Protein turnover and growth of L8 muscle cultures in serum from rats fed clenbuterol.

Beta-adrenergic agonists stimulate muscle growth in chronically treated animals. The response may be primary (receptor binding directly stimulates anabolism) or secondary (circulating levels of hormones or other factors regulate muscle mass). Although B-receptors are functional in L8 muscle cells in culture, a direct effect of B-agonists on protein turnover in culture has not been demonstrated. To determine whether the stimulation of muscle growth is a secondary effect, we assessed the anabolic activity of sera from both clenbuterol-treated rats and normal rats in L8 cultured muscle cells. Rats were fed either normal diet or diet containing ten parts per million clenbuterol for seven days. Blood was collected from the inferior vena cava of rats under metofane anaesthesia and serum prepared. The anabolic activities of serum were measured in cultures of either myoblasts (still dividing) or fused myotubes (non-dividing) to distinguish mitogenic action from alteration in rates of protein turnover. Sera from both normal and clenbuterol-treated rats gave the same stimulation of protein synthesis in myotube cultures. Sera from both groups of rats contained factors which inhibited protein degradation, stimulated amino isobutyric acid (AIB) and thymidine uptake (myoblasts only). These processes were not augmented by sera from rats treated with clenbuterol. Protein accumulation was equivalent after 48 hours of exposure to either test sera. In conclusion, adult rat serum contained anabolic factors but there was no evidence for an increase in activity in serum from clenbuterol-treated rats.