The influence of ration size and feeding frequency on ammonia excretion by juvenile Atlantic cod, Gadus morhua L.

Small groups of juvenile Atlantic cod, Gadus morhua L., were kept at 14°C in through-flow tanks and were fed known quantities of a compounded diet of natural food. The cod were fed single and multiple meals with ration size in the range 0.5 to 4.1% of total wet fish body weight. Ammonia production in each feeding experiment was monitored continuously.
For single-meal experiments, significant relationships were derived between ration size and (a) total ammonia excreted, (b) total exogenous ammonia excreted above endogenous excretion levels, (c) duration of the elevated phase of ammonia excretion, (d) maximum rate of ammonia excretion, and (e) time delay after feeding to reach maximum rate of ammonia excretion. Relationships between nitrogen loss as ammonia and nitrogen intake were examined and estimates of endogenous excretion rate and maintenance ration made.
Repetitive feeding resulted in cyclical variation in ammonia excretion. At the lowest ration size, ammonia excretion rates had nearly returned to the pre-feeding level within 24 h. At higher feeding levels, the effect of each successive meal tended to be cumulative, resulting in increasingly higher ammonia excretion rates which only stabilized towards the end of the experiments.     

Bioenergetics of growth of a cyprinid, Phoxinus phoxinus: the effect of ration, temperature and body size on food consumption, faecal production and nitrogenous excretion

Food consumption, faecal production and nitrogen excretion by minnows, Phoxinus phoxinus , weighing 1–5.5 g were studied at five rations ranging from starvation to ad libitum and four temperatures ranging from 5 to 15°C.
The maximum rate of food consumption (C_max) was related to body weight ( W ) and temperature ( T ) by the relationship: C _max= aW^b1T^b2 . There were significant daily variations in C_max, which tended to decline over time. Absorption efficiency increased with increasing ration size and decreasing temperature. Body weight had no significant effect on the faecal production. The equation F = a C^b1e^b2 T described the relationship between faecal production ( F ), food consumption ( C ) and temperature. Ammonia-N predominated over urea-N in the excreta of most experimental fish. The proportion of urea-N in the total nitrogen excreted was generally higher at lower rations than at higher rations. Rates of nitrogen excretion increased with increased ration size and were, to a lesser extent, influenced by temperature. Body weight had no significant effect on the nitrogen excretion by feeding minnows. The equation N = a+b_lT+b₂C described the effects of food consumption and temperature on nitrogen excretion ( N ) other than urea-N excretion. The relationship between urea-N excretion ( N_u ), food consumption and temperature was described by the equation N_u= ae^b1T ((C+1) ^b 2.
On the average, 11 % of food energy was lost in faecal production and nitrogen excretion by minnows feeding on whiteworms, Enchytraeus spp.     

Quantifying the consequences of nutritional strategies aimed at decreasing phosphorus excretion from pig populations: a modeling approach

There is a global imperative to reduce phosphorous (P) excretion from pig systems. In this study, a previously validated deterministic model was modified to be stochastic, in order to investigate the consequences of different management strategies on P excretion by a group of growing pigs. The model predicts P digestion, retention and excretion from feed composition and growth parameters that describe a specified pig phenotype. Stochasticity was achieved by introducing random variation in the latter. The strategies investigated were: (1) changing feed composition frequently in order to match more closely pig digestible P (digP) requirements to feed composition (phase feeding) and (2) grouping pigs into light and heavy groups and feeding each group according to the requirements of their group average BW (sorting). Phase feeding reduced P excretion as the number of feeding phases increased. The effect was most pronounced as feeding phases increased from 1 to 2, with a 7.5% decrease achieved; the increase in phases from 2 to 3 was associated with a further 2.0% reduction. Similarly, the effect was more pronounced when the feed targeted the population requirements for digP at the average BW of the first third, rather than the average requirements at the mid-point BW of each feeding sequence plan. Increasing the number of feeding phases increased the percentage of pigs that met their digP requirements during the early stages of growth and reduced the percentage of pigs that were supplied <85% of their digP requirements at any stage of their growth; the latter may have welfare implications. Sorting of pigs reduced P excretion to a lesser extent; the reduction was greater as the percentage of pigs in the light group increased from 10% to 30% (from 1.5% to 3.0% reduction, respectively). This resulted from an increase in the P excreted by the light group, accompanied by a decrease in the P excreted by the remaining pigs. Sorting increased the percentage of light pigs that met their dig P requirements, but only slightly decreased the percentage of heavy pigs that met these requirements at any point of their growth. Exactly the converse was the case as far as the percentage of pigs that were supplied <85% of their digP requirements were concerned. The developed model is flexible and can be used to investigate the effectiveness of other management strategies in reducing P excretion from groups of pigs, including precision livestock feeding.     

Effect of ration level on nitrogen excretion, nitrogen retention and efficiency of nitrogen utilization for growth in largemouth bass (Micropterus salmoides)

Largemouth bass ( Micropterus salmoides ) with an average weight of 45.5 g were used to study ration level effects on nitrogen excretion, nitrogen retention, and gross efficiency of utilization of nitrogen for growth. Bass were starved 3 to 4 days and then each bass was placed into an aquarium containing a known volume of water. One day after the fishes were placed in aquaria, nitrogen excretion rates were determined; this rate is the maintenance nitrogen excretion. Each fish was then fed one or more shiners ( Notropis cornutus ); fish were fed only once. Nitrogen excretion measurements were made daily until the rates were similar to maintenance rates. The nitrogen excretion rates for each day after feeding which were above the maintenance nitrogen excretion were combined and reflect the total nitrogen excretion for a given ration level. All ration levels were converted to nitrogen consumption in mg and nitrogen absorption was calculated from subtracting the average faecal nitrogen from nitrogen consumption. From data on nitrogen consumption, nitrogen absorption, nitrogen excretion, and faecal nitrogen, calculations were made for nitrogen retention and gross efficiency of utilization of nitrogen for growth.
As ration level and nitrogen absorption increased, nitrogen excretion increased and is described by the equation, Y = 8.56+0.40 X , where Y is total nitrogen excretion and X is nitrogen absorption. Nitrogen retention also increased with nitrogen absorption and is described by the equation, Y =–8.57+0.60 X , where Y is nitrogen retention and X is nitrogen absorption. Efficiency increases rapidly above maintenance, but levels off at higher ration levels and approaches an asymptote of 60%.     

Seasonal variability in feeding behaviour, metabolic rates and carbon and nitrogen balances in the Sydney oyster, Saccostrea glomerata (Gould)

To establish if nutrients limit the growth of bivalves requires information not only on the quality of food available, but also the animals' feeding behaviour and endogenous metabolic demands. We hypothesized that growth of the Sydney rock oyster (Saccostrea glomerata) would vary in response to seasonal changes in food quality rather than quantity. We also predicted that the oysters would show feeding preferences for nitrogen over carbon, and this behaviour would result in carbon/nitrogen (C/N) ratios for ingested and absorbed matter that would be lower than the C/N of both the seston and the oysters' estimated metabolic maintenance requirements. The experiments were done in two phases under natural conditions. In phase 1, feeding behaviour was assessed on a single occasion and the results used to pose hypotheses for testing in phase 2, which included measurements made on three occasions encompassing autumn, winter and spring conditions. Growth rates varied with changes in ambient food quality and not with the concentration of total suspended matter. Feeding behaviour responded to food quality and, in most cases, resulted in nitrogen enrichment. For example, when nitrogen was potentially limiting to growth and/or maintenance, due to high food C/N (July) or high nitrogen demand (March), pre-ingestive selection ensured nitrogen enrichment of ingested matter and C/N ratios of ingested matter which were below the maintenance requirement. However, in November, when endogenous demands indicated an increased requirement for carbon, feeding behaviour resulted in carbon enrichment, an increase in carbon conversion efficiency, and ingested C/N ratios greater than the maintenance requirement. The results support the assertion of variable feeding physiology in oysters, responsive to both exogenous (seasonal differences in carbon and nitrogen availability) and endogenous (cycles of reproduction and growth) factors.     

The influence of feeding on the metabolic activity of Antarctic krill (Euphausia superba Dana)

Summary The influence of feeding on the metabolic activity of juvenile krill was assessed from 24h experiments in which krill were incubated with various concentrations of diatoms (Chaetoceros calcitrans, Phaeodactylum tricornutum, Thalassiosira eccentrica, Fragilariopsis vanheurkii), newly hatched Artemia nauplii and latex beads. Krill fed on the larger food more efficiently, with reluctant feeding on latex beads. Feeding of krill expressed as clearance rates was poorly correlated with their oxygen uptake rates. Instead, a positive correlation was found between the oxygen uptake rates and ingestion rate (except for latex beads). The result implies that the specific dynamic action is the major cause of the increased oxygen uptake of krill. Krill fed diatoms increased both ammonia and phosphate excretion with increasing ingestion rate, but only phosphate excretion was increased in parallel with ingestion rate for those fed Artemia nauplii. Assuming the daily ration of krill in the field is 5% of the body weight, and the major food source is phytoplankton, oxygen uptake, ammonia excretion and phosphate excretion rates of wild krill are estimated to be 1.6, 4.5 and 7.8, respectively, times the rates of non-feeding krill in 24h laboratory experiments. Krill offered various kinds of food showed different metabolic quotients (O/N, N/P and O/P ratios). While no functional relationship was seen between the metabolic quotient and the ingestion rate of krill fed Artemia nauplii, those fed Fragilariopsis showed a progressive decrease in O/N, N/P, and O/P ratios as their ingestion rates increased.     

Nitrogen excretion in rats on a protein-free diet and during starvation

Nitrogen balances (six days) were determined in male Wistar rats during feeding a diet with sufficient protein or a nearly protein-free diet (n = 2 x 24), and then during three days of starvation (n = 2 x 12). The objective was to evaluate the effect of protein withdrawal on minimum nitrogen excretion in urine (UN), corresponding to endogenous UN, during feeding and subsequent starvation periods. The rats fed the protein free-diet had almost the same excretion of urinary N during feeding and starvation (165 and 157 mg/kg W(0.75)), while it was 444 mg/kg W(0.75) in rats previously fed with protein, demonstrating a major influence of protein content in a diet on N excretion during starvation. Consequently, the impact of former protein supply on N losses during starvation ought to be considered when evaluating minimum N requirement necessary to sustain life.     

Bacterial protein meal in diets for pigs and minks: comparative studies on protein turnover rate and urinary excretion of purine base derivatives

The effect of increasing the dietary content of bacterial protein meal (BPM) on protein turnover rate, and on nucleic acid and creatinine metabolism in growing minks and pigs was investigated in two experiments. In each experiment, 16 animals were allocated to four experimental diets. The diets containing no BPM served as controls, i.e. for minks diet M1, for pigs P1; the experimental diets contained increasing levels of BPM to replace fish meal (minks) or soybean meal (pigs), so that up to 17% (P2), 20% (M2), 35% (P3), 40% (M3), 52% (P4), and 60% (M4) of digestible N was BPM derived. Protein turnover rate was measured by means of the end-product method using [15N]glycine as tracer and urinary nitrogen as end-product. In minks, protein flux, synthesis, and breakdown increased significantly with increasing dietary BPM. In pigs, diet had no observed effect on protein turnover rate. The intake of nucleic acid nitrogen (NAN) increased from 0.15 g/kg W0.75 on M1 to 0.26 g/kg W0.75 on M3 and M4 in the mink experiment, and from 0.08 g/kg W0.75 on P1 to 0.33 g/kg W0.75 on P4 in the pig experiment. Increased NAN intake led, in both experiments, to increased allantoin excretion. Analysis of species effects showed that minks excreted 1.72 mmol/ kg W0.75 of allantoin, significantly more than the 0.95 mmol/kg W0.75 excreted by pigs. In minks, approximately 96% of the excreted purine base derivatives consisted of allantoin, whereas in pigs approximately 93% did. Thus, increasing the dietary content of BPM increased protein turnover rate in minks but not in pigs, and allantoin excretion increased with increasing dietary BPM although it seemed that mink decomposed purine bases to their end-product more completely than pigs did. Collectively these data show that BPM is a suitable protein source for pigs and mink, and recorded differences between species were to a large extent due to differences in protein retention capacity and muscle mass.     

Effect of feeding time on postprandial nitrogen excretion and energy expenditure in rainbow trout

The effect of feeding time (dawn or midnight) on nitrogen excretion and energy expenditure was studied in immature rainbow trout using measurements of respiratory gas exchange. Fish (mean individual weight 70 g) were maintained indoors under natural photoperiod and fed by hand (commercial food pellets) at a rate of 1% weight/day⁻¹. Rates of ammonia and CO₂ excretion and O₂ uptake were measured every hour. Ammonia excretion increased immediately after feeding in fish fed at midnight, and 2h after feeding in fish fed at dawn. Ammonia excretion and energy supply from protein catabolism, were higher in trout fed at midnight than in those fed at dawn, while total energy expenditure was the same in both groups. The results suggested that trout fed in phase with their natural feeding rhythm use dietary protein more efficiently for growth than do trout fed out of phase with the natural rhythm.     

The combined effect of feeding time and ration on growth performance and nitrogen metabolism of greenback flounder

Greenback flounder Rhombosolea tapirina ( c. 2 g) fed to satiation had significantly ( P <0·01) higher feed consumption in the evening than in the morning whereas there was no difference between feeding times for flounder fed restricted rations (1 or 2% body weight per day) because they consumed all of the ration. Differences in growth performance were due to feeding time and ration. Carcass moisture, lipid and energy content were significantly ( P <0·001) different between rations; length gain was significantly affected by feeding time ( P <0·05) and ration ( P <0·001); weight gain showed a significant ( P <0·001) interaction between feeding time and ration. The relationship between feed consumption and specific growth rate showed that the exponential gradient was significantly higher ( P <0·01) for the evening fed fish and indicated feed efficiency for evening fed fish increased as feed consumption increased. Urea excretion increased from 12–20 to 58–63% of total nitrogen excretion at the 1 and 3% rations, respectively. Ammonia and urea excretion were significantly affected by ration ( P <0·001) and feeding time ( P <0·05). Fish fed the 2% ration in the evening had higher growth efficiency and significantly ( P <0·01) lower rates of urea excretion than fish fed 2 or 3% ration in the morning. It is suggested that the higher energetic costs associated with differences in ammonia and urea excretion contributed to differences in growth efficiency.     

The dogfish shark (Squalus acanthias) increases both hepatic and extrahepatic ornithine urea cycle enzyme activities for nitrogen conservation after feeding

Urea not only is utilized as a major osmolyte in marine elasmobranchs but also constitutes their main nitrogenous waste. This study investigated the effect of feeding, and thus elevated nitrogen intake, on nitrogen metabolism in the Pacific spiny dogfish Squalus acanthias. We determined the activities of ornithine urea cycle (O-UC) and related enzymes in liver and nonhepatic tissues. Carbamoyl phosphate synthetase III (the rate-limiting enzyme of the O-UC) activity in muscle is high compared with liver, and the activities in both tissues increased after feeding. The contribution of muscle to urea synthesis in the dogfish body appears to be much larger than that of liver when body mass is considered. Furthermore, enhanced activities of the O-UC and related enzymes (glutamine synthetase, ornithine transcarbamoylase, arginase) were seen after feeding in both liver and muscle and were accompanied by delayed increases in plasma urea, trimethylamine oxide, total free amino acids, alanine, and chloride concentrations, as well as in total osmolality. The O-UC and related enzymes also occurred in the intestine but showed little change after feeding. Feeding did not change the rate of urea excretion, indicating strong N retention after feeding. Ammonia excretion, which constituted only a small percentage of total N excretion, was raised in fed fish, while plasma ammonia did not change, suggesting that excess ammonia in plasma is quickly ushered into synthesis of urea or protein. In conclusion, we suggest that N conservation is a high priority in this elasmobranch and that feeding promotes ureogenesis and growth. Furthermore, exogenous nitrogen from food is converted into urea not only by the liver but also by the muscle and to a small extent by the intestine.     

Ascorbic acid feeding of rats reduces copper absorption,causing impaired copper status and depressed biliary copper excretion

The feeding of diets enriched with ascorbic acid (10 g/kg) to rats has previously been shown to lower plasma and liver copper concentrations. The present studies corroborate this. We hypothesized that ascorbic acid initially reduces copper absorption, this effect being masked later by the stimulatory effect on copper absorption of the impaired copper status. We also hypothesized that the impaired copper status as induced by ascorbic acid feeding is followed by a diminished biliary excretion of copper in an attempt to preserve copper homeostasis. Our hypotheses are supported by the present studies. Ascorbic acid feeding initially reduced apparent copper absorption, and in the course of the experiment this effect tended to turn over into a stimulatory effect. Copper deficiency, as induced by feeding a diet containing 1 mg Cu/kg instead of 5 mg Cu/kg, systematically increased copper absorption. Biliary excretion of copper in rats given ascorbic acid was unaffected initialy but became depressed after prolonged ascorbic acid feeding. A similar time course was seen for fecal endogenous copper excretion that was calculated as the difference between true and apparent copper absorption. Copper deficiency systematically reduced biliary copper excretion and fecal endogenous copper loss.     

The influence of dietary fibre on protein digestion and utilization in monogastrics

Current knowledge of the effects of dietary fibre and associated components on protein digestibility and utilization are discussed. Based on the literature it could be shown that the implications and mechanisms behind the effect of soluble and insoluble dietary fibre on protein digestibility and utilization are quite different. Insoluble dietary fibre will increase faecal bulk and faecal nitrogen excretion is primarily due to and increased excretion of cell wall bound protein. Contrary to this, soluble dietary fibre increase faecal bulk and faecal nitrogen due to an increased excretion of microbial nitrogen. A matter of controversy is the influence of dietary fibre on endogenous nitrogen excretion and factors affecting the losses of nitrogen in this way. It is not known if fibre acts as a secretogogue.     

Reducing the CP content in broiler feeds: impact on animal performance,meat quality and nitrogen utilization

Reducing the dietary CP content is an efficient way to limit nitrogen excretion in broilers but, as reported in the literature, it often reduces performance, probably because of an inadequate provision in amino acids (AA). The aim of this study was to investigate the effect of decreasing the CP content in the diet on animal performance, meat quality and nitrogen utilization in growing-finishing broilers using an optimized dietary AA profile based on the ideal protein concept. Two experiments (1 and 2) were performed using 1-day-old PM3 Ross male broilers (1520 and 912 for experiments 1 and 2, respectively) using the minimum AA:Lys ratios proposed by Mack et al. with modifications for Thr and Arg. The digestible Thr (dThr): dLys ratio was increased from 63% to 68% and the dArg:dLys ratio was decreased from 112% to 108%. In experiment 1, the reduction of dietary CP from 19% to 15% (five treatments) did not alter feed intake or BW, but the feed conversion ratio was increased for the 16% and 15% CP diets (+2.4% and +3.6%, respectively), while in experiment 2 (three treatments: 19%, 17.5% and 16% CP) there was no effect of dietary CP on performance. In both experiments, dietary CP content did not affect breast meat yield. However, abdominal fat content (expressed as a percentage of BW) was increased by the decrease in CP content (up to +0.5 and +0.2 percentage point, in experiments 1 and 2, respectively). In experiment 2, meat quality traits responded to dietary CP content with a higher ultimate pH and lower lightness and drip loss values for the low CP diets. Nitrogen retention efficiency increased when reducing CP content in both experiments (+3.5 points/CP percentage point). The main consequence of this higher efficiency was a decrease in nitrogen excretion (−2.5 g N/kg BW gain) and volatilization (expressed as a percentage of excretion: −5 points/CP percentage point). In conclusion, this study demonstrates that with an adapted AA profile, it is possible to reduce dietary CP content to at least 17% in growing-finishing male broilers, without altering animal performance and meat quality. Such a feeding strategy could therefore help improving the sustainability of broiler production as it is an efficient way to reduce environmental burden associated with nitrogen excretion.     

Modelling interactions between phytoplankton and bacteria under nutrient-regenerating conditions

The interactions of phytoplankton and bacteria in a nitrogen-limitedsteady-state system with an organic nitrogen compound or ammoniumas the sole nitrogen source were modelled. The effects of variousalgal excretion rates and two different mathematical representationsof excretion were examined. The model predicted that higherexcretion elevated the bacterial steady-state biomass, and loweredthe algal biomass. Bacterial respiration, which directly determinednitrogen regeneration, had an important effect on the system.The bacterial growth yield in the model was mainly a functionof the growth rate, and not of the nitrogen:carbon ratio ofthe substrate. In one version of the model, where the excretionof organic carbon increased with decreasing growth rate, themodel started to oscillate when the multiplication product ofmaximum specific excretion of excreted organic carbon (EOC)and the bacterial yield on EOC exceeded the dilution rate, irrespectiveof the form of nitrogen (ammonium or dissolved organic nitrogen)in the medium. The model results were compared with chemostatexperiments with the alga Emiliania huxleyi and a bacterialisolate in pure and mixed culture at two different dilutionrates. The carbon and nitrogen biomass of the bacteria was {smalltilde}1.5 times higher in mixed culture than in pure culture.In the experiments with low dilution rate, the recovery of nitrogenin the form of biomass, ammonium or amino acids was low, suggestingthe excretion by the algae of a refractory nitrogen-containingproduct which the bacteria could not use.     

The influence of ambient temperature and the energy and protein content of food on nitrogenous excretion in the Egyptian fruit bat (Rousettus aegyptiacus)

The diets of frugivorous and nectarivorous vertebrates contain much water and generally have high energy but low protein contents. Therefore, we tested the prediction that to save energy under conditions of high energy demands and high water intake, frugivorous Egyptian fruit bats (Rousettus aegyptiacus) will increase both the absolute quantity and the proportion of ammonia in their urine. We also examined whether such changes occur when protein intake is low and water intake is high. We did three feeding trials. In trials 1 and 2, bats were fed one of four liquid diets containing constant soy protein concentrations but varying in sucrose concentration and were kept at ambient temperatures (T(a)) of 30 degrees Celsius and 12 degrees Celsius, respectively. In trial 3, bats were kept at Ta=12 degrees Celsius and fed one of four liquid diets with equal sucrose concentrations but varying protein concentrations. In trial 1, food intake at a sucrose concentration of 256 mmol/kg H(2)O was initially high but decreased to a constant rate with further increases in sucrose concentration, while in trial 2, food intake decreased exponentially with increasing sucrose concentration. As predicted, at 12 degrees Celsius with varying sucrose concentration, both the absolute quantity and the fraction of ammonia in the bats' urine increased significantly with food intake (P<0.02), while the absolute quantity of urea and the fraction of urea nitrogen excreted decreased significantly with food intake (P<0.03). Varying sucrose concentration had no significant effect on nitrogen excretion at Ta=30 degrees Celsius. Varying protein concentration had no significant effect on nitrogen excretion at Ta=12 degrees Celsius. We suggest that Egyptian fruit bats can increase ammonia excretion in response to increased energetic demands, and we calculate that they can save energy equal to approximately 2% of their daily metabolic rate by doing so.     

Temporal focusing of nitrogen release by a periodically feeding herbivorous reef fish

The reef-dwelling jewel damselfish Plectroglyphldodon lacrymatus (Quoy & Gaimard) exhibited a marked diel periodicity in nitrogenous excretion and defecation rates. This temporal “focusing” of soluble and particulate-faecal nitrogen release on the latter part of the day correlated strongly with the pattern of feeding activity. There was a lag after feeding began each day of ≈ 5–6 h before defecation accelerated, but no such lag was apparent in the case of excretion. Both excretion and defecation, however, did lag behind feeding at the end of the day, for they remained above their respective minimum rates for ≥ 8 h after feeding had ceased. Gut evacuation rates were variable, while the excretion of nitrogenous waste appeared to take ≈24 h and to be subject to a physiological rhythm. Defecation, most of which occurred at a single site in each territory, was far more important in the generation of nitrogen by the fish than was excretion. In laboratory experiments the faeces took up ammonium, but in the field most faeces were probably removed quickly by ophiocomine brittlestars and pulled down into interstices of the reef. Nearly all of the nitrogen egested, but perhaps only one-third of that excreted, was transferred initially to the reef framework below the fish's territory.     

Effect of exercise on protein metabolism in humans as explored with stable isotopes

Exercising for 3.75 h on a treadmill at 50% VO2 max in the fed state induced an increased excretion of 71 mg nitrogen/kg over the 18 h after exercise. However, measurements of the time course of changes in 13CO2 excretion from ingested [1-13C]leucine indicated that all of this increased nitrogen production occurs during the exercise period. Because of the reduced renal clearance and slow turnover of the urea pool, urea excretion lags behind urea production. Measurements of nitrogen flux from the plateau labeling of urinary ammonia achieved by repeated oral doses of 15N-labeled glycine indicated that the nitrogen loss resulted from an increase in protein degradation and a decrease in protein synthesis. Further studies with [1-13C]leucine indicated that a 2-h treadmill exercise induced an increase in the nitrogen loss from 5.4 to 16 mg . kg-1 . h-1 measured with a primed constant infusion of [1-13C]leucine. This resulted from a fall in whole-body protein synthesis. Glucose given at the rate of 0.88 g . kg-1 . h-1 depressed the rate of whole-body protein degradation and appeared to suppress the exercise-induced increase in nitrogen excretion. When leucine oxidation rates were measured at increasing work rates, a linear relationship between percentage of VO2 max and leucine oxidation was observed up to 89% VO2 max when 54% of the flux of leucine was oxidized. These changes may involve nonmuscle as well as muscle tissue. Thus the source of the increased nitrogen losses is probably liver. In muscle, protein degradation is actually decreased judged by methylhistidine excretion, whereas in liver, protein degradation may be increased. Also the fall in whole-body protein synthesis may reflect changes in nonmuscle tissues because in running rats protein synthesis in muscle is maintained. As far as leucine metabolism is concerned, because the increase in leucine oxidation occurs when leucine and its keto acid concentration falls, exercise must specifically activate the 2-oxoacid dehydrogenase.     

Changes in endogenous urea recycling and the handling of renal urea in pregnant and lactating Sardi sheep kept on a constant feeding level

The kinetics of endogenous urea were compared during the last month of pregnancy, lactation, and a nonpregnant, nonlactating control period in Sardi sheep kept on a constant feed level. Urea entry rate estimated by injections of [14C]urea rose by 36% during pregnancy. Renal urea excretion was reduced by 40% during pregnancy and by 28% during lactation. Consequently, fractional urea recycling was greater during pregnancy and, to some extent, during lactation than during the control period. In a second series of experiments, glomerular filtration rate increased by 48% and urea filtration rate rose by 17% during pregnancy. During lactation, both glomerular filtration rate and urea filtration rate were close to control levels. It appears that the decreased renal urea excretion during pregnancy and lactation was due mainly to increased tubular reabsorption of urea. Rumination time increased by 15% during pregnancy. Rumen ammonia concentration was elevated in both pregnant and lactating ewes above the control period level. The results suggest that Sardi sheep possess a high potential for the conservation of nitrogen during pregnancy and lactation periods.     

Bacterial protein meal in diets for pigs and minks: Comparative studies on protein turnover rate and urinary excretion of purine base derivatives

Abstract

The effect of increasing the dietary content of bacterial protein meal (BPM) on protein turnover rate, and on nucleic acid and creatinine metabolism in growing minks and pigs was investigated in two experiments. In each experiment, 16 animals were allocated to four experimental diets. The diets containing no BPM served as controls, i.e. for minks diet M1, for pigs P1; the experimental diets contained increasing levels of BPM to replace fish meal (minks) or soybean meal (pigs), so that up to 17% (P2), 20% (M2), 35% (P3), 40% (M3), 52% (P4), and 60% (M4) of digestible N was BPM derived. Protein turnover rate was measured by means of the end-product method using [¹⁵N]glycine as tracer and urinary nitrogen as end-product. In minks, protein flux, synthesis, and breakdown increased significantly with increasing dietary BPM. In pigs, diet had no observed effect on protein turnover rate. The intake of nucleic acid nitrogen (NAN) increased from 0.15 g/kg W^0.75 on M1 to 0.26 g/kg W^0.75 on M3 and M4 in the mink experiment, and from 0.08 g/kg W^0.75 on P1 to 0.33 g/kg W^0.75 on P4 in the pig experiment. Increased NAN intake led, in both experiments, to increased allantoin excretion. Analysis of species effects showed that minks excreted 1.72 mmol/kg W^0.75 of allantoin, significantly more than the 0.95 mmol/kg W^0.75 excreted by pigs. In minks, approximately 96% of the excreted purine base derivatives consisted of allantoin, whereas in pigs approximately 93% did. Thus, increasing the dietary content of BPM increased protein turnover rate in minks but not in pigs, and allantoin excretion increased with increasing dietary BPM although it seemed that mink decomposed purine bases to their end-product more completely than pigs did. Collectively these data show that BPM is a suitable protein source for pigs and mink, and recorded differences between species were to a large extent due to differences in protein retention capacity and muscle mass.