Response of C2C12 mouse and turkey skeletal muscle cells to the beta-adrenergic agonist ractopamine

The effects of ractopamine (RAG) and ractopamine stereoisomers (RR, RS, SR, and SS) on cyclic AMP (cAMP) production, total protein, and PNA concentrat ions in mouse skeletal muscle cells (C2C12) were evaluated. The RAC (10 mu M) caused an similar to 30% increase in cell number, protein, and DNA conce ntrations in myoblasts after 48 h; no differences were found in myotubes. T he RAG-stimulated increase of these variables in myoblasts was blocked by t he presence of equimolar concentrations of propranolol. At a later passage, myoblasts failed to exhibit an increase in cell number, protein, or DNA up on exposure to RAG. Both myoblasts and myotubes increased cAMP production i n response to 10 mu M RAG. The RAC isomers ranked RR >> SR > RS similar to SS in ability to stimulate cAMP production, with essentially no response to SS. The SR produced about 50% of the RR response. Coincubation of proprano lol (10 mu M) and RAC (10 mu M) prevented RAG-stimulated cAMP production in myotubes but not in myoblasts (similar to 35% of cAMP produced by RAC alon e). Turkey satellite cells (derived from biceps femoris of 12-wk-old toms) produced essentially no increased cAMP when exposed to 10 mu M RAG stereois omers. Stability of RAC was evaluated under laboratory storage and culture conditions. The RAC was stable for more than 4 mo when stored in deuterated DMSO (>98% purity) at room temperature or in aqueous solutions at -80 degr ees C, as determined from sequential nuclear magnetic resonance studies. Ra diolabeled RAC was incubated for 72 h in the presence of serum-containing m edium, with or without C2C12 cells. Ninety-eight percent of the parent comp ound found in the medium at time zero was present in the medium as parent a t the end of 72 h. The cellular cAMP response to RAC through beta-adrenergi c receptors seems to be stereospecific. If the state of myoblasts and myotu bes in vitro reflects the in vivo state, then the ractopamine effect in viv o on cellular processes (including cell division and protein and DNA accumu lation) may be independent of beta-adrenergic receptors in muscle.