MEASUREMENT OF PROTEIN-TURNOVER IN THE SMALL-INTESTINE OF LAMBS .1. DEVELOPMENT OF AN EXPERIMENTAL-MODEL

The objective of this study was to develop an experimental model to me asure both fractional and absolute rates of protein synthesis in the s mall intestine of lambs. Six male castrate lambs (similar to 6 months old, mean liveweight 26 kg) were offered, via continuous feeders, 900 g/day lucerne chaff. They were prepared with catheters in the cranial mesenteric vein (CMV), femoral artery (FA), jugular vein and abomasum, and a blood flow probe around the CMV. Cr-EDTA (0 . 139 mg Cr/ml, sim ilar to 0 . 2 ml/min) was infused abomasally for 24h and L-[2,6-H-3]ph enylalanine (Phe) (441 +/- 33 . 8 mu Ci into the abomasum) and L-[U-C- 14]phenylalanine (43 . 9 +/- 4 . 08 mu Ci into the jugular) were also infused during the last 8 h. Blood from the CMV and FA was sampled dur ing isotope infusions. At the end of infusions, lambs were killed and tissue and digesta samples removed from four sites along the small int estine (SI). Transfers of labelled and unlabelled Phe were measured be tween SI tissue, its lumen and blood, enabling both fractional and abs olute rates of protein synthesis and gain to be estimated. The total S I protein pool was 84 (+ 1 . 7) g and fractional gain rate was 7 . 5 ( +/- 5 . 5)% per day. Mean protein synthesis and fractional synthesis r ates (FSR) were calculated from the mean retention of C-14 and H-3 in SI tissue. FSR tended to increase caudally along the SI (although P > 0 . 05). The choice of free Phe pool for estimating precursor specific radioactivity (SRA) for protein synthesis had a significant effect on FSR. Assuming that tissue free Phe SRA represented precursor SRA gave a mean FSR of 129 (+/- 24)% per day. Corresponding estimates for free Phe SRA in the FA and CMV were 20 (+/- 1 . 8) and 30 (+/- 3 . 1)% per day respectively. The correct value for protein synthesis was therefo re in doubt, although indirect evidence suggested that blood SRA (eith er FA or CMV) may be closest to true precursor SRA. This evidence incl uded (i) comparison with flooding dose estimates of FSR, (ii) comparis on of H-3:C-14 ph, SRA in free Phe pools with this ratio in SI protein , and (iii) the proportion of SI energy use associated with protein sy nthesis. Advantages of the present experimental model compared to othe r methods included (i) measurements of both protein synthesis and gain , and hence, all components of turnover, (ii) measurement of absolute as well as fractional rates of synthesis and gain, (iii) inclusion of proteins which are synthesised and exported, and (iv) concurrent measu rement of protein synthesis and energy utilization by the small intest ine.