tissues to produce Insulin-like Growth Factor-I (IGF-I), also known as somatomedin C.
- IGF-I Actions: IGF-I then mediates many of the growth-promoting effects of HGH:
- Bone Growth: IGF-I further stimulates bone growth and remodeling.
- Muscle Growth: It promotes muscle cell proliferation and differentiation.
- Cartilage Growth: IGF-I is vital for cartilage formation and growth.
- Organ Growth: IGF-I promotes the growth of various organs.
3. Regulation of HGH Release:
The release of HGH from the anterior pituitary gland is carefully regulated by several factors:
- Growth Hormone-Releasing Hormone (GHRH): GHRH, produced by the hypothalamus, stimulates the release of HGH.
- Somatostatin: Somatostatin, also produced by the hypothalamus, inhibits the release of HGH.
- IGF-I Feedback: High levels of IGF-I in the blood can inhibit HGH release, creating a negative feedback loop.
- Other Factors: Other factors, such as sleep, stress, exercise, and nutrition, can also influence HGH secretion.
Summary Table:
| Mechanism | Description |
|---|---|
| Direct Effects | HGH directly stimulates bone growth, protein synthesis, fat mobilization, and increases blood glucose. |
| Indirect Effects | HGH stimulates the release of IGF-I, which promotes bone, muscle, cartilage, and organ growth. |
| Regulation | HGH release is regulated by GHRH (stimulates), somatostatin (inhibits), IGF-I (negative feedback), and other factors like sleep and exercise. |
In essence, human growth hormone promotes growth by directly influencing bone and metabolism and indirectly through the action of IGF-I, all while being carefully regulated by a complex hormonal feedback system.
