Chapter 9 Nutrient fluxes and regulation in fish intestine

The role of environmental salinity on Na+-dependent intestinal amino acid uptake in rainbow trout (Oncorhynchus mykiss)

Na⁺/K⁺-ATPases (NKA) in the basolateral membrane of the intestinal enterocytes create a Na⁺-gradient that drives both ion-coupled fluid uptake and nutrient transport. Being dependent on the same gradient as well as on the environmental salinity, these processes have the potential to affect each other. In salmonids, L-lysine absorption has been shown to be higher in freshwater (FW) than in seawater (SW) acclimated fish. Using electrophysiology (Ussing chamber technique), the aim was to explore if the decrease in L-lysine transport was due to allocation of the Na⁺-gradient towards ion-driven fluid uptake in SW, at the cost of amino acid transport. Intestinal NKA activity was higher in SW compared to FW fish. Exposure to ouabain, an inhibitor of NKA, decreased L-lysine transport. However, exposure to bumetanide and hydrochlorothiazide, inhibitors of Na⁺, K⁺, 2Cl^--co-transporter (NKCC) and Na⁺, Cl^--co-transporter (NCC) respectively, did not affect the rate of intestinal L-lysine transport. In conclusion, L-lysine transport is Na⁺-dependent in rainbow trout and the NKA activity and thus the available Na⁺-gradient increases after SW acclimation. This increased Na⁺-gradient is most likely directed towards osmoregulation, as amino acid transport is not compromised in SW acclimated fish.

Absorption of protein in teleosts: a review

Teleost is a widely diverse group of fishes and so do their feeding habits. From aquaculture points of view, there have been un-interrupted efforts to optimize feeding rates with protein as the chief ingredients in the supplementary diet. However, knowledge on its protein absorption is incomplete so far, to acquire absolute feeding design to mobilize enhanced production of animal-source protein as fish biomass. In this review, the variable protein absorption across digestive tract (DT) in this group of fish has been highlighted. Emphasis is given to outline how DT components, like enterocyte specific absorptive mechanisms, are different in anterior and posterior regions of DT or from the absorptive transporter system. The existence of a transporter-based absorption mechanism brings more variability in the protein absorption in teleosts. At least two such transport systems (Na⁺-dependent and Na⁺-independent) with within-system differences impart more variability to protein absorption. Further, shifting from one stage to another stage of development involves considerable modification of the protein absorptive mechanism in teleosts. Gut microbes may also indirectly facilitate protein absorption in teleosts. Overall, the present review projects a comprehensive understanding of the protein absorption in teleosts that will help to strategize the modulation of feeding technology in fish culture.

An in vitro study of urea and ammonia production and transport by the intestinal tract of fed and fasted rainbow trout: responses to luminal glutamine and ammonia loading

Digestion of dietary protein in teleosts results in high ammonia levels within the intestinal chyme that may reach concentrations that are many-fold greater than blood plasma levels. We used in vitro gut sac preparations of the ammoniotelic rainbow trout (Oncorhynchus mykiss) to investigate the role of the intestine in producing and transporting ammonia and urea, with specific focus on feeding versus fasting, and on responses to loading of the lumen with 2 mmol L^-1 glutamine or 2 mmol L^-1 ammonia. Feeding increased not only ammonia production and both mucosal and serosal fluxes, but also increased urea production and serosal fluxes. Elevated urea production was accompanied by an increase in arginase activity but minimal CPS III activity, suggesting that urea may be produced by direct arginolysis. The ammonia production and serosal fluxes increased in fasted preparations with glutamine loading, indicating an ability of the intestinal tissue to deaminate glutamine and perhaps use it as an energy source. However, there was little evidence of urea production or transport resulting from the presence of glutamine. Furthermore, the intestinal tissues did not appear to convert surplus ammonia to urea as a detoxification mechanism, as urea production and serosal flux rates decreased in fed preparations, with minimal changes in fasted preparations. Nevertheless, there was indirect evidence of detoxification by another pathway, as ammonia production rate decreased with ammonia loading in fed preparations. Overall, our study suggests that intestinal tissues of rainbow trout have the ability to produce urea and detoxify ammonia, likely via arginolysis.

Growth hormone regulates intestinal gene expression of nutrient transporters in tilapia (Oreochromis mossambicus)

Among the various ways that growth hormone (GH) underlies the growth physiology of teleost fishes, GH stimulates transport pathways that facilitate the absorption of nutrients across intestinal epithelia. The current study investigated the effects of GH on the gene expression of nutrient transporters in an omnivorous teleost, the Mozambique tilapia (Oreochromis mossambicus). We employed pituitary gland removal (hypophysectomy) and hormone replacement to assess whether GH directs the gene expression of the GH receptor (ghr2), the peptide transporters, pept1a, pept1b and pept2, the amino acid transporter, slc7a9, the Na+/glucose cotransporter, sglt1, the glucose transporter, glut2, and the myo-inositol transporter, smit2, in anterior, middle, and posterior intestine. ghr2 was predominantly expressed in posterior intestine, while pept1a, pept1b, slc7a9, sglt1, glut2, and smit2 exhibited the highest mRNA levels in anterior and/or middle intestine. While hypophysectomized tilapia exhibited diminished expression of ghr2, pept1a, pept1b, slc7a9, and glut2 compared with intact and sham-operated controls, only ghr2, pept1a, pept1b and glut2 levels were restored by GH replacement. Our findings indicate that GH supports growth, at least in part, by stimulating the gene expression of its cognate receptor and key nutrient transporters in the intestine.

Selection for high muscle fat in rainbow trout induces potentially higher chylomicron synthesis and PUFA biosynthesis in the intestine

Two lines of rainbow trout divergently selected for muscle fat content, fat line (F) and lean line (L) were used to investigate the effect of genetic selection on digestion, intestinal nutrient transport and fatty acid bioconversion, in relation to dietary starch intake. This study involved a digestibility trial for 2 weeks using Cr(2)O(3) as inert marker, followed by a feeding trial for 4 weeks. For the entire duration, juvenile trout from the two lines were fed diets with or without gelatinized starch. Blood, pyloric ceca, midgut and hindgut were sampled at 24 h after the last meal. Transcripts of the proteins involved in nutrient transport and fatty acid bioconversion were abundant in the proximal intestine. GLUT2 transcripts were slightly higher in the F line ceca than in the L line. Dietary starch intake did not enhance the transcription of intestinal glucose transporters, SGLT1 and GLUT2; but it was associated with the higher expression of ApoA1 and PepT1 in the midgut. Significantly, the F line exhibited higher intestinal mRNA levels of MTP, ApoA4, Elovl2, Elovl5 and D6D than the L line, linked to chylomicron assembly and fatty acid bioconversion. Apparent digestibility coefficients of protein, lipid and starch were high in both lines, but not significantly different between them. In conclusion, we found a higher potential of chylomicron synthesis and fatty acid bioconversion in the intestine of F line, but no adaptive transcriptional response of glucose transporters to dietary starch and no genotypic differences in nutrient digestibility.

Transcriptomic and physiological responses to fishmeal substitution with plant proteins in formulated feed in farmed Atlantic salmon (Salmo salar)

BACKGROUND

Aquaculture of piscivorous fish is in continual expansion resulting in a global requirement to reduce the dependence on wild caught fish for generation of fishmeal and fish oil. Plant proteins represent a suitable protein alternative to fish meal and are increasingly being used in fish feed. In this study, we examined the transcriptional response of Atlantic salmon (Salmo salar) to a high marine protein (MP) or low fishmeal, higher plant protein replacement diet (PP), formulated to the same nutritional specification within previously determined acceptable maximum levels of individual plant feed materials.

RESULTS

After 77 days of feeding the fish in both groups doubled in weight, however neither growth performance, feed efficiency, condition factor nor organ indices were significantly different. Assessment of histopathological changes in the heart, intestine or liver did not reveal any negative effects of the PP diet. Transcriptomic analysis was performed in mid intestine, liver and skeletal muscle, using an Atlantic salmon oligonucleotide microarray (Salar_2, Agilent 4x44K). The dietary comparison revealed large alteration in gene expression in all the tissues studied between fish on the two diets. Gene ontology analysis showed, in the mid intestine of fish fed PP, higher expression of genes involved in enteritis, protein and energy metabolism, mitochondrial activity/kinases and transport, and a lower expression of genes involved in cell proliferation and apoptosis compared to fish fed MP. The liver of fish fed PP showed a lower expression of immune response genes but a higher expression of cell proliferation and apoptosis processes that may lead to cell reorganization in this tissue. The skeletal muscle of fish fed PP vs MP was characterized by a suppression of processes including immune response, energy and protein metabolism, cell proliferation and apoptosis which may reflect a more energy efficient tissue.

CONCLUSIONS

The PP diet resulted in significant effects on transcription in all the 3 tissues studied. Despite of these alterations, we demonstrated that high level of plant derived proteins in a salmon diet allowed fish to grow with equal efficiency as those on a high marine protein diet, and with no difference in biometric quality parameters.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

Effects of dietary methylmercury on juvenile Sacramento blackfish bioenergetics

Although much is known about the biogeochemical cycling of mercury in the environment, relatively little is known about methylmercury (MeHg) bioaccumulation in fishes and how chronic sub-lethal exposures affect their functioning. Several species of fish in Clear Lake, California have high MeHg tissue levels, including Sacramento blackfish, Orthodon microlepidotus, a large native cyprinid that is fished commercially. We fed juvenile blackfish one of four diets containing MeHg (0.21 mg/kg control; 0.52 mg/kg low; 22.2 mg/kg medium; and 55.5 mg/kg high treatments) for 70 days. There were no statistical differences (P > 0.05) in food consumption among the treatment groups. By 35 days the high treatment group had a significantly depressed growth rate when compared to the control group (P < 0.05) and by 70 days both the medium and the high groups had significantly lower growth rates (P < 0.05). The high-dose group had a significantly (P < 0.05) lower specific growth rate (SGR) compared all other treatment groups at 35 days, although by 70 days these differences were not significant. The wet/dry muscle mass and muscle mass/total mass ratios, condition factor, and resting routine metabolic rates at both 35 and 70 days were statistically indistinguishable (P > 0.05) between treatment groups. All treatment groups assimilated the dietary MeHg into muscle tissue in a dose-dependent fashion. Percent assimilation was significantly lower (P < 0.05) in the high-dose group compared to the low-dose group at 35 days, (control 53%, low-dose 61%, medium-dose 50%, and high-dose 40%) but at 70 days assimilation was lower (35, 43, 42, and 32%, respectively) and statistically indistinguishable (P > 0.05) among the treatment groups. Dietary MeHg concentrations and bioaccumulation rates were correlated (r2 = 0.98 at 35 days, 0.99 at 70 days). These results may contribute to construction of ecosystem mercury models and more informed natural resources management at Clear Lake.

Effects of soybean meal and salinity on intestinal transport of nutrients in Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss)

Groups of fresh- and seawater-adapted Atlantic salmon (Salmo salar L.) and rainbow trout (Oncorhynchus mykiss) were fed diets with (SBM diet) or without (control diet) extracted soybean meal (30% of protein substituted with SBM) for 3 weeks. Average fish size per group ranged from 597 to 1763 g. One tank or net pen per species, dietary group and water salinity was used. In vitro nutrient transport (D-glucose, the L-amino acids aspartate, lysine, methionine, phenylalanine and proline, and the dipeptide glycyl-sarcosine) was measured using intact tissue (everted sleeve method) from the different postgastric intestinal regions. The dimensions of the different intestinal regions were also measured for each treatment group. Results indicate that SBM causes decreased carrier-mediated transport and increased permeability of distal intestinal epithelium for the nutrients, and the capacity of this region to absorb nutrient was diminished. Salinity may also affect the relative contribution of carrier-mediated and independent uptake to total nutrient absorption.

Intestinal absorption of water-soluble vitamins in rainbow trout (Onchorhynchus mykiss)

The intestinal uptake of water-soluble vitamins, nicotinamide, riboflavin, biotin and folic acid, was studied in isolated everted intestinal sleeves of the cold-water teleost rainbow trout (Onchorhynchus mykiss). The presence of a carrier-mediated transport mechanism was determined by competitive inhibition and by Michaelis-Menten kinetics. The uptake of riboflavin, biotin or folic acid was not only subject to competitive inhibition but also a saturable function of increasing vitamin concentration in the incubation medium. The kinetic constants of the saturable mechanism were for riboflavin: K(m), 2.32 +/- 0.76 microM; Vmax, 0.26 +/- 0.04 pmol/mg min; for biotin: K(m), 9.70 +/- 3.76 microM; Vmax, 0.31 +/- 0.07 pmol/mg min; and for folic acid: K(m), 32.9 +/- 21.2 microM; Vmax, 3.63 +/- 0.99 pmol/mg min. In contrast, the uptake of nicotinamide was not subject to competitive inhibition and was a linear function of concentration (Kd, 0.140 +/- 0.012 pmol/mg min microM). Folic acid was absorbed more rapidly than and was not inhibited by its derivative, 5-methyl-tetrahydrofolate. Thus, the intestinal uptake of riboflavin, biotin and folic acid is carrier-mediated while that of nicotinamide occurs by simple diffusion. These mechanisms are similar to those found in the channel catfish for the same vitamins, except for folic acid, which is absorbed by diffusion in this warm-water omnivorous species.