HGH is produced in the anterior pituitary gland and is circulated throughout the body. HGH interacts with a variety of cells while in the bloodstream including muscle and bone cells. It eventually reaches and stimulates the growth of all major organs (particularly the liver where it stimulates the production of growth factors) with the exception of the brain.
HGH is produced within the cell as a pre-protein, consisting of a 191-amino acid single chain polypeptide associated with a 26 amino acid signal peptide. Human growth hormone (HGH) plays a vital role in growth and development. It is naturally produced by somatotropic cells in the anterior pituitary gland. The hormone is produced as a 217 amino acid precursor protein. The 26 N-terminal amino acids correspond to a signal peptide, which is essential for hormone secretion. This signal peptide is cleaved during the secretion process to yield the mature, 191 amino acid form of HGH. Mature HGH travels through the bloodstream and interacts with a specific HGH-receptor on the surface of various cells, including muscle, bone, and cartilage. Binding of HGH to its receptor causes dimerization and signal transduction, which ultimately stimulates cellular division. HGH also indirectly influences growth by stimulating the liver to produce additional growth factors, such as insulin-like growth factor-1. Synthetic versions of HGH produced by recombinant DNA technology are used to treat growth disorders associated with HGH deficiencies.
In addition to increasing height in children and adolescents, growth hormone has many other effects on the body:
- Increases calcium retention, and strengthens and increases the mineralization of bone
- Increases muscle mass through sarcomere hypertrophy
- Promotes lipolysis
- Increases protein synthesis
- Stimulates the growth of all internal organs excluding the brain
- Plays a role in homeostasis
- Reduces liver uptake of glucose
- Promotes gluconeogenesis in the liver
- Contributes to the maintenance and function of pancreatic islets
- Stimulates the immune system
- Increases deiodination of T4 to T3
In order to facilitate this behavior as a hormone somatotropin binds to two receptors on the outside of a cell known as Human Growth Hormone Binding Proteins (HGHpb). Once the Human Growth hormone binds both receptors (first one, then the second), it causes a shift in the receptor protein, which in turn causes an internal signaling cascade. This cascade is how somatotropin is able to effect cell growth and function. In addition, it can cause the release of other growth factors, like Insulin Growth Factor.