Under the conditions of regularly increasing near-maximal loads, several extremely effective signaling systems that stimulate the synthesis of proteins on ribosomes were revealed. It also increases the muscle nuclei, which in turn can increase the number of “working” genetic material.
The complex of protein kinases – enzymes, phosphorylating activating other enzymes, triggers the synthesis of protein molecules on ribosomes. This complex can be inhibited by the antibiotic rapamycin, hence the name mTOR (mammalian target of rapamycin), that is, the target for rapamycin, characteristic of mammals.
In turn, mTOR can be activated by three possible ways: the action of an insulin-like growth factor I (IGF-I) through a chain of intermediary enzymes, an excess of amino acids, and direct action of a mechanical load through phosphatidic acid molecules. The latter is synthesized by the phospholipase D (PLD) located in the region of the Z-disk, the cytoskeletal proteins of which are well perceived by the mechanical stress transmitted from the actomyosin bridges.
The growth factor (IGF-I) in a conditions of relative rest is formed in the liver cells (and in a small amount in the skeletal muscle) under the control of pituitary growth hormone and circulates with the bloodstream. With intensive physical work, it is synthesized predominantly in the skeletal muscle fibers, even if the pituitary gland in the experimental animal is removed.
The growth factor is regulated by another important process. In the muscle there are mononuclear cells, myosatellites, which adhere to muscle fibers and remain inactive for the time being. If the muscle is subjected to intensive strength training, the IGF-I secreted by it (and some other molecules, for example, nitrogen oxide) cause them to multiply and then merge with the parent fiber, increasing the number of nuclei in the fiber. Such are the summary of the molecular mechanisms underlying the growth of muscles during training.
It is known that the passive stretching of the muscle, when the stretched state is fixed by a gypsum bandage applied to the joint, allows to reliably protect the muscle from atrophic changes. At the same time, the tension force in the stretched muscle is sometimes so great that the thickness of the muscle fibers of the soleus muscle in the posted animal is significantly greater than the thickness of the muscle in the control animal with normal motor activity.
Preliminary data during the experiments show that exposure to rapamycin (an inhibitor of the mTOR system) can only hold the dimensions of the stretched fibers at the control level. The very antiatrophic effect of stretching is not eliminated by the action of this antibiotic. The data indicate that the mTOR-system is activated only under such mechanical influences, which exceed the normal activity of the soleus muscle. It is possible that in order to maintain its structure under normal conditions, some other, yet unknown, signal mechanisms that ensure the necessary level of protein synthesis are used.
Protein Synthesis Diagram