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The effects of dietary inclusions of size-fractionated peptides and free amino acids (FAAs) on Peptide Transporter 1 (PepT1) mRNA levels were assessed along the length of the intestine of juvenile Atlantic cod (Gadus morhua). Five groups of fish (10-15g) were fed for 46days on diets containing approximately 42% protein, provided either as fish meal (FM, control diet) or as a combination of FM with whole fish hydrolysate (FH), retenate after ultrafiltration of FH (UFR), nanofiltered retenate of FH (NFR), or a mix of FAAs, at a 30% level of FM substitution. PepT1 mRNA expression was assessed in pyloric caeca (S1) and the remainder of the intestine divided into four equally long segments (S2-S5). PepT1 transcripts were found in all segments, indicating that the whole intestine is involved in peptide absorption. Differences in the regional expression profile of PepT1 were found. Under control diet (FM diet) conditions, fish exhibited a reduced expression in S5 compared to S2. In fish fed FAA and UFR diets, PepT1 mRNA levels were higher in S2 and S3 compared to other regions. These data suggest that PepT1 may be variably recruited along the whole intestine, including the most distal part, in response to changes in the luminal protein source content. This adaptive response might be functional to keep a maximal efficiency of protein absorption at the intestinal level., (2010 Elsevier Inc. All rights reserved.)

We previously reported that juvenile Atlantic salmon with mean initial BW 11.5 g offed a methionine deficient diet had lower weight gain due to a reduced protein accretion, while lipid gain was unaffected. Muscle of the fish fed the methionine deficient diet was depleted for sulphur amino acids, while in liver, the concentration of these metabolites was maintained within narrow limits. We speculated whether this could be due to an increased muscle proteolysis to support a prioritized liver metabolism in fish fed the low methionine diets. In this study, we assessed whether genes associated with muscle proteolysis increased under methionine deficiency. The composition of the diets was similar to those used previously containing 1.6 or 2.1 g Met/16 g N. We confirmed that the fish fed the low methionine diet gained less protein compared to fish fed the DLmethionine enriched diet (P = 0.014), but growth did not reduce significantly. Also the deficient fish maintained the concentrations of liver sulphur amino acids and reduced muscle free methionine. Several of the other free amino acids within muscle increased. Further, methylation capacity was maintained in liver but reduced in the muscle (P = 0.78 and 0.04, respectively). Gene expression of muscle IGF-1 was lower (P = 0.008) and myosin light chain 2 tended (MLC2, P = 0.06) to be reduced in fish fed low methionine diet, concurrently the activity of cathepsins B+L increased (P = 0.047) in muscle of fish fed the low methionine diet. Gene expression of the muscle-specific E3 ubiquitine ligases (Murf and MaFbx) was not affected by treatment. Thus, the lower protein gain observed in fish fed the low methionine diet may be caused by reduced protein synthesis in line with the reduced IGF-1 gene expression in the white trunk muscle. Thus, to support metabolism, the dietary protein needs to be balanced in amino acids to support metabolism in all compartments of the body and secure maximal protein gain. [ABSTRACT FROM AUTHOR]

The appetite-stimulating effect of krill hydrolysate ( KH), free amino acids and specific water-soluble low molecular N-compounds was explored in a short feeding trial with Atlantic salmon. A 100 g kg−1 fish meal positive ( MFM), a 30 g kg−1 fish meal negative control ( LFM) and six more LFM diets were produced added known attractants for fish- KH at two levels: low ( LAK) or high ( HAK), AMP, choline chloride ( CC); an amino acid mix resembling the free amino acid composition of KH ( AA) or a mix of AMP, CC and AA ( ACA). HAK, ACA and AA showed highest feed intake and significantly higher plasma phospholipids and cholesterol. The lower performing treatments ( LFM, AMP) showed higher liver lipids and hepatosomatic index. CC induced reduced liver lipids and increased plasma phospholipids and cholesterol. Appetite regulating neuropeptide gene expression analysis (q PCR) was performed in fish fed LFM, HAK and AA. Pyy showed the highest postprandial expression in LFM, whereas the expression of the anorexigenic neuropeptides cart, pomca1, pomca2 and pomca2s was low. These apparently contradictory results may be explained by initial appetite stimulation by HAK and AA, resulting in higher feed intake, followed by satiation and appetite downregulation. [ABSTRACT FROM AUTHOR]

Abstract: For the majority of marine fish larvae a fully developed digestive tract, including gastric digestion, is acquired weeks to months (depending on species) after onset of exogenous feeding. Still, the processing capacity (capability to degrade and absorb dietary nutrients) of the larval gut is sufficient to support fast larval growth by digesting prey naturally available in the sea. However, the physiological constraints of the gut with respect to digestion of cultivated live prey and particularly formulated starter feeds still remain to be elucidated. This paper reviews some recent findings in the areas of control and efficiency of digestive function of marine fish larvae. For studies on the hormonal control, the peptide hormone cholecystokinin (CCK) has been targeted, since it is believed to play an important role in controlling digestion in vertebrates. Recent work on the processing capacity include studies of the digestibility and transfer kinetics of macronutrients from live prey and experimental work on absorption of protein, peptides and free amino acids from the lumen of the digestive tract into the tissues of larval fish and how this changes during ontogeny. The molecular and in vitro characterization of transporters is currently being integrated with ongoing in vivo studies. [Copyright &y& Elsevier]

Ontogenetic changes in the capacity of Senegalese sole (Solea senegalensis Kaup, 1858) larvae to digest and metabolise Artemia protein and amino acids (AA) were studied using 12, 22 and 35 days after hatching (DAH) larvae that were fed Artemia metanauplii radiolabelled with a [U-14C] protein hydrolysate. About 82% and 18% of the label was incorporated into the Artemia trichloroacetic acid (TCA) precipitate (mostly protein) and soluble (mainly free AA) fractions, respectively. The digestibility of Artemia was high at all tested ages, with label absorption varying between 77% and 83% at 24 h after feeding (HAF). A rapid digestion, absorption and catabolism of Artemia AA were noted, with most of the absorption into the body occurring during the first 3 HAF. Traces of label were already found in the metabolic-CO2 trap at 1 HAF. Furthermore, label was largely and almost immediately incorporated into the TCA precipitate fraction (mostly protein) of gut and body tissues. Slight differences were noted in diet utilization between larvae at different ages. At 12 DAH larvae had a lower catabolism and evacuation of the label, as well as the highest accumulation in the gut. However, except for the amount of catabolised label, the results were not significantly different from those for 35 DAH larvae. Moreover, no significant differences were found in the amount of label incorporated into the body, although it seemed to be higher in 12 DAH larvae. Taken together, these results reveal a higher absorption and a significantly higher retention of the absorbed label in 12 DAH larvae. In addition, 12 DAH larvae appeared to have a slower absorption of the label, which, in continuously feeding larvae, might result in overall lower food absorption efficiency. Therefore, it seems that young larvae have the ability to compensate for a possible lower digestive capacity with higher body retention of absorbed AA. This study confirms that sole larvae, even young stages, have a high capacity for digesting live preys. [ABSTRACT FROM AUTHOR]

In marine fish larvae, the sum of protein deposition, turnover and catabolism necessary for their rapid growth dictates a high amino acid (AA) requirement. Once the yolk is exhausted, the digestive tract becomes the vital organ that ensures a steady supply of dietary AA to the growing larval tissues. In this paper, we discuss the demand and availability of AA (free and polymerised pools) in relation to larval digestive capacity. The sources of AA from compound and live diets are described, and the early regulatory roles of cholecystokinin (CCK) and a retrograde peristaltic activity are highlighted. [Copyright &y& Elsevier]

With the aim of improving the formulation of diets for early stages of marine fish, post-larval Hippoglossus hippoglossus (56 days after first feeding; 23 mg dry weight) were tube-fed liquid diets of radiolabelled L-(14C) free amino acids (FAA diet), peptides (PEPT diet) and protein (PROT diet), in order to compare the kinetics of transfer from gut into the liver and carcass. The test diets were prepared using equal concentrations of amino acids (AA) and fed as a single pulse (0.3 µl, 2.2 µg AA). The PROT diet consisted of methylated-14C-labelled (mainly lysine) bovine serum albumin (BSA), the PEPT diet was the PROT diet digested with trypsin and the FAA diet consisted of an amino acid mixture based on BSA composition with added 14C-lysine. The common labelling with 14C-lysine permitted a comparison of the three diets. Based on dissection and distribution of the tracer in each compartment, the FAA diet was almost completely absorbed 1 h after the tube-feeding; PEPT and PROT diets required more time before they were processed by the digestive tract. The FAA diet was absorbed about eight times as fast as protein and about six times as fast as peptides from the gut. A low retention of label in the liver for all diets at all sampling times suggests that the liver does not store the AA at these absorption rates. The FAA diet had a higher assimilation efficiency (about 90%) than the PEPT diet (about 12%) and PROT diet (about 32%). FAAs, thus, appear to be superior to protein and peptides as a dietary source of amino acids in post-larval Atlantic halibut. [ABSTRACT FROM AUTHOR]

Free amino acids (FAA) and protein have been measured in whole laboratory-readed halibut larvae and on dissected individuals separated into yolk and body compartments. At hatching both FAA and protein are mainly located in the yolk compartment. During the first 12 d of the yolk-sac stage more than 70% of the FAA pool disappeared from the yolk without any significant changes in the yolk protein pool. This suggests different uptake mechanisms for FAA and protein from the yolk, and a sequential utilisation of the endogeneous reservoirs of free and protein amino acids in Atlantic halibut larvae. The data suggest that in the early yolk-sac stage FAA enter the embryo from the yolk and are utilised both for energy and protein synthesis. Later on when the free pool cannot fulfil the nutritional requirements, additional amino acids are recruited from yolk protein. Of the total amino acids (free + protein amino acids) present at hatching ca. 60% will be used as precursors for body protein synthesis while the remaining 40% are used as fuel in the larval energy metabolism. [ABSTRACT FROM AUTHOR]

Comments on the article "Assimilation of Dietary Free Amino Acids, Peptides and Protein in Post-Larval Atlantic Halibut," published in previous issue. Study conducted on methylated bovine serum albumin (BSA) using the protein and peptide test solutions. Demonstration of the study in vitro hydrolysis rates of methylated BSA by two mammalian cytosolic proteases; Verification of the results for assimilation of protein and peptides with the use of non-methylated compounds.

Leucine has been identified to modulate feed intake and energy homeostasis in fish as in other vertebrates. Under allostatic conditions, energy expenditure may change, and adjustments to the processes that govern the energy homeostatic system may be necessary. We investigated the responsiveness of appetite-related neuropeptides involved in feed intake regulation in Atlantic salmon (Salmo salar) reared with high (35 g/kg leucine) or control (27.3 g/kg leucine) leucine-supplemented diets and/or under chronic stressor conditions (chasing) for eight weeks. We also analysed the response of amino acid transporters potentially involved in uptake of branched-chain amino acids (BCAA), including leucine, into areas of the brain where nutrient sensors may signal locally or to other areas involved in appetite control. At the end of the experiment, all fish were subjected to a novel-acute stressor (confinement). Our results show that fish fed with high leucine diet had a lower feed intake, growth, and hepatosomatic index (HSI) when compared to fish fed control leucine diet. In addition, increased mRNA expression of amino acid solute carrier (slc) genes in the diencephalon, and genes related to appetite control, such as proopiomelanocortin a1 (pomca1), in both the diencephalon and telencephalon, imply their involvement in leucine anorectic effect. Stress, as high leucine, reduced feed intake, growth and HSI of fish fed control or high leucine diet and antagonized the high leucine effect on the slc genes mRNA expression. An increase of neuropeptide y a1 (npya1) was observed both due to high dietary leucine and/or stress treatment which may represent a compensatory regulatory mechanism with the aim to reverse the decrease in feed intake. In summary, our results confirm an anorectic role of high dietary leucine via the activation of amino acid sensing mechanisms in the brain. Further, corticotropin-releasing hormone 1 b1 (crh1b1) and npya1 showed to play a role in the regulation of appetite in Atlantic salmon under stress conditions and/or high leucine levels. • High dietary leucine acts as anorectic factor in the control of energy balance in Atlantic salmon. • Both slc transporters and pomca1 neuropeptide are involved in the leucine anorexigenic control. • The same mechanisms do not seem to be responsible for such disruption under stressed conditions. • Instead, crf1b1 and npya1 play a role in the regulation of appetite under stressful conditions. [ABSTRACT FROM AUTHOR]