Historical text by DatBTrue, written between 2008 and 2016 and reproduced verbatim. It records what was argued then; it is not dosing guidance and not medical advice. Read the disclaimer.

To quickly summarize the difference in function, MGF stimulates muscle stem cells (satellite cells) to reenter the cell cycle and proliferate, whereas IGF-1 is necessary for the differentiation of newly generated muscle precursor cells into myoblasts and myofibers.

Growth Hormone bound to its receptor in the liver activates the STAT5b signaling pathway which promotes movement of that activated signaling protein to the nucleus where it induces transcription of the IGF-1 gene. From studies in mice and humans, it is evident that GH induces expression of both the endocrine form of IGF-1, the muscle form of IGF-1 and MGF. 1 Mice deficient in GH (lit/lit mice) respond to administration of GH with an acute increase in MGF, but not IGF-1, in skeletal muscle, although IGF-1 in liver is increased. 2 However the precise mechanism by which GH has distinct effects on IGF-1 and MGF expression in muscle is not completely known.

What is known is that in response to stretch overload and the presence of growth hormone combined gene transcription in muscle and its subsequent “blueprint” assembly” are induced to create mechano growth factor (MGF also known as IGF-1Ec in humans or IGF-1EB in mice) instead of muscle IGF-1. 3

How does the behavior of IGF-1 and MGF differ?

“MGF is a non-secreted form of IGF-1 that can be found in the nucleus of cells in culture or in a perinuclear location in hippocampal cells after ischemia (restriction in blood supply)”.

In other words MGF never leaves the cell it was created in. For emphasis I quote from another source:

“…MGF… is not normally secreted.”

Geoffrey Goldspink has written “that MGF increases myoblast proliferation via a different signalling pathway” then IGF-1.

To reiterate & expand upon this concept I quote from another source:

“IGF-1 isoforms differ in the signaling pathways they activate. By over-expressing IGF-1Ea and MGF in muscle, it has been shown that both isoforms can activate IGF-1R and AKT phosphorylation. In addition, MGF was shown to induce phosphorylation of ERK, a property not shared with IGF-1Ea.”

So IGF-1 and MGF work through different pathways not receptors. There is only one receptor, IGF-1-receptor (also insulin, IGF-II & hybrid receptors which are not relevant to this discussion). There is no MGF receptor. The reason why it would be nonsensical to have an MGF receptor is that MGF does not leave the cell.

This contrasts with IGF-1 which is released from the liver into circulation and which is created in muscle and translocates to the cell surface. Both events result in IGF-1 binding to the IGF-1 receptor.