Eh, GH does not cause cells to multiply; it makes them enlarge in size. It works by stimulating the liver and other cells (particularly the liver) to produce insulin-like growth factor. Essentially, it causes cells to grow in size; thus, it will cause muscles as well as bones to grow. It does not increase the number of cells per say. Here is a quote from UPTODATE"
ACTIONS OF GH
GH receptor — GH acts by binding to a specific receptor, located mostly in the liver, and inducing intracellular signaling by a phosphorylation cascade involving the JAK/STAT (signal transducing activators of transcription) pathway. Its predominant action is to stimulate hepatic synthesis and secretion of IGF-I, a potent growth and differentiation factor [1].
The GH receptor is a 70 kd protein that is a member of the class of receptors common to the cytokine/hematopoietin superfamily [1]. The receptor consists of an extracellular ligand-binding domain, a single membrane-spanning domain, and a cytoplasmic signaling component. Circulating GH-binding proteins are fragments of the receptor that bind GH [26].
A single GH molecule complexes with two GH receptor molecules, followed by rapid binding and activation of JAK2 tyrosine kinase, leading to phosphorylation of several cytoplasmic signaling molecules.
The STAT proteins comprise important signaling components for GH action. These cytoplasmic proteins are phosphorylated by JAK2 and directly translocated to the cell nucleus, where they elicit GH-specific target gene effects by binding to nuclear DNA [1]. Alternatively, STAT proteins 1 and 5 may interact more directly with the GH receptor molecule [27].
Other actions of GH include stimulation of c-fos and IRS-1 phosphorylation. As a differentiating and growth factor, IGF-I is a critical protein induced by GH, and is likely responsible for most of the growth-promoting activities of GH [28]. Furthermore, IGF-I also directly inhibits GH secretion [28] and GH receptor function [29] by a negative feedback loop.
Mutations of the GH receptor are associated with partial or complete GH insensitivity and growth failure (Laron dwarfism) [30]. These syndromes (Laron dwarfism) are associated with normal or high serum GH concentrations, low serum GH-binding protein concentrations, and low serum IGF-I concentrations. Over 30 different homozygous or heterozygous exonic and intronic mutations of the receptor have been described, most of which are found in its extracellular ligand-binding domain. STAT mutations have also reported leading to short stature [27].
Specific effects — GH stimulates linear growth in children by acting directly and indirectly (via the synthesis of IGF-I) on the epiphyseal plates of long bones. GH also has specific metabolic actions including:
* Increased lipolysis and lipid oxidation, which leads to mobilization of stored triglyceride
* Stimulation of protein synthesis
* Antagonism of insulin action
* Phosphate, water, and sodium retention
The physiology of IGF-I, which mediates most of the peripheral actions of GH, is discussed separately. (See "Physiology of insulin-like growth factor I").
Dennis did not compete again after leukemia because of the fear that the leukemia would be re-activated if he took HGH. Thus, quite possibly the dude with the best genetics ever for bodybuilding went by the wayside as he had equivalent genetics for cancer. Sucks to be him. He may very well have been the next Steve Reeves; ND speak on this. I am curious if you concur.
