I. The fractional synthetic rates of tissue proteins were studied in growing pigs using the constant-infusion technique of tracer-labelled amino acids ( [l4C]1eucine and [14C]lysine) and the mathetmatical model for calculation, employed in rats by Garlick, Millward &James (1973).2 . During a 6 h infusion, samples were taken from blood and muscle and at the end of the infusion from liver, muscle, pancreas, heart, duodenum, jejunum, ileum, colon, and skin. The specific radioactivity of free and protein-bound leucine and lysine was estimated. 3. A quasi-steady-state in the specific radioactivity of free plasma leucine and lysine was reached within approximately z h, the rate-constants being 35 and 48/d respectively.4. The specific radioactivity of free leucine and lysine in plasma was used to calculate the flux of these amino acids. It was found to be higher than the daily intake.5. The average fractional rate of protein synthesis in muscle and heart was 8.1 %/d, in small and large intestine the values were 50 and 33 %/d respectively and in liver and pancreas more than Ioo%/d.6. The calculation of protein synthetic rate in pig tissue using the constant-infusion method of labelled amino acids seems to be a suitable tool for study of this species.

1. The effects of thyroid hormones on the range of tissue protein synthesis in growing pigs using the constant infusion technique with [14C]leucine and [I4C]lysine were studied.2. During a 6 h infusion, samples were taken from blood and, at the end of the infusion, from liver, pancreas, stomach, small and large intestines, kidney cortex, kidney medulla, muscle and skin.3. Lower relative specific radioactivities of free leucine and lysine in several tissues were observed in the hormone-treated group than in the untreated one.4. The range of protein synthesis rate and the daily amount of protein synthesized in tissues was higher in all tissues after application of thyroid hormones.5. Assuming that the organs analysed represented 70 % of the total trichloroacetic acid-precipitable protein of the pig, the estimated range of daily protein synthesis was 251490 and 312-88Og in untreated and hormone-treated pigs respectively.

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A trial was performed with 2 fistula pigs (each with 2 fistulas, one located about 30 cm below the pyloric orifice and the other at the end of the small intestine). Animal A received a casein diet containing 14% crude protein for a period of 2 weeks before the tracer amino acid was administered. Animal B received the same diet for a period of 10 days and was then fed a diet (at the same protein level) containing gluten as sole protein source. The two tracer amino acids, 14C-U-L-leucine and 3H-4,5-(N)-L-lysine, were injected intravenously. The passage rates for dry matter, organic matter and N measured at the beginning of the small intestine were higher than the rate of intake. The rate of passage of amino acids was also found to be increased relative to the rate of intake. In general, this increase involved the non-essential amino acids to a much larger extent. A considerable proportion of the amino acids passing into the large intestine is not excreted with the faeces but is probably converted in catabolic processes. It is for this reason that any values for the efficiency of amino acid absorption calculated on the basis of data on the faecal excretion of amino acids will not provide conclusive evidence for the availability of dietary amino acids in processes of the intermediate metabolism. The rate of secretion of 3H and 14C radioactivity into the digesta of the small intestine was found to increase rapidly within 1-2 hrs after administration of the tracer amino acids. The 14C radioactivity detected was found to be almost exclusively derived from 14C leucine while only about 60% of the 3H activity found in the digesta of fistula I were shown to be bound to lysine. Labelled lysine and leucine (of endogenic origin) are absorbed into the small intestine at a slower rate (i.e. endogenic proteins are less efficiently digested) than the non-radioactive amino acids (of exogenic origin) so that a process of concentration of endogenic amino acids is observed towards the end of the small intestine.

Three pigs, of 34 kg live weight, were each fitted with re-entrant cannulas both in the duodenum and terminal ileum and catheters in the jugular vein and in the carotid artery. Pigs received a diet based on wheat and dried skimmed milk in equal amounts at 12 h intervals. During the preliminary period the digesta flowing from both duodenal and ileal cannulas were collected over 12 h after feeding on two consecutive days and half of them were reintroduced into the gut and half were stored at -20 degrees C. During the experimental period 15N-urea was infused into the jugular vein for 12 hours starting with the morning meal. Total amount of urea infused was 5 g containing 1.22 g 15N-excess. The digesta from both proximal duodenal and ileal cannulas were collected and stored, while the digesta from the preliminary period were reintroduced into the respective distal cannulas. Blood samples were taken at different time of infusion. At the end of infusion period the animals were sacrificed and samples of the contents of the digestive tract and tissues were taken. Urea flux calculated according to atom-% 15N-excess of urea N in plasma was 1.23 to 2.37 g/kg body weight/day. In the duodenal digesta 94.5 +/- 0.2 and in ileal digesta 57.1 +/- 7.39 per cent of 15N were in the TCA soluble fraction. The total amount of 15N in the duodenal digesta was 1.7 to 6.3 times greater than in the ileal digesta. Only small amount of 15N was found in the caecum and almost none in the contents of colon and rectum. It is concluded that urea is secreted into all parts of the digestive tract, the main sites of urea secretion being pancreatic juice and/or bile as well as the small intestine. The total amount of urea secreted is assumed to be similar to the daily urea excretion.

Short-term trials with a triazin derivative and a peptide aldehyde were carried out to investigate the in-vivo administration of proteinase inhibitors in rats for improving the N balance of the animals. The results of N balances suggest that the peptide aldehyde may be regarded as a promising substance that may be used to increase the rate of utilization of the absorbed N in the intermediary metabolism. The short-term use of both active compounds did not affect the true N digestibility. The two compounds did not influence the concentration of free amino acids in the liver and muscle tissue under the experimental feeding conditions used. Similarly, tests of the biological half-life of proteins taken from the small and large intestine did not reveal any changes due to the presence of the inhibitors. Animals receiving a protein-free diet exhibited longer half-life periods for proteins from the small and large intestine compared with animals of the other groups. A protein-free nutrition of the animals caused a general increase in the levels of free non-essential amino acids in the liver. This was accompanied by a simultaneous decrease in essential amino acids. The same type of nutrition led to a general decline in the concentration of free amino acids in muscular tissue, particularly of the essential amino acids. High-protein nutrition led especially to an increase in the concentration of branched-chain amino acids in both types of tissues investigated.

Two pigs of 35 kg live weight were fitted with re-entrant duodenal cannulas anterior to the pancreatic duct and fed barley-soya or casein-wheat starch diets prior to the experiment. After 24 hours fasting they were given a single meal of 15N-labelled free amino acids-wheat starch (A) or 15N-labelled wheat (B) diets and digesta leaving the stomach was collected during 12 hours. The proportion of TCA soluble N in total N of digesta increased with time from 10 to 40% with diet B and decreased from 90 to 47% with diet A. Total N leaving the stomach within 12 hours accounted for 96% and 106% of N ingested on diet A and B, while the amount of 15N accounted for only 66 and 86% of 15N given in diets A and B, respectively. The content of endogenous N in the digesta was 1.22 g/12 h after feeding diet A and 1.67 g/12 h after feeding diet B. It was concluded that considerable amounts of N are secreted and absorbed in the part of the digestive tract proximal to the opening of the pancreatic duct.

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