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°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 acretion 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 [³⁵S]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 [³⁵S]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 heavy-chain 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.