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Centre Especial de Recerca en Nutrició i Ciència dels Aliments,
* Departament de Fisiologia-Divisió IV and
Departament de Nutrició i Bromatologia-CeRTA, Universitat de Barcelona, Barcelona, Spain
5To whom correspondence should be addressed. E-mail: mmoreto{at}farmacia.far.ub.es.
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ABSTRACT |
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 TOP ABSTRACT MATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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KEY WORDS: • jejunum • glucose • lysine • methionine • fatty acids • chickens
The intestine of the chicken is capable of high rates of nutrient absorption. This is of special importance for commercial lines of Gallus gallus selected for rapid growth and performance because they must cope with extraordinary energetic and anabolic demands. Thus, the intestine of Leghorn chickens has a maximal D-glucose uptake capacity of 49 µmol/(d · g) (1 ), but this figure can be tripled in young broiler birds (2 ). The capacity to absorb nutrients depends mainly on the development of the mucosal surface area, the passive permeability properties of the epithelium and the functional properties of the specific nutrient transporters present in the brush border and the basolateral membranes.
Throughout life, the intestinal mucosa undergoes changes in nutrient absorption that are genetically programmed; in addition, this tissue has a remarkable capacity to adapt its structure and function in response to dietary composition (3 ). For example, dietary supplementation of chickens with L-methionine downregulates specific transporters of L-methionine uptake in the small intestine (4 ). Although several studies have examined the effects of fat supplementation on intestinal lipid composition and function in rats, no such studies have been done in chickens. Because enterocytes can carry out a significant phospholipid synthesis (5 ), dietary fatty acids will eventually be incorporated into the membrane and a steady-state fatty acid profile will be rapidly attained (6 ). Our first objective was to study how dietary supplementation with fats of different composition [enriched with saturated, (n-3) or (n-6) fatty acids] affects the fatty acid composition of the brush border of jejunal enterocytes of broiler chickens.
Dietary lipids can also modify the functions of enzymes and nutrient transporters present in the mucosal membrane. In the rat intestine, the degree of dietary fat saturation affects the lipid composition and fluidity of brush border membranes (7 ,8 ) and this can modify the transport and diffusion of certain nutrients across the intestine (9 ). In rats, the effects of dietary lipids on the intestinal capacity to absorb nutrients can be either unspecific, due to the changes in the physicochemical properties of the membrane (7 ), or specific, modulating the transcription of sodium glucose cotransporter 1 (SGLT1) mRNA (10 ). Studies in chickens fed commercial diets showed that the brush border membrane fluidity and fatty acid composition of the jejunum change during development (11 ), and that there are also regional differences in the small intestine (12 ). However, these differences in lipid composition do not correlate well with the observed changes in apical D-glucose transport (11 ,12 ). Because our diets presumably induced significant changes in membrane lipid composition, we examined whether this would also affect the functional properties of the brush border membrane, as described for rats.
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MATERIALS AND METHODS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Male Ross chickens (Gallus gallus domesticus L.) supplied by Petaluma (Caldes de Montbui, Spain) consumed a standard diet (CTL diet, Table 1 ) ad libitum during the first 2 wk of life and then were randomly divided into four groups and fed either the same diet or a diet enriched with fats of different composition (Table 1) for two additional weeks, i.e., the LAR diet, supplemented with 60 g/kg lard [rich in saturated fatty acids (SFA)]; the LSO diet, supplemented with 60 g/kg linseed oil [rich in (n-3) PUFA]; and the SFO diet, supplemented with 60 g/kg sunflower oil [rich in (n-6) PUFA]. Food consumption and body weight were measured twice a week throughout the experiment. The calculated metabolizable energy was 11.3 MJ/kg for the control diet and 12.7 MJ/kg for the diets supplemented with 60 g/kg fat. The formulation and preparation of diets was performed by the Institut de Recerca i Tecnologia Agroalimentàries Mas Bové (Reus, Spain).
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Isolation of BBMV.
Studies on fatty acid composition and nutrient transport were carried out on BBMV. The vesicles were prepared using the Mg2+ precipitation method (13
) as previously described (14
). The composition of the intravesicular medium was as follows: 300 mmol/L mannitol, 0.1 mmol/L MgSO4, 20 mmol/L N-2-hydroxyethyl-piperazine-N’-2-ethanesulfonic acid (Hepes)- tris(hydroxy-methyl) amino-methane (Tris) (pH 7.4) and 4.1 mmol/L LiN3 as antimicrobial agent. Vesicles were diluted to a final protein concentration of 20–30 g/L, and then frozen and stored in liquid nitrogen in 150-µL aliquots. Each isolation batch corresponds to the jejunum of one chicken and in the Results section, "n" indicates the number of chickens or membrane preparations. The purity of the membrane preparations was routinely assayed by measuring the activity of brush-border 
-D-glucohydrolase (sucrase; EC 3.2.1.48) by the method of Dahlquist (15
), and basolateral Na+-K+-adenosinetriphosphatase (Na+-K+-ATPase; EC 3.6.1.3) activity according to Del Castillo and Robinson (16
). Alkaline phosphatase (EC 3.1.3.1) activity was determined using p-nitrophenyl phosphate as substrate (17
). Vesicle orientation was checked by the method of Del Castillo and Robinson (16
). Protein content was determined using the BioRad (Hercules, CA) protein assay, with bovine serum albumin as standard.
Fatty acid analyses.
Total lipids were extracted from the brush border membrane following the procedure proposed by Folch et al. (18
). Vesicles were homogenized in ice-cold chloroform:methanol (2:1 v/v) containing 0.9 mmol/L BHT as antioxidant, using a Polytron model PT-2000 (Kinematica, Luzern, Switzerland). Fatty acid from lipid samples were derivatized as methyl esters (FAME) according to Slover and Lanza (19
). Samples containing 
100 mg of lipid were saponified with 2 mL of sodium methoxide in methanol (0.5 mol/L) at 100°C in a water bath for 10 min; the solution was cooled to room temperature and 2 mL of 1.78 mol/L of boron trifluoride in methanol was added. The solution was heated for a further 10 min in a boiling-water bath. After cooling, 1 mL of hexane and 2 mL of a 103 mmol/L NaCl were added and the mixture vigorously shaken. The upper layer was transferred to screw-capped test tubes using a Pasteur pipette. The organic FAME solution was dried with anhydrous sodium sulfate and finally concentrated under a stream of nitrogen.
FAME were analyzed by gas chromatography (GC) with a flame ionization detector. The sample volume (0.5–1 µL) was injected into a Hewlett-Packard 5890 series II Gas Chromatograph (Little Falls, Wilmington, DE) fitted with a 60-m SP 2380 capillary column (Supelco, Bellefonte, PA) coated with methyl silicone (0.25 mm i.d. x 0.2 µm film thickness). The oven temperature was at 177°C for 11.3 min, and then raised to 235°C at a rate of 7°C/min. The final oven temperature was held for 10 additional min. Injector and detector temperatures were 250 and 300°C, respectively. Helium (Carburos Metálicos, Barcelona, Spain) was used as carrier gas at a pressure of 30 kPa and the split ratio was set at 1:30. FAME were identified by comparison of their retention time and equivalent chain length with standard FAME (20 ) and by their mass spectrum obtained by GC-mass spectrometry (MS). FAME samples were quantified according to their percentage area, obtained by integration of the peak as a semiquantitative method.
The GC-MS of FAME was performed by injecting 2 µL samples into the same gas chromatograph as before, but now connected to a Hewlett-Packard 5989A Mass Spectrometer. The GC was equipped with the same capillary column as used before. The oven temperature was programmed as follows: 177°C for 11.3 min, then raised to 235°C at a rate of 2°C/min, and the temperature maintained for 15 additional min. Helium was used as carrier gas at a flow rate of 1 mL/min. FAME were detected using electron impact ionization with an ion source temperature of 200°C. The same extractive and analytical procedures were applied to the study of fatty acid composition of diets.
The saturation index was calculated as 
a/ (b · c), where a is the percentage number of saturated residues, b is the percentage number of each type of unsaturated residue and c is the number of double bonds in the residue (21
).
Nutrient uptake.
Experiments on nutrient transport were carried out at 37°C for incubation periods ranging from 1 s to 30 min, using the rapid filtration technique previously described (14 ). The vesicles were incubated under isotonic conditions (320 mOsm/kg) in an incubation medium containing 100 mmol/L mannitol, 0.2 mmol/L MgSO4, 4.1 mmol/L LiN3, 20 mmol/L Hepes-Tris (pH 7.4), 100 mmol/L NaSCN or KSCN, and the appropriate amount of labeled and unlabeled D-glucose, L-methionine or L-lysine. Incubation was terminated by the addition of 2 mL ice-cold stop-solution containing 150 mmol/L KSCN, 4.1 mmol/L LiN3 and 20 mmol/L Hepes-Tris (pH 7.4). Samples were rapidly filtered under negative pressure through 0.22-µm cellulose acetate/nitrate filters (Millipore GSWP02500, Bedford, MA) prewetted with chilled buffer, then rinsed four times with 2 mL ice-cold stop-solution and dissolved in Biogreen-6 cocktail (Scharlau, Barcelona, Spain). Radioactivity was determined by liquid scintillation counting (Packard Tri-Carb 1500, Wellesley, MA). Nonspecific tracer fixation to the filters was obtained by adding the stop-solution immediately before addition of the vesicles. Experiments were performed with at least four different membrane preparations, each in triplicate.
Kinetic studies.
The kinetic characterization of D-glucose, L-methionine and L-lysine uptake by the BBMV was performed from self-inhibition curves using 8 µmol/L D-[U-14C]glucose, 0.5 µmol/L L-[3H]methionine or 0.25 µmol/L L-[3H]lysine as substrate and 11 different concentrations of unlabeled D-glucose, L-methionine or L-lysine, respectively, on the cis side ranging from 0 to 20 mmol/L. The incubation time tested was 5 s for D-glucose and 3 s for L-methionine and L-lysine because transport was linear up to 7 and 3 s, respectively, for all dietary conditions (data not shown). To facilitate comparison of the kinetic constants, the Vmax and Kd values were normalized to 1 s incubation.
The self-inhibition curves of D-glucose uptake were fitted to a model of a single transport system plus diffusion because in the chicken jejunum only system SGLT1 is involved in Na+-dependent D-glucose transport (1 ,22 ). The analysis of amino acid transport was more complex because the uptake of L-lysine takes place through two systems and L-methionine is mediated by four transport systems, two of them shared by cationic and neutral amino acids [systems b0,+ and ym+; (14 )], and two specific systems for neutral amino acids [systems L and B; (23 )]. Dietary effects on systems b0,+ and ym+ were assessed from L-lysine transport kinetics and the effects on systems B and L from L-methionine transport (with Na+ and L-lysine present), by fitting self-inhibition curves as described elsewhere (23 ). Because systems B and L have been shown to have very similar half-saturation constants (23 ), it is very difficult to distinguish between the two systems by only kinetic means; for this reason, the kinetic analysis was performed by assuming that L-methionine is transported by a single neutral transport system.
The rates of mediated D-glucose, L-lysine, and L-methionine transport were fitted by nonlinear regression analysis from plots generated by the Enzfitter statistical package (Biosoft, Cambridge, UK). The best fit was assigned to the fit showing the lowest as well as significantly different residual sums of squares (P 
0.05), according to the criteria of Motulsky and Ransnas (24
). Because errors associated with experimental fluxes were roughly proportional to their values, we considered it appropriate to apply a proportional weighting to the data.
Data analysis.
The effects of dietary fatty acid treatment on the variables studied (i.e., animal characteristics, enzyme activity and fatty acid composition of brush border membrane) were analyzed statistically using one-way ANOVA. The homogeneity of variances was tested with Levene’s test. Differences among treatment groups were assessed by Tamhane’s multiple range test. Kinetic parameters were compared using Student’s t test. In both cases, a P < 0.05 value was considered statistically significant. Statistical analyses were performed using the version 10 of the SPSS software (SPSS, Chicago, IL).
Reagents and standards.
Reagents were from Sigma Chemical (St. Louis, MO) and from Panreac (Barcelona, Spain). Labeled substrates were obtained from New England Nuclear Research Products (Dreieich, Germany).
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RESULTS |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Sucrase activity was not significantly affected, whereas alkaline phosphatase activity in the homogenate of the LSO and SFO groups was higher than in the LAR and CTL groups (Table 3
). Vesicles prepared from the mucosa of chickens fed all four diets had a high apical membrane purity because the enrichment factor for sucrase and alkaline phosphatase was 10-fold. Contamination by the basolateral membrane was low because the enrichment factor of the basolateral marker was only 0.3-fold (results not shown). Examination of vesicle orientation indicated that 92–95% of the vesicles were right-side out.
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Fatty acid membrane composition.
The total SFA present in the membrane resulted in small but significant differences among groups, with the highest content in the LAR group, as expected. The MUFA accounted for 11–16% of the total fatty acids present in the brush border membrane. As with SFA, the quantitative differences in membrane MUFA were much lower than differences in dietary MUFA composition (Tables 4 and 5 ).
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4–5 g/100 g of total fatty acids in the CTL, LAR and SFO groups.
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Dietary fatty acid and intestinal nutrient transport.
The effects of diet enrichment with lipids containing fatty acids of different series on the transport of D-glucose, L-lysine and L-methionine is shown in Table 6 . None of the diets modified the Michaelis constant (Km) of transporters for their substrates, but they affected some Vmax and Kd. The main effects on Vmax were an 18–22% reduction in D-glucose Vmax by the LAR and SFO diets, and a 55% reduction in L-lysine Vmax across b0,+ in chickens fed the three supplemented diets. However, the LSO diet increased L-methionine Vmax across systems B and L relative to the three other groups. These results indicate that there is not a single pattern of diet effects on the mediated uptake.
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DISCUSSION |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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Analysis of the brush-border lipid composition shows that some fatty acids were present in a proportion that clearly reflected their dietary intake, whereas others were independent of dietary sources. The amount of SFA was within a margin that was narrower than that in the experimental diets. Thus, the proportion of SFA in the membrane is a well-regulated variable, as reported for other species such as rats (6 ), guinea pigs (25 ) and fish (26 ). The total MUFA content in the membrane was similar (comprised between 11 and 16%) while in the diets there was much variability (ranging between 19 and 38%), and similar conclusions can be drawn for (n-6) PUFA. However, the (n-3) PUFA content showed a wider variability in the membrane, clearly reflecting the differences in dietary intake of this kind of fatty acid.
The CTL and SFO diets had a high 18:2(n-6) content, but with very different (n-6)/(n-3) ratios, and the membrane content in both groups was much higher than in the LAR and LSO groups, indicating a relationship between intake and composition. High intake of 18:3(n-3) (LSO diet) also resulted in a significantly higher membrane proportion, which supports the hypothesis that certain dietary fatty acids strongly affect membrane composition. The high 18:3(n-3) content of the LSO diet may stimulate the synthesis of longer and more desaturated members of the series (27 ). Indeed, there was a fivefold increase in 20:5(n-3) [eicosapentaenoic acid (EPA)] and a twofold increase in 22:5(n-3) (docosapentaenoic acid) in the membrane of chicks fed the LSO vs. CTL diet. This indicates that the higher availability of 18:3(n-3) in the LSO diet (6- to 18-fold that of the other diets) enhances the synthesis of members of the fatty acid (n-3) family. However, the proportion of 22:6(n-3) (DHA) in the brush border membrane of the LSO group was the same as in CTL- and LAR-fed chickens (see Fig. 1 ), suggesting that the metabolic steps from 22:5(n-3) are independently regulated.
Dietary enrichment with 18:3(n-3) (LSO diet) reduced the incorporation of certain metabolites of the (n-6) series into the membrane, which has been attributed to the inhibitory effects of 20:5(n-3) (EPA) on both 
6 and 5 desaturases (28
,29
). Our results indicate that this also occurs in chickens because the amount of 18:3(n-6) in the membrane [the product of 18:2(n-6) desaturation] and membrane 20:4(n-6) (AA) were significantly lower in LSO than in CTL chickens (Fig. 1)
. Chickens fed diets with PUFA predominating over SFA, such as the CTL, LSO and SFO groups (all having P/S ratios > 3), had membrane P/S ratios between 1.04 and 1.19, indicating that the lipid metabolic pathways tend to keep the membrane P/S ratio close to unity.
Mucosal function.
Although membrane lipid composition has been shown to affect mucosal enzymes, there is no consistent pattern in the effect of dietary lipids on intestinal hydrolases (3 ); for example, corn oil supplementation has been shown to decrease sucrase activity (30 ) or have no effects on sucrase and phosphatase activities (31 ). Our results showed that dietary supplementation with LSO and SFO increases alkaline phosphatase activity in mucosal homogenates without affecting sucrase activity. The effects of fat-supplemented diets on chicken mucosal enzymes were slight, indicating that there is no direct link between membrane fatty acid composition and enzymatic activity.
Isocaloric changes in dietary lipids alter both passive and active transport processes (32 ). High fat intake has specific effects on D-glucose absorption in rats (9 ,33 ), and SFA specifically induce expression of the SGLT1 protein in the brush border membrane (10 ), supporting the hypothesis that both unspecific and specific mechanisms are involved in the intestinal fat-induced increase in the uptake of nutrients. The LSO-rich diet did not affect the D-glucose transport kinetics, whereas in the LAR and SFO groups, the Vmax decreased and the Kd increased. These results indicate that at low luminal hexose concentrations (e.g., 0.1 mmol/L), glucose absorption was reduced only in the LAR- and SFO-fed chickens because the brush border membrane has a lower D-glucose Vmax. In the rat intestine, SFA have specific effects on glucose transport that point to a distinct pattern of adaptation (10 ). At high substrate concentrations (e.g., 20 mmol/L), the passive influx prevails and both the LAR and SFO diets increased the intestinal D-glucose absorbing capacity. This result is consistent with the observations of Thomson et al. (34 ) in rats at high luminal glucose concentrations.
The kinetic study of the transport of L-lysine and L-methionine showed that dietary lipids affect amino acid Vmax and Kd constants. The changes that may be nutritionally important are the Vmax y+m reduction in the LAR group and the increased uptake in the PUFA-supplemented chickens, whereas the component involved in L-methionine transport was affected only by the LSO diet, with a 35% Vmax increase. The LSO diet also decreased the Kd of both L-lysine and L-methionine. Moreover, lipid supplementation reduced the Vmax of the low capacity b0,+ system, although the nutritional relevance of this effect is limited because the contribution of the b0,+ system to the overall L-lysine uptake is extremely low (23 ).
The effects of dietary lipids on nutrient transport do not seem to follow a single pattern of adaptation but several patterns according to specific lipid-protein interactions. For example, the LAR diet reduced the Vmax of D-glucose and L-lysine (across y+m), whereas the LSO diet increased the Vmax of both L-lysine and L-methionine. This suggests that a low brush border P/S ratio is associated with a reduction in the permeability of nutrients across specific transport pathways. The measurement of steady-state fluorescence depolarization of DPH indicates that there are no differences in membrane fluidity, in spite of changes in fatty acid profiles (35 ). However, the variation in membrane composition affects the physicochemical properties of the membrane and hence its passive permeability to nutrients. Because consistent changes in the Kd for D-glucose, L-lysine and L-methionine were observed only in chickens fed the LSO-enriched diet, we suggest that the higher relative membrane content of (n-3) PUFA is involved in such changes in passive nutrient permeability. Rats fed a diet enriched with linolenic acid have also a reduced Kd for D-glucose in the ileum (36 ).
In this study we have demonstrated that dietary lipids strongly affect the lipid composition of the brush border membrane of the jejunum and the nutrient transporters present in the membrane, supporting the view that the intestinal mucosa is a dynamic structure that is shaped in response to changes in the luminal nutrient contents. Once the chicken intestinal glucose and amino acid transporters are cloned, it would be interesting to determine whether dietary lipids regulate the message or the protein levels in the membrane and to what extent such alterations are responsible for the observed changes in nutrient uptake.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Supported by grant ALI96–0910 from the Spanish Ministerio de Educación y Cultura and grant 1999-SGR-00271 from the Generalitat de Catalunya (Spain).
3 C. Ferrer and E. Pedragosa contributed equally to this work.
4 E. Pedragosa and M. Torras-Llort were research fellows of the Recerca i Docència Program, Universitat de Barcelona, Barcelona, Spain.
6 Abbreviations used: AA, arachidonic acid; BBMV, brush border membrane vesicles; CTL, control group; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; FAME, fatty acid methyl esters; GC, gas chromatography; LAR, lard group; LSO, linseed group; MS, mass spectrometry; P/S, polyunsaturated/saturated ratio; SFA, saturated fatty acids; SFO, sunflower group; SGLT1, sodium glucose cotransporter 1.
Manuscript received 17 July 2002. Initial review completed 18 August 2002. Revision accepted 16 December 2002.
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LITERATURE CITED |
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TOPABSTRACTMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
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