MGF is produced from the IGF-I gene through alternative splicing and features distinct 3' exons compared to the liver or systemic type (IGF-IEa). In humans, it includes a 49 base pair insertion, while rodents have a 52 base pair insertion within the E domain of exon 5. This insertion causes a reading frame shift, resulting in a different carboxy-terminal sequence compared to systemic IGF-IEa. Both MGF and other IGF isoforms share the same 5' exons that encode the ligand-binding domain of IGF-I. The processing of the pro-peptide generates a mature peptide that plays a role in enhancing protein synthesis. Additionally, evidence suggests that the carboxy-terminal of the MGF peptide functions as a distinct growth factor, promoting the division of mononucleated myoblasts or satellite (stem) cells, which increases their numbers for local repair.
MGF exhibits a dual function in that, similar to other IGF-I isoforms, it enhances protein synthesis while also activating satellite cells. However, the latter function of MGF likely holds greater significance since the majority of the mature IGF-I will be produced from IGF-IEa during the secondary phase of repair. Nevertheless, studies have demonstrated that MGF is a powerful inducer of muscle hypertrophy in experiments where MGF cDNA was inserted into a plasmid vector and delivered via intramuscular injection. This approach led to a 20% increase in the weight of the injected muscle within two weeks, and analyses showed that this was due to hypertrophy of the muscle fibers. Other research groups have conducted similar experiments using a viral construct featuring the liver type of IGF-I, resulting in a 25% increase in muscle mass, but this improvement took over four months to manifest (Musaro et al. 2001). Therefore, the dual role of MGF in promoting satellite cell activation alongside protein synthesis indicates it is significantly more effective than the liver type or IGF-IEa in inducing rapid hypertrophy.
The peptide is typically administered in dosages of 200-400 mcg per day and works synergistically with other substances such as IGF and PEG.