Abomasal casein infusion enhances the mitogenic activity of serum from protein-restricted steers.

Holstein steers fed protein-restricted diets were used to evaluate protein realimentation and site of serum collection on the ability of calf serum to affect proliferation, protein synthesis and degradation in L6 myoblast cell culture bioassay. In Experiment 1, five steers (average weight 227 kg) received continuous abomasal infusion of 4 L of water or casein (50% of basal dietary nitrogen intake) in a switchback design. Serum was collected 2 d before and after 1, 3, 5, 7 and 9 d of infusion. Abomasal casein infusion increased serum mitogenic activity, nitrogen retention (119%) and total post-ruminal amino acid flow (78%). In Experiment 2, serum was collected from the jugular and femoral veins and the carotid artery before and after 7 d of abomasal casein infusion. Serum from calves abomasally infused with casein increased myoblast proliferation (jugular > femoral > carotid) and protein synthesis and decreased protein degradation in cultured myotubes. The addition of test calf serum inhibited the mitogenic activity of control calf serum. Results suggest that post-ruminal amino acid flow and site of serum collection alter the ability of serum to influence cell culture bioassays.