Protein synthesis in trout liver is stimulated by both feeding and fasting

Environmental variation associated with overwintering elicits marked metabolic plasticity in a temperate salmonid, Salvelinus fontinalis

Poleward winters commonly expose animals, including fish, to frigid temperatures and low food availability. Fishes that remain active over winter must therefore balance trade-offs between conserving energy and maintaining physiological performance in the cold, yet the extent and underlying mechanisms of these trade-offs are not well understood. We investigated the metabolic plasticity of brook char (Salvelinus fontinalis), a temperate salmonid, from the biochemical to whole-animal level in response to cold and food deprivation. Acute cooling (1°C day-1) from 14°C to 2°C had no effect on food consumption but reduced activity by 77%. We then assessed metabolic performance and demand over 90 days with exposure to warm (8°C) or cold winter (2°C) temperatures while fish were fed or starved. Resting metabolic rate (RMR) decreased substantially during initial cooling from 8°C to 2°C (Q10=4.2-4.5) but brook char exhibited remarkable thermal compensation during acclimation (Q10=1.4-1.6). Conversely, RMR was substantially lower (40-48%) in starved fish, conserving energy. Thus, the absolute magnitude of thermal plasticity may be masked or modified under food restriction. This reduction in RMR was associated with atrophy and decreases in in vivo protein synthesis rates, primarily in non-essential tissues. Remarkably, food deprivation had no effect on maximum oxygen uptake rates and thus aerobic capacity, supporting the notion that metabolic capacity can be decoupled from RMR in certain contexts. Overall, our study highlights the multi-faceted energetic flexibility of Salvelinus spp. that likely contributes to their success in harsh and variable environments and may be emblematic of winter-active fishes more broadly.

Using 15N to determine the metabolic fate of dietary nitrogen in North Pacific spiny dogfish (Squalus acanthias suckleyi)

Marine elasmobranchs are ureosmotic, retaining large concentrations of urea to balance their internal osmotic pressure with that of the external marine environment. The synthesis of urea requires the intake of exogenous nitrogen to maintain whole-body nitrogen balance and satisfy obligatory osmoregulatory and somatic processes. We hypothesized that dietary nitrogen may be directed toward the synthesis of specific nitrogenous molecules in post-fed animals; specifically, we predicted the preferential accumulation and retention of labelled nitrogen would be directed towards the synthesis of urea necessary for osmoregulatory purposes. North Pacific spiny dogfish (Squalus acanthias suckleyi) were fed a single meal of 7 mmol l-1 15NH4Cl in a 2% ration by body mass of herring slurry via gavage. Dietary labelled nitrogen was tracked from ingestion to tissue incorporation and the subsequent synthesis of nitrogenous compounds (urea, glutamine, bulk amino acids, protein) in the intestinal spiral valve, plasma, liver and muscle. Within 20 h post-feeding, we found labelled nitrogen was incorporated into all tissues examined. The highest δ15N values were seen in the anterior region of the spiral valve at 20 h post-feeding, suggesting this region was particularly important in assimilating the dietary labelled nitrogen. In all tissues examined, enrichment of the nitrogenous compounds was sustained throughout the 168 h experimental period, highlighting the ability of these animals to retain and use dietary nitrogen for both osmoregulatory and somatic processes.

A potential role of nettle (Urtica dioica) extract on growth, biochemical changes and reproductive performance of convict cichlid (Amatitlania nigrofasciata)

This study was carried out to investigate the effects of methanolic extract of nettle (Urtica dioica) on growth, reproduction, biochemical and immunological parameters of female convict cichlid (Amatitlania nigrofasciata). For this purpose, 54 fish were distributed in 3 treatments included: without supplementation (control), 0.1 g (NE0.1), and 0.5 g (NE0.5) nettle extract per kilogram feed over 56 days. The highest final weight (4.2 ± 0.1 g), weight gain (2.8 ± 0.1 g), and specific growth rate (2.0 ± 0.0% day^-1) were recorded in NE0.1 group. Higher and lower feed conversion ratio were obtained in control and NE0.1 treatments, respectively. Hepatosomatic and viscerosomatic indices in NE0.1 treatment were significantly lower compared with control treatment. Fish fed NE0.1 showed significantly lower glucose (55.2 ± 6.5 mg g^-1), cholesterol (28.4 ± 3.3 mg g^-1), and triglyceride (211.5 ± 39.0 mg g^-1) levels. Total protein (36.3 ± 3.4 mg g^-1) and albumin (2.7 ± 0.1 mg g^-1) showed a marked increase in NE0.1 treatment. Same trend was observed in C3, C4, and IgM concentrations. NE0.1 showed the highest number of eggs per female (183.7 ± 10.2), hatching rate (97 ± 0.7%), and larval survival rate at 3 days post hatch (86.3 ± 0.6%) compared with the other treatments. These findings indicated that 0.1 g methanolic extract of nettle kg feed^-1 could enhance growth, improve metabolic, and immune function of convict cichlid. Moreover, this study confirmed that appropriate dose of nettle can positively promote reproductive performance which makes it as a valuable and cost-effective herb in aquaculture industry.

Physio-metabolic response of rainbow trout during prolonged food deprivation before slaughter

Fish normally undergo periods of food deprivation that are longer than non-hibernating mammals. In aquacultured rainbow trout (Oncorhynchus mykiss), it is unclear how fasting may affect their physiological adaptative response, especially when they are normally fed daily. In addition, that response may vary with temperature, making it necessary to express fasting duration in terms of degree days. In the current study, trout were fasted for 5, 10, and 20 days (55, 107, and 200 degree days (°C d), respectively). To assess the physiological response of fish to fasting, different biometric, blood, plasma, and metabolic parameters were measured, as well as liver fatty acid composition. The fish weight, condition factor, and the hepato-somatic index of 5-day fasted trout were not significantly different from those of control fish. Gastric pH increased as fasting progressed while plasma concentrations of glucose, triglycerides, and total proteins decreased significantly after 10 days of fasting, while the percentage of non-esterified fatty acids increased. There were no significant differences in plasma ions (sodium, potassium, and calcium), except for chloride ion which decreased after 5 days of fasting. Liver glycogen decreased after 5 days of fasting while glycogen concentration in muscle did not decrease until 20 days of fasting. Liver color presented a higher chroma after 5 days of fasting, suggesting a mobilization of reserves. Finally, acetylcholinesterase activity in the brain was not affected by food deprivation but increased after 10 days of fasting in liver and muscle, suggesting the mobilization of body reserves, but without severely affecting basal metabolism.

Post-prandial changes in protein synthesis in red drum (Sciaenops ocellatus) larvae

Protein synthesis is one of the major energy-consuming processes in all living organisms. Post-prandial changes in protein synthesis have been studied in a range of animal taxa but have been little studied in fish larvae. Using the flooding-dose method, we measured post-prandial changes in whole-body rates of protein synthesis in regularly fed red drum Sciaenops ocellatus (Linnaeus) larvae for 24-28 h following their daily meal. Fractional rates of protein synthesis increased from a baseline (pre-feeding) rate of 16% day(-1) to a post-prandial peak of 48% day(-1) ca. 8 h after feeding before declining to 12% day(-1) after 24-28 h. The overall mean daily rate of protein synthesis was calculated as 27% day(-1). Although suggested as energetically impossible in larval poikilotherms, our results show that rates in excess of 30% day(-1) can be attained by larval fishes for a few hours but are not sustained. The average daily energetic cost of protein synthesis was estimated as 34% of daily total oxygen consumption, ranging from 19% immediately before feeding to 61% during the post-prandial peak in protein synthesis. This suggests that during the post-prandial peak, protein synthesis will require a large proportion of the hourly energy production, which, given the limited metabolic scope in fish larvae, may limit the energy that could otherwise be allocated to other energy-costly functions, such as foraging and escape responses.

Postprandial gastrointestinal blood flow, oxygen consumption and heart rate in rainbow trout (Oncorhynchus mykiss)

The present study is the first to simultaneously and continuously measure oxygen consumption (MO(2)) and gastrointestinal blood flow (q(gi)) in fish. In addition, while it is the first to compare the effects of three isoenergetic diets on q(gi) in fish, no significant differences among diets were found for postprandial MO(2), q(gi) or heart rate (f(H)) in rainbow trout, Oncorhynchus mykiss. Postprandial q(gi), f(H) and MO(2) were significantly elevated above baseline levels by 4 h. Postprandial q(gi) peaked at 136% above baseline after 11 h, f(H) peaked at 110% above baseline after 14 h and MO(2) peaked at 96% above baseline after 27 h. Moreover, postprandial MO(2) remained significantly elevated above baseline longer than q(gi) (for 41 h and 30 h, respectively), perhaps because most of the increase in MO(2) associated with feeding is due to protein handling, a process that continues following the absorption of nutrients which is thought to be the primary reason for the elevation of q(gi). In addition to the positive relationships found between postprandial MO(2) and q(gi) and between postprandial MO(2) and f(H), we discovered a novel relationship between postprandial q(gi) and f(H).

Protein metabolism in marine animals: the underlying mechanism of growth

Growth is a fundamental process within all marine organisms. In soft tissues, growth is primarily achieved by the synthesis and retention of proteins as protein growth. The protein pool (all the protein within the organism) is highly dynamic, with proteins constantly entering the pool via protein synthesis or being removed from the pool via protein degradation. Any net change in the size of the protein pool, positive or negative, is termed protein growth. The three inter-related processes of protein synthesis, degradation and growth are together termed protein metabolism. Measurement of protein metabolism is vital in helping us understand how biotic and abiotic factors affect growth and growth efficiency in marine animals. Recently, the developing fields of transcriptomics and proteomics have started to offer us a means of greatly increasing our knowledge of the underlying molecular control of protein metabolism. Transcriptomics may also allow us to detect subtle changes in gene expression associated with protein synthesis and degradation, which cannot be detected using classical methods. A large literature exists on protein metabolism in animals; however, this chapter concentrates on what we know of marine ectotherms; data from non-marine ectotherms and endotherms are only discussed when the data are of particular relevance. We first consider the techniques available to measure protein metabolism, their problems and what validation is required. Protein metabolism in marine organisms is highly sensitive to a wide variety of factors, including temperature, pollution, seasonality, nutrition, developmental stage, genetics, sexual maturation and moulting. We examine how these abiotic and biotic factors affect protein metabolism at the level of whole-animal (adult and larval), tissue and cellular protein metabolism. Available gene expression data, which help us understand the underlying control of protein metabolism, are also discussed. As protein metabolism appears to comprise a significant proportion of overall metabolic costs in marine organisms, accurate estimates of the energetic cost per unit of synthesised protein are important. Measured costs of protein metabolism are reviewed, and the very high variability in reported costs highlighted. Two major determinants of protein synthesis rates are the tissue concentration of RNA, often expressed as the RNA to protein ratio, and the RNA activity (k(RNA)). The effects of temperature, nutrition and developmental stage on RNA concentration and activity are considered. This chapter highlights our complete lack of knowledge of protein metabolism in many groups of marine organisms, and the fact we currently have only limited data for animals held under a narrow range of experimental conditions. The potential assistance that genomic methods may provide in increasing our understanding of protein metabolism is described.

Growth, oxygen consumption, and protein and RNA synthesis rates in the yolk sac larvae of the African catfish (Clarias gariepinus)

Rapidly growing African catfish yolk sac larvae were investigated during the first 22 h after hatching. Body compartment protein concentration increased fourfold yet oxygen consumption remained constant (mean=21.3 +/- 3.2 nmol O2 mg(-1) protein h(-1)), suggesting fast growth results mainly from yolk sac protein absorption. The protein synthesis rates at 1-2 and 5-6 h also equaled the highest conceivable rates of muscle protein synthesis; 11.6-11.9% and 7.4-7.9% day(-1), respectively. Therefore the corresponding energetic costs of protein synthesis were almost the theoretical minimum; 13.0 +/- 1.7-16.3 +/- 2.8 micromol O2 mg(-1) protein synthesised. Total protein synthesis expenditure (74.5-77.7 micromol O2 g(-1) protein h(-1)) was also less than other yolk sac larvae. These protein synthesis rates resulted from high RNA concentrations (113.2 +/- 3.4 microg RNA mg(-1) protein) and were also correlated with RNA translational efficiency. High translational efficiency (1 h; 1.2+/-0.1 mg protein synthesised microg(-1) RNA day(-1)) equaled high synthesis rate (36.8 +/- 5.4 microg RNA microg(-1) DNA day(-1)) and both declined over 22 h. This investigation suggests rapid growth combines growth efficiency and compensatory energy partitioning. This accommodates the ontogenetic and phylogenetic standpoints imposed by energy budget limitations.

Protein growth rate in rainbow trout (Oncorhynchus mykiss) is negatively correlated to liver 20S proteasome activity

The efficiency with which fish and other animals add and maintain body proteins is a balance between synthesis of proteins and their degradation. In fish that have similar food consumption and protein synthesis rates, a greater ratio of synthesis to degradation would be expected to produce more efficient conversion of food into growth. In addition, we hypothesised that high activities of the proteasome, a major pathway of protein degradation, would be negatively correlated with growth rate. In order to test this hypothesis we maintained rainbow trout for 62 days, during which repeat measurements of food consumption and growth were made. We selected fish for high and low growth efficiencies. Protein degradation was estimated from the difference between protein synthesis (determined by 15N flux) and protein growth. We found that protein synthesis rates were significantly higher in the low growth efficiency group, as were estimated protein degradation rates. In another group of fish that also did not differ in food consumption, the activity of the proteasome in the liver, but not in the muscle, was negatively correlated with growth rates. These two experiments showed that high proteasome activity is linked to decreased growth efficiency.

Influence of oxygen partial pressures on protein synthesis in feeding crabs

Many water-breathing animals have a strategy that consists of maintaining low blood PO2 values in a large range of water oxygenation level (4-40 kPa). This study examines the postprandial changes in O2 consumption, arterial blood PO2, and tissue protein synthesis in the shore crab Carcinus maenas in normoxic, O2-depleted, and O2-enriched waters to study the effects of this strategy on the O2 consumption and peptide bond formation after feeding. In normoxic water (21 kPa), the arterial PO2 was 1.1 kPa before feeding and 1.2 kPa 24 h later. In water with a PO2 of 3 kPa (arterial PO2 0.6 kPa), postprandial stimulation of protein synthesis and O2 consumption were blocked. The blockade was partial at a water PO2 of 4 kPa (arterial PO2 0.8 kPa). An increase in environmental PO2 (60 kPa, arterial PO2 10 kPa) resulted in an increase in protein synthesis compared with normoxic rates. It is concluded that the arterial PO2 spontaneously set in normoxic Carcinus limits the rates of protein synthesis. The rationale for such a strategy is discussed.

Selective changes in the protein-turnover rates and nature of growth induced in trout liver by long-term starvation followed by re-feeding

We report upon the effects of a cycle of long-term starvation followed by re-feeding on the liver-protein turnover rates and nature of protein growth in the rainbow trout (Oncorhynchus mykiss). We determined the protein-turnover rate and its relationship with the nucleic-acid concentrations in the livers of juvenile trout starved for 70 days and then re-fed for 9 days. During starvation the total hepatic-protein and RNA contents decreased significantly and the absolute protein-synthesis rate (A(S)) also fell, whilst the fractional protein-synthesis rate (K(S)) remained unchanged and the fractional protein-degradation rate (K(D)) increased significantly. Total DNA content, an indicator of hyperplasia, and the protein:DNA ratio, an indicator of hypertrophy, both fell considerably. After re-feeding for 9 days the protein-accumulation rates (K(G), A(G)) rose sharply, as did K(S), A(S), K(D)), protein-synthesis efficiency (K(RNA)) and the protein-synthesis rate/DNA unit (K(DNA)). The total hepatic protein and RNA contents increased but still remained below the control values. The protein:DNA and RNA:DNA ratios increased significantly compared to starved fish. These changes demonstrate the high response capacity of the protein-turnover rates in trout liver upon re-feeding after long-term starvation. Upon re-feeding hypertrophic growth increased considerably whilst hyperplasia remained at starvation levels.

Protein synthesis, growth and energetics in larval herring (Clupea harengus) at different feeding regimes

Rates of growth, protein synthesis and oxygen consumption were measured in herring larvae, Clupea harengus, in order to estimate the contribution that protein synthesis makes to oxygen consumption during rapid growth at 8°C. Protein synthesis rates were determined in larvae 9 to 17 d after hatching. Larvae were bathed in (3)H phenylalanine for several hours and the free pool and protein-bound phenylalanine specific radioactivities were determined.Fractional rates of protein synthesis increased 5 to 11 fold with feeding after a period of fasting. Efficiencies of retention of synthesized protein were approximately 50% during rapid growth. Rapid growth in herring larvae thus appears to be characterized by moderate levels of protein turnover similar to those obtained for larger fish. Increases in growth rate occurred without changes in RNA concentration, i.e., the larvae increased the efficiency of RNA rapidly. Oxygen consumption rates were not correlated with growth rates. Protein synthesis was estimated to account for 79% of the oxygen consumption, and energy costs of protein synthesis were high, i.e., about 98 mmole O2 g(-1) protein synthesized.

Effect of feeding on the tissue free amino acid concentrations in rainbow trout (Oncorhynchus mykiss Walbaum)

This study investigates whether tissue free amino acid (FAA) pools in rainbow trout,Oncorhynchus mykiss (Walbaum), are altered following feeding and the relationships between the amount of food consumed and the FAA pool size. Trout were starved for 7 days to provide baseline data and then refed on day 8. Individual food intake was measured by radiography and the consumption of amino acids (AA) calculated from dietary protein consumption. Total FAA concentrations in the stomach, liver and white muscle were little changed at various times after the meal and this pattern was repeated for the majority of individual FAA. Overall, the most notable change was a reduction in essential FAA concentrations (principally in valine, leucine and isoleucine) in the white muscle following feeding. However, in the caeca total FAA, total essential FAA and a number of individual FAA were significantly elevated at 4, 9 and 15h following feeding. There were few significant correlations between dietary amino acid consumption and total tissue FAA and essential FAA concentration in the stomach, caecum and white muscle; correlations were stronger in the liver. In order to explain the relative constancy of total FAA concentrations in the tissues following food intake (which represents over 100% of the total FAA pool) a model is presented that quantifies the AA flux through the free pools and considers the role of protein turnover in regulating FAA pool size.

Cost of growth in cells and organisms: general rules and comparative aspects

In a crude fashion it can be said that metabolizable energy (M) is partitioned into metabolic work, paid for by 'oxidations' (R), and 'assimilation', i.e. production (P), so that M = R+P. However, a fraction of R is required to meet the expenses of production and if these expenses represent, Joule for Joule, a constant proportion of the amount produced, then Rt = Rm+cP, where Rt = total metabolic expenditures, Rm = metabolic expenditures for maintaining the non-producing organism, and cP = Rp = metabolic expenditures connected with the processes of production. The partitioning of metabolizable energy into R and P as well as into Rm and Rp may vary depending on the phylogeny and life-history of the species concerned and on ecological circumstances. Thus selection is expected to act on both ratios, R/P and Rm/Rp. By comparing the ratios P/(P+Rp) (the apparent efficiency of production) and Rp/P (the apparent metabolic cost of production) in different types of organisms, one finds that a value of P/(P+Rp) = 0.75, equal to 75% efficiency, 10 mgdbm/mmol ATP, and 16 mumolO2/mg dbm (when I mg identical to 22 J), can be used as a 'consensus value' for the average efficiency, or cost, of the transformation of metabolizable energy into production in a wide range of organisms, from bacteria to mammals. This value corresponds to about three times the theoretical cost of synthesizing the same amount of tissue on the basis of known biochemical principles. The reasons why the empirical costs of production are higher than the theoretical costs of synthesis by what appears to be a common factor may be quite different in bacteria, small ectothermic and large endothermic organisms. Deviations from the consensus value may be due to differences in energy density of the nutrients assimilated and the tissues synthesized. Further complications arise because of interactions between P, Rp, and Rm. In microorganisms the existence of a constant and a variable component of maintenance metabolism has been postulated, the latter decreasing with increasing rate of production. In small ectothermic metazoans, on the other hand, the nonlinear relationship between growth metabolism and growth rate has led to the speculation that above a critical value of Pg certain energy consuming functions of maintenance are suppressed and the energy thus gained used for fuelling growth processes. There is some evidence that, at least in ectothermic metazoans, the apparent cost of growth decreases with the rate of growth, reaching a low plateau of about 10 mumolO2/mgdbm at growth rates exceeding about 8 mgdbm/g/h.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein-nitrogen flux and protein growth efficiency of individual Atlantic salmon (Salmo salar L.)

Protein-nitrogen flux (the proportions of consumed and absorbed protein-nitrogen partitioned into protein synthesis and growth) was examined in Atlantic salmon, Salmo salar L. Salmon were held in groups and fed high or low rations or starved. Individual food consumption rates were measured using radiography. Fish varied widely in protein growth efficiency (protein growth divided by protein consumption), but this did not correlate with consumption rate, digestive capacity (as measured by absorption efficiency, trypsin levels and pyloric caecal size) or feeding hierarchy rank. Protein synthesis rates, measured in whole-animals, were linearly correlated with protein consumption and assimilation. There was a significant correlation between protein growth efficiency and the efficiency of retention of synthesised proteins. The capacity for protein synthesis and RNA activity were positively correlated with rates of food consumption and growth but were not correlated with protein growth efficiency. It was concluded that individual differences in protein growth efficiency related to differences in synthesis retention efficiency, but not to differences in the capacity for protein synthesis, RNA activity, digestive capacity or feeding hierarchy rank.

Protein metabolism during sexual maturation in female Atlantic salmon (Salmo salar L)

Body composition and fractional rates of protein synthesis (percentage of the protein mass synthesized per day) were determined in female Atlantic salmon returning to the River Tay, Scotland in July and in October after a 95 day period without food, during which time the animals became sexually mature. During the 95 day period of starvation/sexual maturation the ventricle and red muscle remained as a constant proportion of fresh weight whereas the liver, gill and ovary increased and the stomach and white muscle decreased. Fractional rates of protein synthesis increased markedly in the liver, stomach and ovary during the period of starvation/sexual maturation. In the gill, ventricle and white muscle fractional protein synthesis rates increased slightly or remained constant. From the estimated rates of protein loss or gain in the various tissues it is concluded that there is considerable protein turnover and repartitioning of amino acids during the period of starvation and sexual maturation. The absolute rate of protein synthesis rates in the ovary indicates that this tissue made the largest contribution to the energy and amino acid demands of the fish, whilst most of the amino acids required for maturation of the ovary were derived from white muscle, principally as the result of increased muscle protein degradation.