[Properties of skeletal muscle fibers. II. Hormonal influences]

The skeletal muscle contains fibers with various contractile and metabolic properties. These populations of muscle fibers differ in their sensitivity and their response to circulating hormones which also affect the muscular differentiation (multiplication and fusion of myoblasts into myotubes). This review deals with the regulations of energy metabolism and of protein synthesis in muscles by several hormones acting either directly, or in association with other hormones, or by induction of growth factors. In most cases, hormonal effects seem to depend on the type and level of activity of the constitutive muscle fibers. The muscle fiber types involved in the anabolic properties of estrogens have not yet been clearly described. In the case of growth hormone and insulin, the slow fiber type is mainly affected; their effects are partially mediated through an increased secretion of somatomedins (IGFs) or by interaction on IGF receptors. The other reported hormones or factors induce a shift toward a more potent fast contracting activity, ultimately increasing the percentage of fast glycolytic fibers. Androgens, catecholamines and beta-agonists are anabolic and produce an enlargement of these fibers, whereas thyroid hormones or glucocorticoids in excess increase their catabolism.