Effect of growth hormone, testosterone and serum concentration on actin synthesis in cultured satellite cells

Because most of the DNA in a mature muscle accumulates during postnatal life and is derived from satellite cells, cellular regulation of protein accumulation in muscle fibers originating from satellite cells is an important aspect of muscle growth control. These experiments were designed to study the modulation of a alpha-actin accumulation in satellite cell-derived myotubes by serum and two anabolic hormones commonly assumed to be involved in muscle growth regulation; growth hormone and testosterone. Satellite cells were cultured from rats ranging in age from 5 d to 1 yr. After fusion into myotubes, various levels of pig serum, 3 to 20%, were added to culture medium, and the amount of alpha-actin per myotube nucleus was determined 4 d later. The quantity increased with increasing percentages of serum in the medium. For an assessment of the extent to which serum stimulatory activity was due to growth hormone or testosterone in the serum, similar experiments were conducted with medium containing 10% pig serum plus various levels of porcine growth hormone or testosterone. Neither of these hormones stimulated actin accumulation at concentrations from 10(-9) to 10(-6) M, a range encompassing physiological levels for each hormone. These experiments do not support the premise that either growth hormone and (or) testosterone are the blood-borne agents that interact directly with muscle to stimulate protein accretion, because neither growth hormone nor testosterone acted directly on muscle cells in vitro to stimulate muscle protein accumulation.