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3 Departments of Food Science and Human Nutrition and Animal Science, Iowa State University, Ames, IA 50011 and 4 JBS United, Sheridan, IN 46069
* To whom correspondence should be addressed. E-mail: mspurloc{at}iastate.edu.
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ABSTRACT |
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 TOP ABSTRACT IntroductionMaterials and MethodsResultsDiscussionLITERATURE CITED |
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Introduction |
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TOPABSTRACTIntroductionMaterials and MethodsResultsDiscussionLITERATURE CITED |
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In all mammalian species, (n-3) and (n-6) PUFA are essential for growth and development. As acyl moieties of phospholipids in cell membranes, they act directly, or as precursors to other molecules, to modulate cell growth, metabolism, communication, and the expression and function of myriad proteins and genes (7–9). Docosahexenoic acid (DHA) [22:6(n-3)] and eicosapentaenoic acid (EPA) [20:5(n-3)], the major (n-3) PUFA in marine-based products, have received much attention because of their antiinflammatory activities that encompass direct and indirect regulation of inflammatory pathways in multiple cell types (10–12).
Multiple studies demonstrate that maternal diet impacts tissue concentration of DHA and EPA of both the dam and offspring (13–15). Furthermore, intestinal adaptation and nutrient absorption are susceptible to modification by dietary manipulations. The small intestine of piglets consuming long chain (n-3) PUFA recover more completely from lesions and biochemical alterations caused by malnutrition (16). Additionally, the activation of AMP-activated protein kinase (AMPK), a key sensor of energy status within the cell, resulted in an upregulation of GLUT2 in mice (17). Mechanistically, this is perhaps quite important, because AMPK is upregulated by dietary (n-3) PUFA, at least in rats (18). However, the effects of long-term consumption of (n-3) PUFA by the dam throughout gestation and lactation on piglet intestinal nutrient uptake and energy stores in liver and muscle at weaning have not been determined. Therefore, this study was designed to test the hypothesis that enhancing the (n-3) PUFA concentration of piglet tissues via the maternal gestation and lactation diets would improve nutrient uptake and tissue glucose and glycogen stores at weaning. The data presented herein indicate that long chain (n-3) PUFA supplementation of maternal diets throughout gestation and lactation increases intestinal glucose absorption through GLUT2 and SGLT1 and improves tissue glycogen stores postweaning.
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Materials and Methods |
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TOPABSTRACTIntroductionMaterials and MethodsResultsDiscussionLITERATURE CITED |
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150 d prior to farrowing (Table 1). The 4 experimental dietary treatments consisted of: 1) basal corn/soybean meal control (no added fat) (CONT); 2) the basal diet supplemented with protected fish oil (PFO) (Gromega365; JBS United); 3) the basal diet supplemented with DHA from Schizochytrium algae (DHAGF) (S-Type Gold Fat; Advanced BioNutrition); and 4) the basal diet supplemented with dried coconut fat (COCO). The fatty acid profiles of the diets are presented in Table 2. Both the PFO and S-Type Gold Fat ingredients had high (n-3) PUFA concentrations and contained 29 and 43% total fat, respectively. The rest of these ingredients were comprised of protein and carbohydrate as per the manufacturer's description. S-Type Gold Fat contains 40% DHA and 2% EPA by percentage of fat and PFO had 13% EPA and 13% DHA. The total fat of the 4 diets differed. However, the DHA percentage of the DHAGF diet was equal to the DHA percentage in the PFO diet. The raw COCO fat ingredient was comprised of 88% saturated fat (high in SFA C10:0-C16:0) as a percentage of fat. This COCO diet was included to augment potential differences between (n-3) and SFA and because medium chain fatty acids are commonly used in the pig industry in weanling pig diets, because medium chain fatty acids are absorbed and utilized at a faster rate than long chain fatty acids (19,20). All diets (Table 1) met and exceeded the nutrient requirements for gestating and lactating sows (21) and all piglets had access to water at all times.
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Fatty acid analysis.
One week postfarrowing, 40 mL of mid-lactation milk was obtained from 4 sows of each dietary treatment following an i.v. injection of 10 IU of oxytocin-S (Intervet) to induce milk secretion. At random, several udders from each sow were milked, pooled together, and snap-frozen on dry ice pending fatty acid analysis. Lipids from milk, muscle, and liver samples were extracted by the method of Lepage and Roy (22) with minor modifications. Briefly, 0.5 g of tissue or 300 µL of milk was homogenized in 2.5 mL 4:1 methanol:hexane and 200 µL of 3.7 mmol heptadecanoic acid/L methanol was added to each sample as an internal standard. FAME were analyzed by GC on a Hewlett-Packard model 6890 (Hewlett-Packard) fitted with a Omegawax 320 (30-m x 0.32-mm i.d., 0.25 µm) capillary column (Sigma-Aldrich). Hydrogen was the carrier gas. The temperature program ranged from 80 to 250°C with a temperature rise of 5°C/min. The injector and detector temperatures were 250°C and 1 µL of sample was injected and run splitless. Fatty acids were identified by their retention times on the column with respect to appropriate standards.
Small intestine morphology. Formalin-fixed jejunum sections were embedded in paraffin, sectioned, mounted on a glass slide, and hematoxylin/eosin stained. Three digital images were captured and 10 villi and 10 crypts were randomly measured per image (30 per piglet) after calibrating pixels to microns using a digital image of the calibration slide. Means of the parameters were determined for each pig and treatment.
Ussing chamber. Proximal jejunum samples starting 40 cm from the stomach consisting of a 20- to 30-cm segment of proximal jejunum were removed and placed in chilled Krebs-Henseleit buffer (consisting of, in mmol/L: 25 NaHCO3, 120 NaCl, 1 MgSO4, 6.3 KCl, 2 CaCl, 0.32 NaH2PO4; pH 7.4) for transport back to the laboratory (<40 min) under constant aeration until clamped in the Ussing chambers. Two jejunal segments per pig were then stripped of outer muscle layers and immediately mounted in Ussing Chambers (DVC 1000 World Precision Instruments). Each segment was bathed on its mucosal and serosal surfaces (opening area 1.0 cm2) with 8 mL Krebs solution and gassed with 95% O2-5% CO2 mixture. The intestinal segments were voltage clamped at 0 mV by an external current after correction for solution resistance. After a 30-min period to allow the tissues to stabilize, they were challenged independently with 10 mmol/L D-glucose and 10 mmol/L L-glutamine, which was added to serosal buffer, with equimolar (10 mmol/L) mannitol added on the mucosal side. The potential difference across the tissue was measured for 30 min after each challenge by open circuit conditions every 10 s due to a short-circuit current being delivered by a voltage clamp apparatus. The change in maximal current was recorded and the tissue conductance was calculated from the short-circuit current and potential difference using Ohm's law. This was repeated on 4 different days with a total of 4 pigs per treatment.
Determination of muscle and liver glycogen. Samples of muscle (longissimus dorsi) or liver tissue (0.5 g) were extracted in ice-cold perchloric acid (0.5 mol/L) using a Tissue Tearor homogenizer. Duplicate samples (300 µL) of each homogenate were then prepared for glycogen hydrolysis with 0.3 g/L amyloglucosidase (Sigma-Aldrich) for 120 min at 38°C. The incubation was stopped by the addition of 0.6 mol/L perchloric acid and the samples clarified by centrifugation (1500 x g; 15 min at 4°C). Glucose (HK) assay kits (Sigma-Aldrich) were used to determine total micromolar glycosyl units (glucose, glucose-6-P, and glucose from glycogen) from the clarified samples and from the original homogenate (glucose, glucose-6-P only). Results were expressed as milligrams glycosyl units per gram wet tissue.
RNA extraction and quantitative PCR.
Total RNA was recovered from cells using Trizol reagent (Invitrogen), DNase treated using the Turbo DNase (Ambion), and total RNA (2 µg) was reverse transcribed using the iScript cDNA synthesis kit (Bio-Rad). Primer sequences were (sense and anti-sense): porcine AMPK
2, 5'-cgacgtggagctgtactgctt-3' and 5'-cataggtcaggcagaacttgc-3'; porcine SGLT1, 5'-cgtgctgtttccagatgatg-3' and 5'-atcagctccatgaccagctt-3'; and porcine ribosomal protein L32 (housekeeper), 5'-tggaagagacgttgtgagcaa-3' and 5'-cggaagtttctggtacacaatgtaa-3'. Thermal cycler conditions for PCR were 95°C for 3 min followed by 40 cycles of 95°C for 15 s, 65°C for 30 s, and 72°C for 30 s. PCR products amplified were cloned into pGEMT vector (Promega) and sequenced for verification. Real-time reactions were carried out on an iCycler real-time machine using the IQ SYBR Green Supermix kit (Bio-Rad). We calculated abundance of gene product by regressing against the standard curve generated in the same reaction by their respective plasmids and gene values normalized to ribosomal protein L32 housekeeper gene, which was not affected by the dietary treatment (P > 0.10).
Protein expression. Whole frozen jejunum sections (1 g) were homogenized on ice in 700 µL Buffer A (50 mmol/L Tris-HCl, pH 7.5, 50 mmol/L NaF, 5 mmol/L sodium pyrophosphate, 1 mmol/L EDTA, 1 mmol/L dithiothreitol, 0.1 mmol/L phenylmethylsulfonyl fluoride, 10% glycerol) containing 1% Triton X-100, 5 µmol/L aprotinin, leupeptin, and pepstatin. The lysates were centrifuged at 6000 x g; 20 min at 4°C to remove insoluble material. Supernatants were collected and protein was quantified using BCA reagents (Pierce) and frozen until assayed. Jejunum lysates were used for both the AMPK assay and western-blot analysis.
The abundance of GLUT2 (60 kDa) and SGLT1 (70 kDa) protein was determined by western-blot analysis. Briefly, supernatant containing 250 µg protein was immunoprecipitated at room temperature for 2 h using the Catch and Release v2.0 Reversible Immunoprecipitation System (Upstate Cell Signaling Solutions). Both GLUT2 and SGLT1 were immunoprecipitated with 1:100 primary antibody (Chemicom International) dilutions. Immunoprecipitated proteins was separated by SDS-PAGE using a 12% resolving gel and transferred to a nitrocellulose membrane and probed with primary antibody for GLUT2 or SGLT1 (1:1000) overnight. Then the membranes were probed with goat-anti-rabbit immunoglobulin G–horseradish peroxidase antibody (Pierce) at 1:20,000 for 1 h at room temperature. Blots were developed using the SuperSignal West Pico Chemiluminescent Substrate system (Pierce), captured onto micro-film for analysis, and densitometry of the protein was determined using Quantity One 1-D analysis software (Bio-Rad).
AMPK activity.
The ability of dietary (n-3) PUFA to upregulate AMPK activity was assayed as previously described (18,23). In brief, supernatants from homogenized jejunum samples (described earlier) containing 700 µg protein were thawed and immunoprecipitated overnight at 4°C with 2.5 µg anti-AMPK using the Catch and Release v2.0 Reversible Immunoprecipitation system. The AMPK activity in the immunoprecipitates was determined as a function of phosphorylation of SAMS peptide as reported by Davies et al. (23). Assay reagents were added directly to the immunoprecipitate and incubated for 20 min at 30°C. Aliquots were removed and spotted onto circle filters and washed 3 times with 1% H3PO4 (once with acetone) and then air dried. Filters were then placed in scintillation vials and counted. Activity data were reported as pmol·min–1·mg jejunum protein–1.
Statistical analyses. All data are presented as means ± pooled SEM. The effects of fatty acids were tested by the PROC MIXED procedure in SAS (version 9.1, SAS Institute) and treatment differences were evaluated using least significant differences, which provided all pair-wise comparisons. Litter/piglet was considered the experimental unit and experimental replicate or day of harvest was considered a random effect. Differences were deemed significant at P < 0.05 and tendencies were noted at P < 0.10.
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Results |
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TOPABSTRACTIntroductionMaterials and MethodsResultsDiscussionLITERATURE CITED |
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Jejunum morphology. Villus height (496 ± 56.9 µm), crypt depth (138 ± 7.0 µm), and the villus height:crypt depth ratios (3.6 ± 0.40 µm) were unchanged by either medium chain saturated or long chain (n-3) PUFA diet exposure.
Postweaning nutrient absorption. Ex vivo jejunal nutrient absorption following the addition of 10 mmol/L D-glucose or L-glutamine was evaluated in 3 ways: 1) short circuit current, which measures change in active ion transport; 2) conductance, which measures changes in total ion transport; and 3) passive ion transport, which is measured by changes in resistance. Active transport was significantly greater following the addition of D-glucose in tissue obtained from piglets of dams fed the DHAGF and PFO diets vs. CONT piglets (P < 0.05; Fig. 1A). However, only the DHAGF treatment glucose transport was significantly higher than in the COCO piglets (P < 0.05; Fig. 1A). Compared with CONT tissues, active D-glucose uptake of tissue from DHAGF piglets was 470% higher, but the PFO and COCO diets also resulted in greater uptake vs. the CONT (320 and 40%, respectively). However, active L-glutamine uptake was only higher in tissue from piglets of dams fed the DHAGF diet compared with CONT and PFO piglets (Fig. 1B). Neither total nor passive ion transport was affected by (n-3) PUFA or COCO dietary supplementation (data not shown).
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20% (P = 0.095; Fig. 2). Moreover, both the PFO and DHAGF diets resulted in higher total protein expression of SGLT1 compared with piglets from dams fed the CONT diet (P < 0.05; Fig. 3), whereas there was no effect of the COCO diet. Quantitative PCR was also conducted to determine whether the mRNA abundance (i.e. log starting quantity) of SGLT1 was also influenced by maternal diet. Dietary fatty acid supplementation did not alter SGLT1 mRNA expression in the jejunum [data presented as log starting quantity for CONT (1.87), PFO (1.71), DHAGF (1.84), and COCO (1.70); pooled SEM = 0.14].
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2 mRNA in tissues of piglets from dams fed the DHAGF diet tended to be higher than those of dams fed the CONT diet (4.0 vs. 3.4 ± 0.3 log starting quantity; P = 0.07; Fig. 4B). However, the COCO diet did not alter AMPK activity or AMPK2 mRNA abundance.
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Discussion |
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TOPABSTRACTIntroductionMaterials and MethodsResultsDiscussionLITERATURE CITED |
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The lipid component of maternal diet during gestation and lactation can influence development of the small intestine in offspring (33). Interestingly, the intestinal uptake of glucose by SGLT1 is influenced by dietary fatty acids (34) and these researchers suggested that small changes in the fatty acid composition of the diet influenced the intestinal uptake of actively and passively transported molecules. Consistent with our findings, Thomson et al. (34) showed that feeding rats PUFA was associated with higher rates of active transport of glucose into the jejunum and increased mobility compared with rats fed saturated fat. Both the PFO and DHAGF piglet jejunum samples had superior absorptive capacity as determined by the Ussing chamber and compared with piglets from dams fed the CONT and COCO diets. This enhanced absorptive capacity could not be explained by alterations in the small intestinal architecture (i.e. villus height and crypt depth) by the (n-3) PUFA treatments. However, long-chain PUFA, and particularly the changes in the ratio of (n-6):(n-3) fatty acids achieved by higher (n-3) intake, enhance the recovery of the intestinal microvilli from starvation-induced damage in piglets (16). Apart from intestinal structure, it should also be noted that the glucose transporter protein abundance in whole tissue jejunum samples tended to increase with PFO and DHAGF feeding, although the abundance of SGLT1 mRNA was unchanged. Glucose absorption by enterocytes is accomplished by either sodium-coupled transport by the energy-dependent glucose transporter, SGLT1, or the diffusion of the glucose molecule through GLUT2. Consequently, it is quite possible that transport was also enhanced by higher transporter abundance.
Mechanistically, the increased glucose transport may relate to AMPK. Walker et al. (17) described a study in which the activation of AMPK in murine jejunum tissue resulted in an increase in net glucose flux. AMPK is a key sensor of energy status within the cell and acts to limit cellular energy depletion by downregulating selective ATP-dependent processes such as fatty acid synthesis (35). Because our experimental design included feed deprivation postweaning to impose a metabolic stress, the tendency for increased AMPK activity associated with the (n-3) fatty acids may have contributed to the increase in absorptive capacity. The sodium ion exits the enterocyte via the Na/K-ATPase to maintain the sodium gradient. However, this sodium pump is energy dependent and it is critical that ATP levels are sustained to support SGLT1 activity. The fact that dietary PUFA increase hepatic AMPK activity in rats (18) seemingly supports this possibility, albeit we readily acknowledge that others (17) found that the cellular levels of SGLT1 were actually decreased in mouse tissue when AMPK was activated by 5-aminoimidasole-4-carboxamide riboside and that localization of GLUT2 to the brush border membrane was enhanced. Because the whole tissue analysis of the glucose transporters precluded recovery of crude basolateral membranes, we cannot rule out the possibility of an increased membrane GLUT2 in our study.
Feeding (n-3) PUFA to females to enrich tissue concentrations in fetal and neonatal piglets is not a new phenomenon. Similar to works by Fritsche et al. (36) and Rooke et al. (37,38), we have shown that in utero and postnatal exposure to dietary (n-3) PUFA via the maternal diet increases piglet tissue concentrations. Additionally, Rooke et al. (38) reported that progeny of dams fed a diet containing tuna oil during lactation had a lesser growth depression at weaning. These authors speculated that the favorable effect on growth stemmed from the antiinflammatory properties of (n-3) PUFA, but nutrient transport was not evaluated.
The lack of an effect of the SFA present in the COCO and CONT diets on piglet muscle and intestinal tissues concentrations was surprising. This may be explained by the fact that these fatty acids can be effectively absorbed and utilized for energy when glycogen is low and there are limited fat stores (20,39). With respect to energy stores, we found muscle glycogen concentrations increased with the DHAGF diet at 24 h postweaning. However, no augmentation of liver glycogen or glucose stores in liver or muscle tissues was achieved by the PFO or COCO diets. Pigs that are abruptly weaned onto a dry diet have low plasma glucose, high FFA, and low liver glycogen (40). Furthermore, a rat cancer cachexia study showed that supplementation of the diets with fish oil preserved tissue glycogen stores, whereas diets with COCO did not change glucose or glycogen levels (41).
In this study, we investigated the ability of SFA and (n-3) PUFA intake in utero and postnatally via the maternal diet to enhance liver and muscle energy stores, glucose metabolism, and small intestine morphology in the transition piglet at weaning. In utero and postnatal exposure to PUFA does indeed alter the fatty acid profiles of piglet tissues. These data tend to suggest that PUFA increases glycogen stores, even 24 h postweaning. Additionally, jejunum glucose absorption is significantly enhanced by adding (n-3) PUFA to maternal gestation and lactation diets, an effect perhaps attributable to parallel increases in membrane GLUT2 and SGLT1 that relate to increased AMPK activity and mRNA abundance for the -2 subunit. Whereas the effects of (n-3) PUFA on these potential mechanisms require further study, supplementing maternal diets with these fatty acids may provide a practical means of alleviating the impact of weaning stress on piglet growth and health.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Author disclosures: N. K. Gabler, M. E. Spurlock, no conflicts of interest; J. D. Spencer, D. M. Webel, employed by JBS United.
5 Abbreviations used: AMPK, AMP-activated protein kinase; COCO, coconut diet; CONT, control diet; DHA, docosahexaenoic acid; EPA, eicosapentaenoic acid; DHAGF, docosahexaenoic acid Gold Fat diet; GLUT2, glucose transporter 2; mRNA, messenger RNA; PFO, protected fish oil diet; SGLT1, sodium glucose transporter 1.
Manuscript received 31 May 2007. Initial review completed 26 June 2007. Revision accepted 6 September 2007.
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