GHRP-6 and the Growth Hormone Secretagogue Receptor
GHRPs are not simply surrogates of GHRH, instead GHRP-6 is an artificial activator of a separate newly discovered receptor called Growth Hormone Secretagogue Receptor (GHS-R). Soon Ghrelin was discovered, the endogenous ligand that binds to the GHS-R. Both Ghrelin and all the synthetic compounds such as GHRP-6 were termed “Growth Hormone Secretagogues” (GHSs). One side effect of GHRP-6 is a significant increase in appetite due to stimulating the release of Ghrelin, a peptide that is released naturally in the lining of the stomach that increases hunger and gastric emptying. Also, GHRP-6 causes stimulation of the anterior pituitary gland which causes an increase in Growth Hormone release. The increased amounts of Growth Hormone can cause the liver to secrete the hormone IGF-1, which improves the animal body’s ability to burn fat and build muscle. Since GHRP-6 acts directly on the feedback loop which signals the inhibition of Growth Hormone release, GHRP-6 can re-stimulate the production of Growth Hormone.
Follistatin (FST) is a secreted glycoprotein that was first identified as a folliclestimulating hormone inhibiting substance in ovarian follicular fluid (1, 2). Human Follistatin cDNA encodes a 344 amino acid (aa) protein with a 29 aa signal sequence, an Nterminal atypical TGF binding domain, three Follistatin domains that contain EGFlike and kazallike motifs, and a highly acidic Cterminal tail. Follistatin is a secreted protein that binds to ligands of the TGF-Beta family and regulates their activity by inhibiting their access to signaling receptors. It was originally discovered as activin antagonists whose activity suppresses expression and secretion of the pituitary hormone FSH (follicle stimulating hormone). In addition to being a natural antagonist, follistatin can inhibit the activity of other TGF-Beta ligands including BMP-2,-4,-6,-7, Myostatin, GDF-11, and TGF-Beta1. Follistatin is expressed in the pituitary, ovaries, decidual cells of the endometrium, and in some other tissues. Recombinant human Follistatin 315 is a kDa protein containing amino acids 30-344 of the FST-344 protein.