Human growth hormone (HGH), also known as somatotropin, is a peptide hormone produced by the anterior pituitary gland. HGH plays a crucial role in growth and development, especially during childhood and adolescence.
The production and release of HGH is regulated by a complex neuroendocrine feedback mechanism involving the hypothalamus and several hormonal and neural factors. Here is an overview of how HGH production is regulated:
- The hypothalamus produces and secretes growth hormone-releasing hormone (GHRH) which stimulates the release of stored HGH from the anterior pituitary. The secretion of GHRH is modulated by neural stimuli and negative feedback from blood levels of HGH and IGF-1.
- The hypothalamus also produces somatostatin, which inhibits GHRH and suppresses HGH release.
- Changes in blood glucose levels, certain amino acids, deep sleep, stress, and exercise also impact GHRH and somatostatin secretion from the hypothalamus.
- GHRH binding to specific receptors on somatotroph cells in the anterior pituitary directly triggers HGH synthesis and secretion.
- New HGH peptides are packaged in secretory vesicles within the somatotroph cells.
- Nerve impulses from the hypothalamus result in depolarization of somatotroph cell membranes and calcium ion influx, causing the vesicles to fuse with cell membranes and release stored HGH into circulation in a pulsatile manner.
- Elevated blood levels of HGH and IGF-1 elicit negative feedback to the hypothalamus and pituitary to decrease further HGH release, forming a regulatory loop.
Key external factors regulating HGH release include:
- Nutrition: Fasting and low blood glucose promote HGH release while eating suppresses release. Certain essential amino acids like arginine also stimulate HGH secretion.
- Sleep: The onset of slow wave deep sleep with delta wave patterns directly triggers intense HGH release. This effect decreases with age.
- Exercise: Intense exercise like weight training elevates HGH for up to 30 minutes post-workout. The magnitude depends on the intensity, duration and rest intervals during exercise.
- Hormones: Rising estrogen levels during puberty spark the adolescent growth spurt in both boys and girls in part due to the synergistic effect of estrogen and testosterone on promoting HGH and IGF-1 release from the liver.
The downstream effects of HGH are mediated by
IGF-1 produced in the liver and peripheral tissues:
- HGH stimulates IGF-1 synthesis and secretion, enhancing its growth-promoting effects on musculoskeletal tissues. Almost all the growth-stimulating effects of HGH are achieved through IGF-1 acting on its receptor.
- Together, HGH and IGF-1 stimulate protein synthesis and cellular proliferation in tissues including muscles, bones and joints during childhood development and continuing into adulthood to maintain musculoskeletal health.
In summary, the intricate neuroendocrine regulation of HGH synthesis and secretion is both complex and elegant. Small disturbances can lead to growth abnormalities during the developmental years. Later in life, an age-related decline reduces the vitality enhancing effects. In select cases, HGH injections may help overcome hormonal deficiencies under medical supervision.