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Diets with protein of inferior quality may increase protein breakdown in skeletal muscle but the experimental results are inconsistent. To elucidate the relationship, pigs were fed isoenergetic and isonitrogenous diets based on soyabean-protein isolate or casein for 15 weeks, with four to six animals per group. A higher plasma level of urea (2.5-fold the casein group value, P = 0.01), higher urinary N excretion (2.1-fold the casein group value, P = 0.01), a postabsorptive rise in the plasma levels of urea, 3-methylhistidine and isoleucine in soyabean protein-fed pigs suggested recruitment of circulatory amino acids by protein breakdown in peripheral tissues. Significant differences between dietary groups were detected in lysosomal and ATP-dependent proteolytic activities in the semimembranosus muscle of food-deprived pigs. A higher concentration of cathepsin B protein was found, corresponding to a rise in the cathepsin B activity, in response to dietary soyabean protein. Muscle ATP-stimulated proteolytical activity was 1.6-fold the casein group value (P = 0.03). A transient rise in the level of cortisol (2.9-times the casein group value, P = 0.02) occurred in the postprandial phase only in the soyabean group. These data suggest that the inferior quality of dietary soyabean protein induces hormonally-mediated upregulation of muscle protein breakdown for recruitment of circulatory amino acids in a postabsorptive state.

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Energy balances were measured by indirect calorimetry in four experiments on male growing rats, fed restrictively on isoenergetic and isonitrogenous (10% CP) diets based on either casein supplemented with methionine, or soy protein isolate (experiments 1, 2 and 3) and soy protein isolate supplemented with methionine (experiment 0), respectively. At the end of experiments the rats were killed for body analysis and determination of thyroid hormones and lipids in blood as well as mitochondrial respiration in liver and heart. Feeding of non-supplemented soy protein resulted in a lower efficiency of energy utilisation as well as a lower protein utilisation compared to casein-fed and supplemented soy protein-fed rats. Chemical body composition was not markedly different between the dietary groups. After long-term feeding of soy protein (experiment 3) mass and mitochondrial protein content of the interscapular brown adipose tissue were increased compared to casein-fed rats. Serum thyroid hormone levels were not changed (T3 and free T3) or were significantly lowered (T4 and free T4) following soy protein feeding in comparison with casein feeding (except for experiment 2). Cholesterol and triglycerides were decreased on an average in response to soy protein or supplemented soy protein feeding. In two of three experiments a significant lower efficiency of hepatic mitochondrial respiration with succinate as substrate, expressed by the ratio of added ADP to oxygen consumed, was observed in soy protein-fed rats compared to casein-fed rats.

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Whole body protein turnover was studied in growing rats fed restrictively on isoenergetic (GE 17.6 MJ/kg DM) and isonitrogenous (104 g CP/kg DM) diets based on soy protein isolate or casein supplemented with D,L-methionine. During each of the three separate experiments six male Fischer rats per group were housed individually in metabolic cages at 24 degrees C. Prefeeding of both dietary groups up to similar body weights at the start of the main experimental periods (105-134 g) lasted up to 16 d for casein-fed rats and up to 30 d for the soy protein-fed rats. Following the energy and nitrogen balance periods whole-body protein synthesis was estimated by the end-product method using a single tracer dose of a mixture of 15N-labelled amino acids. Fractional protein accretion rate [% of the protein pool accreted per day] was significantly lower in soy protein-rats than in casein-fed rats in all three experiments whereas fractional synthesis rate was not significantly lower. Therefore, protein breakdown subsequently calculated as the difference between synthesis and accretion showed a tendency towards higher values in this group. In soy protein-fed rats also a tendency towards higher excretion of 3-methylhistidine as a marker of myofibrillar protein breakdown was observed. It is concluded that increase in lean tissue growth resulting from improved protein quality is brought about by changes of both rates, by small increase of protein synthesis and by reduced rate of body protein breakdown.

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Whole body protein synthesis was investigated in growing male mice which were long-time selected for high carcass protein amount (DU-6P, protein line) or for high body weight (DU-6, growth line) and in the unselected randomly bred control (DU-Ks). Six mice/line were housed singly in metabolic cages for the estimation of N balance, whole body protein synthesis (end-product method, single dose of 15N-labelled amino-acid mixture), and N distribution in the body. Another six mice/line were used for the determination of the body composition. All mice had free access to a commercial stock diet (crude protein 268 g, gross energy 19 MJ/kg dry matter) and to water. Body weight of both selection lines was about twice that of control mice at the same age. Selection for high body weight resulted in higher body fat content. Scaled to the corresponding body protein pools, the protein synthesis rates of selected mice were significantly higher than in controls, but were not significantly different between both selection lines in contrast to the protein deposition rates. The higher protein accretion in the protein line in comparison to the growth line seems to be due to a combination of a lower protein breakdown and an increased protein synthesis rate.

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1. Male and female broiler chickens (144 in total) were given diets supplemented with clenbuterol (CB) at 0 (control) and at 1 mg/kg between 28 and 49 d of age to study the effect of CB on growth, carcase and skeletal muscle. 2. CB improved growth in males by increasing daily weight gain and final live weight and by lowering food conversion ratio. In females it changed the carcase composition by reducing abdominal fat pad and by increasing the proportion of protein. Consequently, carcase protein gain was increased in both sexes (11% and 16%, respectively). 3. Skeletal muscle weights were enhanced by between 6% and 22%. Muscle fibre diameters were increased in extensor hallucis longus (EHL) but not in gastrocnemius (GAS) muscle. This increase was more pronounced in females. EHL total muscle fibre number remained unchanged. The proportion of fast-twitch glycolytic fibres was increased at the expense of fast-twitch oxidative fibres in males only. Nuclear/cytoplasm and DNA/protein ratios tended to be decreased by CB. 4. From the elevated EHL muscle RNA/DNA, unchanged protein/RNA and translation activity it is suggested that CB stimulated protein synthesis at the pretranslational level. Reduced protein degradation is deduced from decreased neutral calcium-dependent proteolytic activity. 5. It is concluded that broiler chickens respond to long-term CB treatment as has been shown in various mammals. However, the sex-specific response in growth, carcase composition and skeletal muscle cellularity is more clearly apparent in broiler chickens.

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Abstract Whole body protein synthesis was investigated in growing male mice which were long-time selected for high carcass protein amount (DU-6P, protein line) or for high body weight (DU-6, growth line) and in the unselected randomly bred control (DU-Ks). Six mice/line were housed singly in metabolic cages for the estimation of N balance, whole body protein synthesis (end-product method, single dose of (15)N-labelled amino-acid mixture), and N distribution in the body. Another six mice/line were used for the determination of the body composition. All mice had free access to a commercial stock diet (crude protein 268 g, gross energy 19 MJ/kg dry matter) and to water. Body weight of both selection lines was about twice that of control mice at the same age. Selection for high body weight resulted in higher body fat content. Scaled to the corresponding body protein pools, the protein synthesis rates of selected mice were significantly higher than in controls, but were not significantly different between both selection lines in contrast to the protein deposition rates. The higher protein accretion in the protein line in comparison to the growth line seems to be due to a combination of a lower protein breakdown and an increased protein synthesis rate.

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The time course of AA digestion, AA balance (sV AS), and AA absorption (wV AS) was estimated on growing rats (Wistar rats, LW = 124 g) in different sections of the intestinal tract using the combination of 15N tracer and TiO2 marker techniques. The animals received once a diet of 15N labelled wheat and yeast as protein sources supplemented by TiO2 as a marker. Up to 6 h after feeding the amino acid composition the 15N excess and the TiO2 content in the digesta of stomach, small and large intestine were determinated in the relation of amino acids resp. of 15N labelled amino acids to the marker. In addition the content of amino acids and the 15N excess of these amino acids were estimated in plasma. From these data the disappearance rates and the relation of exogenous to endogenous amino acids as well as the sV and the wV values of the different amino acids were calculated for the different gut sections. The following results were obtained: The relative disappearance rate for N and TiO2 marker out of the stomach went approximately parallel but with a delay for TiO2 of about 30 minutes. The AA composition of the stomach content, the small and the large intestine content did not vary in dependence of the time. The AA composition of the stomach digesta was nearly identical to that of the diet, while that of the small intestine was between exogenous AA composition (feed) and endogenous AA composition (digesta on protein free feeding). AA composition of the large intestine digesta showed quite big differences (bacterial AA break down and AA synthesis). Considering a delay time (small intestine: 1 h, large intestine: 4 h) the exogenous portion of the different AA remained constant in both of these intestinal sections during the whole experimental time. The exogenous AA part varied for small intestine digesta between 31 and 69% (mean value: 41%), and for large intestine digesta between 13 and 39% (mean value: 22%). The sV AS values in the small intestine (AA balance resp. precaecal digestibility) differed from 61% (threonine) to 86% (proline) with an average of 73.4 +/- 7.4%, those for wV AS (AA absorption) from 81% (lysine) to 94% (proline) with an average of 88.1 +/- 4.1%. There were significant differences between AA, but they are negligible for practical purposes.(ABSTRACT TRUNCATED AT 400 WORDS)

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After a 9 day preparation period 42 Wistar rats (live weight 100 g) were fed a diet of 15N labelled wheat supplemented with the marker TiO2 (impulse labelling). At 7 time intervals (0.5 to 6 h after feeding) 6 animals were killed and thereafter total N, 15N and TiO2 levels were estimated in the digesta of different intestinal sections. The following results were obtained: The transit rate of the marker amounted to 10.3 +/- 0.62% per hour of the intake. The endogenous part of N increased during passage from stomach (3.5%) to duodenum (38.6%), jejunum (59.1%), ileum (64.8%), large intestine (78.3%) and faeces (87.7%). The apparent N digestibility in the stomach increased with time reaching 26% 6 h after feeding. In the whole small intestine it was 66.3%, in the ileum 78.9% and in the large intestine 90.4%. The true digestibility (6 h after feeding) showed the same course, but was always some units higher (stomach 33.3%, whole small intestine 82.5%, ileum 92.2% and large intestine 93.5%). Apparent and true digestibility values in the ileum correspond best to the data of precaecal digestibility; those of the large intestine correspond to the postileal digestibility. The N disappearance rate in the stomach is the sum of absorption rate (16%/h) and transit rate into the small intestine (12.4 +/- 1.6 mg N/h). Most absorption occurred during passage through the small intestine (2/3 of total absorption). The absorption in the small intestine was about 80% of the N amount flowing from the stomach into the intestine. The amount lay between 18.2 and 26.1 mg N/h and half of this was of endogenous origin. The reabsorption rate of endogenous N for the whole intestinal tract was estimated to be 91.4%. The N secretion into the whole intestine increased during the 6 h after feeding up to 85.5 mg (64% of N intake), for which 77 +/- 5.5% was secreted into the small intestine. Secretion into the stomach was relatively small and up to 4 hours after feeding, amounted only to 3.0 ... 6.7 mg N.

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The fractional synthesis rates (FSR) for liver, pancreas, small intestine, large intestine, skeleton muscle and whole body protein were estimated with the help of the large dose method of McNurlan et al. (1979) and Garlick et.al (1980) using simultaneously [14C]lysine and [15N]lysine as flooding substances in growing rats. The following results and conclusions can be drawn: In the case of lysine as flooding substance the incorporation time (time from the injection up to killing the animals) can be fixed at 20 minutes. On principle lysine is suitable as flooding substance. [14C]lysine and [15N]lysine can yield identical values for FSR in the same animal. The variation coefficients of the FSR values were 7.3% (4... 12%) using [14C]lysine and 13.8% (9.5... 36%) using [15N]lysine (emission-spectrometry) as flooding substance. Using the mass-spectrometric method for measuring 15N-excess it is possible to obtain the same accuracy of result as in the case of estimation [14C]lysin. The main advantage of using 15N-labelled amino acids as flooding substances is the absence of any radioactivity. Therefore this method is also suitable for farm animals and--in combination with the indolent muscle biopsy--it may be used for men.

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In 3 experimental groups 9 female Wistar rats (initial live weight 150 g) were fed either the control diet, the control diet supplemented with 5 mg clenbuterol or the combination of 5 mg clenbuterol and 500 mg propranolol per kg diet over a 12-day period. The N-balance was estimated over days 6 to 10 followed by a 15N-tracer experiment for determining the influence of the feed additives on characteristics of protein metabolism on day 12. All differences in the means were concluded to be significant for P < 0.05. Live weight gain and feed efficiency were improved by clenbuterol. The animals treated with clenbuterol had 18%-24% higher muscle weights whereas the combined treatment increased the muscle weights by 10%-16% only. The good correlation between the increase of muscle weights and the total protein content indicates that clenbuterol does not change the relation between protein- and water accumulation. Histological-histochemical investigations showed that the higher muscle weights were achieved through muscle fibre hypertrophy. The number of muscle fibres remained constant. Concerning the distribution of the fibre types, clenbuterol increased the proportion of FTG-fibres (white, fast-twitch, glycolytic) on the expense of the FTO-fibres (fast-twitch, oxidative). While the number of nuclei per muscle fibre did not change, the nucleus-cytoplasm relation decreased by 24%. Compared to the animals fed the control diet, the N-balance in the clenbuterol-treated group was increased by 41%. Feeding the combination of clenbuterol and propranolol resulted in an increase of 24% only. Clenbuterol increased the N-content of the carcass by 6% and reduced the carcass fat content by 30%. In the group fed clenbuterol and propranolol, the N-content of the carcass only tended to be increased and the influence on carcass fat reduction was only 16%. The stimulated N-deposition in the carcass of clenbuterol-treated rats was obtained, since the calculated protein degradation rates were more reduced than protein synthesis rates. In-vitro investigations of the muscle protein synthesis and -protease activities support these results. The clenbuterol-induced increase in muscle protein was accompanied by an inhibition of the Ca-dependent protease activity and an increase of muscle DNA- and RNA-content. The additional application of propranolol reduced these effects of clenbuterol again. Since propranolol partly prevented the effects of clenbuterol on protein metabolism it is suggested that not only the lipolytic but also protein anabolic effects are caused by the beta-adrenergic action of clenbuterol.

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In a series of experiments in two ileo-rectostomized pigs the precaecal apparent and true digestibilities for crude protein of seven 15N-labelled feedstuffs were studied using the mobile bag technique. True digestibility values of crude protein calculated with the help of isotope dilution were 4-14 units higher than the corresponding apparent digestibility values. This is caused by the infiltration of the bag residues with endogenous unlabelled nitrogen. In dependence on the kind of feedstuff the endogenous N portion in the bags amounted to 25-70% of the total N. In addition, the contamination is influenced by differences between the animals (mean 4-7%) and by different treatments of bags after gut passage (3-10%).

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The mobile bag technique (MBT) was used in two ileorectostomized and two intact pigs with duodenal cannulae, in order to estimate the precaecal and total crude protein digestibility and amino acid (lysine, methionine, cystine, threonine, tryptophan) absorption of ten single feedstuffs (field bean, lupin, rapeseed meal, fish meal, barley, wheat) and three diets. For each feed 36 bags were applied per animal. After a peptic predigestion in vitro, application of the bags through the duodenal cannula into the intestine and their passage through the gut, the residues remaining in the bags were pooled to collective samples and just as the original feed material analysed on dry matter, nitrogen and amino acids. The directly determined MBT-digestibilities (BV) and the values obtained by regressive calculations (BVb) have been compared with apparent digestibilities (sV) conducted from conventional digestion experiments. 74 per cent of all BV data was higher than the corresponding sV values. Similarities of BV with sV showed in 58 per cent, those of BVb with sV only in 17 per cent. In all cases differences of > +/- 4 digestibility units showed. Checking additivity of diet-BV by means of calculations from the BV of their single ingredients did not generally lead to the same results as the directly estimated diet-BV; in some cases the results were contrary. Therefore the use of MBT for mixed feed diets has to be further investigated. For single feedstuffs the MBT can be used successfully as a rapid and simple technique when regressive calculations are employed.

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The digestibility, the N balance, the rate of protein synthesis and other parameters, characterising the protein metabolism in dependence on live weight, protein- and energy supply are estimated on Wistar rats (4-5 animals/group). These experiments were done in 5 alternating consecutive growth and energy maintenance periods at 4 different levels of protein (6, 10, 17, 26% CP) during the live weight period of 70 to 230 g. The rate of protein synthesis was calculated from the course of renale 15N excretion by means of the end product method after giving a single dose of a mixture of 17 15N labelled amino acids. N deposition, rate of protein synthesis and flux rate increased with the protein level of the ration. During maintenance these data were much lower, but showed the same dependence on the protein level. The absolute protein synthesis (g/d) increased up to the live weight of 130... 180 g and decreased afterwards according to the age. The reutilization rate varied between 44 and 87% and decreased with increasing dietary protein level by 27% and during proceeding age by 8 ... 12%. In contrast to the absolute metabolism rates (g/d) the fractional rates (%/d) clearly decreased with the age of the animals. The stimulation of these rates by the dietary protein level resembled that for the absolute rates of synthesis. The protein deposition showed the typical course of a growing curve according to the N intake and the protein synthesis showed practically the same course but on a higher level. The break down remained constantly (approximately 140 mg N/d) up to an N intake of about 360 mg and afterwards it increased too.

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The apparent precaecal crude protein digestibility and amino acid absorption as well as the total digestibility of the nutrients organic matter, crude protein, crude fat and crude carbohydrates were determined of rations consisting of barley plus oilmeals of two rapeseed varieties of double quality (DQ) from Canada (Canola) and the GDR as well as of one variety of single quality (SQ) with 10 female pigs weighing between 120 and 140 kg with ileorectal anastomoses (IRA) and 4 intact (INT) female pigs weighing between 150 and 160 kg. The three rapeseed meals (RSM) were furthermore fed to IRA pigs as sole protein source together with protein-free feed. Based on the digestibilities of the crude nutrients determined with INT pigs and calculated with the difference method the energy content of RSM DQ "Canola", DQ GDR and SQ GDR could be calculated as 495, 633 and 500 EFUpig/kg DM (average: 543 EFUpig). The precaecal digestibility of the crude protein of the above mentioned RSM sorts determined with IRA pigs and calculated with the difference method amounts to 73, 76 and 75% resp. and is thus lower than the average absorption of all amino acids. The precaecal digestibilities of crude protein and essential amino acids for the mixtures calculated from the digestibility values of the individual feedstuffs barley and the above mentioned RSM largely agree with the experimentally determined digestibility values, i.e. there is additivity. On an average of the values known from literature for DQ RSM (including our own results), the precaecal digestibility of crude protein amounts to 70, of lysine to 74, of methionine to 82, of cystine to 82, of threonine to 69, of tryptophan to 72 and of isoleucine to 74%.

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In an experiment with 3 pigs (initial live weight 30 kg, each fitted with 2 re-entrant fistulas in duodenum and ileum, one labelled with 15N), the duodenal and ileum digesta was exchanged. The N and 15N contents were estimated in faeces, urine, duodenal and ileum digesta of all experimental animals as well as in special organs and in the contents of different tract sections. The 15N excess (15N') of N compounds secreted into the gut lumen was determined using the 15N' in pancreas, gut mucosa and TCA-soluble blood serum. From measuring the digesta passage through the 3 sections of the digestive tract: 1. mouth ... duodenum, 2. duodenum ... ileum, 3. ileum ... after (Krawielitzki et al., 1989) the absorption and secretion rates of nitrogen were calculated. Secretion into the 1st section amounted to 5.3 g N/d (= 15% of intake) and the absorption to approximately 1% of intake. In the 2nd section the corresponding dates were 8.9 resp. 38.6 g N/d (= 25 resp. 110% of N intake), and in the 3rd one 1.9 resp. 8.4 g N/d (= 5.6 resp. 24% of N intake). Total absorption amounted to 134% of N intake and the over all re-absorption of endogenous N compounds secreted into the gut lumen to about 90%. During the passage the amount of endogenous N (g/d) decreased from 5.3 at the duodenum to 3.8 at the ileum to 1.6 in the faeces, but the relative portion increased (13 resp. 35 resp. 39%). An incorporation into body proteins occurred only from N compounds absorbed in the 1st and in the 2nd section. N (or 15N) absorbed in the large intestine was almost quantitatively excreted by urine. The method of digesta exchange between cannulated labelled and unlabelled pigs seems to be a suitable method to estimate absorption and secretion of exogenous and endogenous N portions in various sections of the digestive tract.

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Methodical recommendations are suggested--predominantly for laboratory and small animals (rats and young chickens)--for the determination of parameters of the protein metabolism of the whole body after single doses of a mixture of 15N labelled amino acids by means of the determination of the temporal course of cumulative 15N excretion in urine and the assessment of the tracer kinetic data in a compartment model. These recommendations are to make it possible to carry out purposefully such experiments under comparable conditions. The advantages of this method are: the non-invasive character of the method; the possibility of repeating the experiment with the same animal; the adaptability to other methods of investigation (e.g. measuring energy metabolism); the relatively low expenditure of labour and requirement of test animals; the relatively good reproducibility of the method. Thus this method is a good supplement to the flooding and permanent infusion methods and should be used wherever the determination of parameters of the protein metabolism of the total body is sufficient.

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In 3 successive experiments with growing rats the suitability of pulse labelling with [15N]glycine, linked with a 14C labelling by means of [14C]lysine (experiment 3), was tested for the determination of kinetic parameters of the protein metabolism of the whole body by the application of the compartment model in comparison with pulse labelling with a 15N amino acid mixture (experiment 2) and long-time labelling with 15N with 15N labelled wheat in the feed (experiment 1) under standardized experiment conditions. In simultaneously carried out measurings of energy metabolism with parallel groups of animals the comparability of the metabolic development was studied. The ascertained values of protein synthesis rate, protein catabolism rate and re-utilization rate showed insignificant differences only between the 3 15N tracer variants (with certain limitations for the 'protein turnover' (P)-group of experiment 2) in comparison with errors of the applied methods, from which conclusions can be drawn for the suitability of [15N] glycine as tracer, at least under the experiment conditions tested. The protein synthesis and degradation rates ascertained from 14CO2 excretion in experiment 3 were clearly below those average values ascertained with 15N. The differences in the average heat production between the main periods of the 3 experiments were statistically insignificant.

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In a tripartite experiment with five rats each the suitability of 51Cr2O3 and TiO2 as markers was tested for estimating the transit time and passage rate as well as the total protein digestibility and the proportion of endogenous nitrogen in the small intestine with very small samples. Both markers are suited for these tests because of their simple analysis, their high recovery rate (94...105% for 51Cr2O3; 98...105% for TiO2) and their quantitative excretion in faeces within 3 days. Depending on the level of crude fibre (5.1 resp. 9.4% of DM) the transit time was found to be 7...8 h for a commercial breeding feed and 5...6 h for this commercial feed +15% straw meal. After the application of a single doses, the marker excretion of 50% appeared within 10 +/- 1 h resp. 8 +/- 1 h, and after the 12th hour there were no significant differences at all. The apparent N-digestibilities estimated by the conventional or the 51Cr2O3 resp. TiO2 indicator method did not show any differences. The proportion of endogenous nitrogen at the end of the small intestine calculated on the basis of the indicator dilution method was 83 +/- 11% for 51Cr2O3 and 83 +/- 12% for TiO2.

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16 male rats of 100 g live weight were given 50 mg of a mixture containing 15N labelled amino acids as a single dose within a protein-free feeding period. Following this the 15N excretion in faeces and urine as well as the development of the 15N excess in different organs and tissues were estimated over 3 days by slaughtering the animals within given 7 time intervals. Using a 3 pool model and the computer program for the interpretation of 15N tracer experiments by Töwe et al. (1984), kinetic parameters such as the rate of protein synthesis, protein breakdown and the rate of reutilization were calculated. Despite a negative N balance (-41.8 mg N/d) under protein-free conditions the protein metabolism of the rat shows high dynamics characterized by a high flux rate (225 mg N/d) and a high rate of body protein synthesis (181 mg/d). The reutilization was 85%. Depending on time the 15N excess in the tested organs and tissues showed significant differences and seems to demonstrate that under these conditions protein synthesis mainly takes place in the most important organs (e.g. intestinal tract, liver). Under protein-free feeding conditions protein synthesis and protein breakdown of the whole body seems to be slightly increased in comparison to N balanced feeding conditions.

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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.

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