IGF is the acting substance that causes the cell hyperplasia, but it is actually very limited in what parts of the body it effects. The only ones are skeletal muscle and the heart. Since MK677 stimulates the creation of HGH, which created IGF, it will also have this effect.
However, when used at the low doses most people use them at, the effects on the heart are minimal to non-existent. If you use them while working out, they body shifts into muscle creation (since it knows it has muscle damage).