THE EFFECT OF POSTRUMINAL AMINO-ACID FLOW ON MUSCLE-CELL PROLIFERATION AND PROTEIN-TURNOVER

An experiment was conducted to characterize the effects of postruminal administration of casein, glutamine, cornstarch, and water on protein turnover and in vitro muscle cell proliferation. Four MARC III steers (205 kg) were fed a protein-restricted bromegrass hay-based diet (2.8 6 Mcal of DE/kg and 13.6 g of N/kg), Using a 4 x 5 Latin square arrang ement balanced for residual effects, casein and glutamine, equal to 50 % of basal. dietary nitrogen intake, cornstarch, isocaloric with casei n infusion, or an equal volume of water was continuously infused into the abomasum of steers. Blood samples, collected every 2 h for 24 h af ter 7 d of infusion, were tested for the effect on cell cycle kinetics and myotube protein turnover. Urine and feces were also collected for 4 d after blood sampling for nitrogen balance and fractional skeletal muscle degradation. The mitogenic activity and ability of serum to in fluence rate of myoblast proliferation in a dose-dependent manner was influenced (P < .05) by infusate: casein > cornstarch > glutamine = wa ter. Abomasal infusion of casein and cornstarch increased (P < .05) in vitro muscle protein synthesis and decreased (P < .05) in vitro muscl e protein degradation, whereas abomasal glutamine infusion only increa sed (P < .05) in vitro muscle protein synthesis. Abomasal glutamine in fusion decreased (P < .05) fractional skeletal muscle protein degradat ion and synthesis; however, fractional muscle protein accretion tended to increase due to a greater decline in fractional muscle protein deg radation. In contrast, abomasal casein infusion increased (P < .05) fr actional skeletal muscle protein synthesis, breakdown, and accretion. These results suggest that muscle hypertrophy may be regulated by seru m constituents whose activity is affected by postruminal amino acid fl ow.