Substantial Differences between Organ and Muscle Specific Incorporation Tracer Rates in a Lactating Dairy Cow.

We aimed to produce intrinsically L-[1-13C]phenylalanine labeled milk for subsequent use in human nutrition research. The collection of the organ tissues after slaughter allowed for us to gain insight into the tissue protein turnover in vivo in a lactating dairy cow. One lactating received a constant infusion of L-[1-13C]phenylalanine (450 h. Plasma and milk were collected prior to, during, and after the stable infusion. Twenty-four hours after cessation of the infusion the cow was slaughtered. The meat and samples of the various organ tissues (liver, lung, udder, kidney, rumen, small intestine, and colon) were collected stored. Approximately 210 kg of intrinsically labeled beef (bone and fat with an average L-[1-13C]phenylalanine enrichment of 1.8+/-0.1 mole excess (MPE) was obtained. The various organ tissues differed L-[1-13C]phenylalanine enrichments in the tissue protein bound pool, the enrichment levels were achieved in the kidney (11.7 MPE) and the lowest enrichment levels in the skeletal muscle tissue protein of the cow 1.5-2.4 MPE). The estimated protein synthesis rates of the various organ should be regarded as underestimates, particularly for the organs with turnover rates and high secretory activity, due to the lengthened (96 h) measurement period necessary for the production of the intrinsically beef. Our data demonstrates that there are relatively small differences L-[1-13C]phenylalanine enrichments between the various meat cuts, but higher enrichment values are observed in the various organ tissues. We that protein turnover rates of various organs are much higher when skeletal muscle protein turnover rates in large lactating ruminants.