RACTOPAMINE INCREASES TOTAL AND MYOFIBRILLAR PROTEIN-SYNTHESIS IN CULTURED RAT MYOTUBES

The ability of the phenethanolamine (beta-adrenergic agonist) ractopamine to stimulate cellular protein accretion and protein synthesis in cultured muscle cells was evaluated. ELC5 myoblasts (a subclone of rat L6 cells) were proliferated in culture (Dulbecco's Modified Eagle Medium plus 10% fetal bovine serum at 37-degrees-C) to confluency and then allowed to differentiate to form myotubes. Myotubes were then further incubated in the presence of 10(-9), 10(-8), 10(-7), 10(-6) or 10(-5) mol/L ractopamine. A significant (p < 0.05) response in cellular protein accretion was observed for the 10(-6) and 10(-5) concentrations when compared to 10(-8) and 10(-9) mol/L ractopamine. Ractopamine at 0 and 10(-6) mol/L was used to examine the effect of the beta agonist on [S-35]methionine incorporation (protein synthesis) into total cellular protein, 43-kDa proteins and myosin heavy-chain (200 kDa) protein. Protein synthesis in response to beta agonist treatment was measured at 4, 24, 48, 72 and 96 h after ractopamine addition to the ELC5 myotubes in culture. Ractopamine (10(-6) mol/L) increased [S-35] methionine incorporation (apparent protein synthesis) at 24 h (p < 0.01), 48 h (p < 0.05), 72 h (p < 0.01) and 96 h (p < 0.05) in cultured ELC5 muscle cells. Ractopamine also increased apparent protein synthesis rate of the 43-kDa proteins (p < 0.05) and myosin heavychain protein (200 kDa) (p < 0.05). These results indicate that ractopamine-enhanced ELC5 myotube protein accretion is mediated, at least in part, by stimulating cellular protein synthesis.