[Tracer studies in rats with 15N labeled wheat for the determination of endogenous and exogenous fecal nitrogen]

Effect of dietary fiber on endogenous nitrogen flows in lactating dairy cows

The effect of dietary fiber on endogenous N secretion was studied using a 15N isotope dilution technique in four fistulated Holstein cows. Two isonitrogenous diets differing only in fiber (NDF and ADF) content were used in a crossover design. One diet (HF) contained 37.4% NDF, while the other (LF) contained 23.3%. A new model was developed to estimate endogenous N secretions and losses for the preintestinal, intestinal, and the total sections of the gastrointestinal tract. Three precursor pools: TCA-soluble fraction of plasma, intestinal mucosa, and milk were compared. Although endogenous losses estimated with the model were numerically different for each precursor pool selected (TCA-soluble fraction > mucosa > milk), treatment effects were similar. As intestinal mucosa is probably closest to the precursor pool, these data are discussed. Non-urea N endogenous secretions contributed 13% of the duodenal N flow but were not affected by the fiber content of the diet. The nonurea N endogenous flow at the duodenum was comprised of approximately equal inputs from endogenous N direct, and that incorporated into the microbial biomass. Total endogenous N flows at the duodenum exceeded, by nearly twofold, estimated inputs of urea-N to microbial biomass. Metabolic fecal output averaged 17% of fecal N and was not affected by level of dietary fiber, but net losses from secretions occurring in the small intestine were higher with the low fiber diet. Overall, endogenous N secretions represented 30% of total digestive tract protein synthesis.

[Determination of a prececal N-absorption from natural feed by 15N-labeled laboratory rats using the isotope dilution method]

60 Wistar rats (5 animals/group) received 12 different feedstuffs over a period of 7 days and were simultaneously labelled with 15N (orally by means of 15NH4Cl in the feed). In the subsequent 5 days faeces were collected in order to determine the apparent and true digestibility of crude protein. On the 13th experimental day the animals were killed 3 hours after the intake of half the daily ration and the atom-% 15N excess (15N') was determined in the TCA-soluble and TCA-precipitable fractions of the blood plasma and the digesta of the 2nd and 3rd thirds of the small intestine. Precaecal N-absorption was calculated with the help of the quotient [formula: see text] the blood plasma and the digesta The following values (in %) were registered in comparison to true N-digestibility (in the following in brackets): casein = 95.8 (99.2), whole egg = 92.1 (97.5), fish meal = 85.8 (93.4), dried skimmed milk = 98.4 (96.1), soybean meal = 79.6 (90.6), assay protein = 94.2 (98.9), wheat = 92.9 (90.7), barley = 84.3 (84.8), yeast, grown on molasses = 85.6 (87.2), yeast, grown on whey = 86.1 (88.5), biomass of liquid manure = 43.6 (68.9), activated sludge = 54.4 (64.1). One can conclude that the isotope dilution technique demonstrated here as an evaluation method is very well suited for the characterization of the N-digestibility of a feedstuff in the small intestine.

[Determination of endogenous nitrogen in feces using the isotope technic]

A ration consisting of wheat gluten and N-free components was supplemented with L-lysine and L-leucine and fed to two groups of growing Wistar rats. Group 1 received 15N Lys and unlabelled Leu, group 2 received unlabelled Lys and 15N Leu in order to study the influence of the utilization of the 15N marker on the labelling quota of faeces and urine as well as various fractions of the body. The good utilization of Lys in group 1 results in a higher 15N excess in faeces and a reduced 15N abundance in urine in comparison to group 2 with a lower utilization of 15N Leu. The results show that the 15N abundance in urine is unsuitable as an indicator of the 15N labelling quota of endogenous metabolic faecal nitrogen.

[Action of proteinase inhibitors. 4. Effect of short-term application of chymostatin on nitrogen metabolism in the digestive tract and protein metabolism in tissue]

Chymostatin is an effective inhibitor of intracellular proteinases in vitro. In the present experiment male rats were injected intraperitonealy during a 3 days period twice daily with a solution containing 0,9 mg Chymostatin per 100 g live weight. Reference animals received a control injection containing the same solvents but no chymostatin. During this period a daily nitrogen balance was made and metabolic faecal nitrogen and true digestibility of nitrogen were estimated using 15N-labelled animals. Furthermore, apparent biological half lives of proteins in liver and intestinal tissues were determined following the decay curves for radioactivity in proteins 48 hours after injection of L-[5-3H]-arginine und L-[guanido-14C]-arginine. The fractional rate of protein synthesis in tissues was measured by a 6 hours continuous infusion technique with L-[U-14C]-tyrosine and L-[U-14C]-leucine. Among the parameters estimated only the apparent biological half lives of proteins in liver and intestinal tissues were influenced by chymostatin. However, the prolonged half lives seemed to be rather an effect of an increased reutilisation of amino acids resulting from the intracellular protein breakdown than a decreased rate of protein degradation. The in vivo effect of the proteinase inhibitor was by far inferior compared with the action in vitro. Factors like distribution, degradation and excretion of the inhibitor could be responsible for the moderate in vivo action of chymostatin.

[A new method for testing the quality of food proteins for maintenance metabolism. 4. Testing of isolated proteins as well as various protein sources of plant and animal origin]

Male adult rats (370 g body weight) were fed on maintenance level (460 kJ ME/kg 0,75). In a 10 days preliminary period they received a casein/methionin diet (95,5 W/W) supplemented with 10 mg N-15-excess per 0,178 kg metabolic body weight in form of ammonium acetate. Thereafter the animals were changed on 8 isonitrogenous diets containing as protein sources casein, soya protein, gelatine, whole egg, fish meal, pea, wheat and yeast. The N-15-excretion via urine and faeces 6th and 10th day after the end of labelling period was used to evaluate the dietary proteins for maintenance. N-15 in urine was lowest in animals fed with the wheat diet and highest after feeding the whole egg diet. From these data a so called "N-15-excretions BV" was calculated, which indicated the highest quality for wheat and soy protein in meeting the needs of the intermediary metabolism in maintenance. On the other hand, dietary protein sources influence the loss of endogenous nitrogen as metabolic faecal nitrogen (MFN). It was found to be lowest in animals fed with diets containing isolated proteins (6 mg MFN/100 g body weight) and highest after feeding protein sources of plant origin with a high content in crude fibre (10 mg MFN/100 g). Both, losses of N-15 via urine and via feces were combined in a parameter called "total BV". According to this parameter the differences in quality for maintenance were only little between the protein sources testet (casein 100, soy protein 100, pea 99, wheat 99, whole egg 92, fish meal 89, gelatin 89). It was concluded that in maintenance the supply with indispensable amino acids is not critical and that the supply with dispensable amino acids (or nonspecific nitrogen) is of great importance.

[Dependence of metabolic fecal amino acids on the amino acid content of the feed. 1. Metabolic fecal amino acids of rats fed with maize]

The amount of metabolic fecal amino acids (MFAA) in dependence on the amino acid intake was determined for graded maize rations with 15N-labelled rats and the quota of labelled endogenous amino acids in faeces was calculated according to the isotope dilution method. The excretion of amino acids and MFAA in faeces are described as functions of the amino acid intake for 17 amino acids and regressively calculated. For all 17 amino acids investigated, there was a more or less steep increase of MFAA according to an increasing amino acid intake. In contrast to MFAA in N-free feeding, MFAA in feeding with pure maize (16.5% crude protein) increase to the 2- to 4.5-fold value. The thesis of the constancy of the excretion of MFAA can consequently be no longer maintained. The true digestibility according to the conventional method is, on an average of all amino acids, 7.3 units below the one ascertained according to the 15N-isotope method. For the limiting amino acids lysine and threonine the difference is biggest (23 resp. 17 units). Tryptophane as first limiting amino acid could not be determined. The true digestibility of nearly all amino acids ascertained for maize according to the isotope method is above 90%. For the limiting amino acids the expenditure resp. the loss of endogenous amino acids is biggest.

[Protein digestibility and amino acid absorption in various segments of the digestive tract of pigs. 5. Results of the application of 35S-methionine]

The intake, excretion and the rate of flow of protein, methionine and 35S were measured at four pigs (live weight 63 kg) with re-entrant cannulae in the duodenum and the ileocecal region after the oral application of 35S-methionine and the feeding with a wheat + wheat gluten + lysine resp. a protein-free diet over a period of 7 days. From the measured values one pattern each for the N- and the methionine metabolism was laid down with the help of the compartment model, and the endogenous and exogenous levels of protein resp. methionine in the duodenum, the ileum and in feces were estimated.

[New method of checking the quality of food proteins required for maintenance. 2. 15N excretion in feces of test rats labelled with 15N after feeding with different protein sources]

For 7 days 37 test rats received a casein diet with an extra of 6.6 mg 15N-excess in the form of ammonium acetate. From the eighth test day onwards 4 resp. 5 rats each received various protein sources under maintenance conditions (115 kcal/kg body mass0,75). The atom-% 15N-excess was determined in feces, blood liver and muscles (urine cf. 1st information). The endogenous quota of N in the feces was calculated as follows: (formula: see text). The numerical value of the TCA-soluble fraction of N in the total blood was corrected by the decrease of the atom-%15N' in the last 12 hours (time for the passage of the fecel matter from small intestines to excretion). Since the endogenously excreted N-amount varied greatly according to different feed, a scale is proposed as biologic value of food proteins, which exclusively refers to the metabolic fecal nitrogen (MFN) under conditions of maintenance (abbr. MFN-BV). A proposal for its definition is: (formula: see text). Above that, a total BV is suggested which also refers to maintenance metabolism. The total BV is calculated as follows: (formula: see text). The following values were ascertained for MFN-BV and total BV: casein = 80 and 82; complete egg=68 and 67; fish meal=61 and 86; Torula yeast=31 and 46; peas=41 and 43; soya (assay protein)=73 and 61; wheat=47 and 71; gelatin=64 and 42. Finally, the recommendation is given to include in feed tables real digestibility values for food proteins ascertained with the 15N method. In the above mentioned order the following values of the real digestibility of proteins were ascertained with the 15N method and classical methods: casein=98.2 and 97.2; complete egg=100.0 and 98.7; fish meal=96.9 and 93.4; Torula yeast=83.0 and 67.6; peas=97.1 and 85.6; soya (assay protein)=98.3 and 96.4; wheat=95.7 and 87.3; gelatin=99.1 and 96.0. *cf. 1st information Bergner et al. (1978)