MGF is produced from the IGF-I gene through alternative splicing and features different 3' exons compared to the liver or systemic type (IGF-IEa). In humans, it contains a 49 base pair insert, while rodents have a 52 base pair insert within the E domain of exon 5. This insertion leads to a reading frame shift, resulting in a distinct carboxy terminal sequence that differs from systemic IGF-IEa. MGF shares the same 5' exons with other IGF isoforms, which encode the IGF-I ligand-binding domain. The processing of the pro-peptide generates a mature peptide that aids in enhancing protein synthesis. Additionally, evidence suggests that the carboxy-terminal of the MGF peptide functions as a separate growth factor, promoting the division of mononucleated myoblasts or satellite (stem) cells, thus increasing the pool available for local repair.
MGF exhibits a dual action, similar to other IGF-I isoforms, as it enhances protein synthesis while also activating satellite cells. However, MGF's role in activating satellite cells may be more crucial since most mature IGF-I during the second phase of repair will come from IGF-IEa. Nonetheless, research indicates that MGF is a strong inducer of muscle hypertrophy. When the cDNA of MGF was incorporated into a plasmid vector and injected intramuscularly, there was a 20% increase in the weight of the injected muscle within two weeks, attributed to the enlargement of the muscle fibers. Other studies using a viral construct of the liver type of IGF-I showed a 25% increase in muscle mass, although over a period of more than four months (Musaro et al. 2001). Thus, the dual role of MGF in stimulating satellite cell activation and protein synthesis implies that it is significantly more effective than the liver type or IGF-IEa in promoting rapid hypertrophy.
The peptide is typically administered at a dosage of 200-400 mcg per day and works synergistically with other substances like IGF and PEG.