1 Division of Nutritional Sciences and 2 Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801
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ABSTRACT |
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Aggressive enteral nutrition and poor intestinal perfusion are hypothesized to play an important pathogenic role in nonocclusive small bowel necrosis. This study tests the hypothesis that glucose and glutamine transport are differentially regulated during hypoxia regardless of the luminal nutrient present. Sprague-Dawley rats (247 ± 3 g; n = 16) were randomized to receive 1 h of intestinal hypoxia or serve as normoxic controls. During this hour, jejunal loops were randomized to receive in situ perfusions of mannitol, glucose, or glutamine. When compared with normoxic groups, glucose but not glutamine transport was impaired (P < 0.001) during hypoxia. Messenger RNA abundance of the sodium glucose cotransporter sodium-dependent glucose cotransporter-1 (SGLT-1) and neutral basic amino acid transporter Bo did not differ with hypoxia or nutrient perfused. Jejunal brush-border SGLT-1 abundance was decreased (P = 0.039) with hypoxia; however, total cellular SGLT-1 protein abundance did not differ among treatment groups. These data indicate that SGLT-1 activity is regulated during hypoxia at the posttranslational level. Additional information regarding the mechanisms regulating nutrient transport in the hypoperfused intestine is critical for optimizing the composition of enteral nutrient formulas.
sodium-dependent glucose cotransporter-1; nutrient absorption; small intestine
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INTRODUCTION |
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THE INCIDENCE OF NONOCCLUSIVE small bowel necrosis is significant in traumatically injured patients, despite adequate systemic resuscitation. Currently, the causative mechanism remains unknown; however, enteral nutrition has been provided in ~90% of cases (21), suggesting that the inappropriate administration of specific nutrients into a poorly perfused small bowel may play a pathogenic role (12, 21, 23, 25). During states of low blood flow to the intestine, enteral nutrients may increase oxygen demand beyond that available, potentially increasing intestinal hypoxia and impairing intestinal function. Gastrointestinal function has been shown to be impaired during hypoperfusion or decreased oxygenation of the intestine (28). Previous in vivo rat studies from our lab indicate that, during hypoperfusion, impaired barrier function, increased lactate concentration, decreased ATP concentration, and altered nutrient transport are characteristic (15). The detrimental effects of hypoperfusion may be minimized with an enteral formula composed of nutrients easily processed by the hypoperfused intestine.
The purpose of the current study was to determine how nutrient perfusion alters sodium-dependent glucose and glutamine transport activity and their regulation at the cellular level during hypoperfusion. Glutamine transport occurs via system Bo, which is part of a family of transporters for neutral amino acids (14). Glucose transport occurs via the sodium-glucose cotransporter [sodium-dependent glucose cotransporter-1 (SGLT-1)]. We previously reported that, during hypoxia, brush-border glucose transport is impaired, but glutamine transport remains unaltered following luminal perfusion of hexoses (15). It is possible that glutamine transport was maintained in this previous study, because the jejunum had not been exposed to luminal glutamine. Therefore, the current study was designed to examine the effect of luminal glucose or glutamine on jejunal nutrient transport during hypoxia. We hypothesize that glucose and glutamine transport are differentially regulated at the cellular level during hypoxia regardless of the luminal nutrient present.
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METHODS |
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Animals. Sixteen male Sprague-Dawley rats (Harlan Sprague Dawley, Indianapolis, IN) weighing 247 ± 3 g were acclimatized and housed in a temperature- and humidity-controlled facility with a 12:12-h light-dark cycle in individual cages. Animals were maintained on a standard rodent diet and were given free access to water. The University of Illinois at Urbana-Champaign Laboratory Animal Care Advisory Committee approved all procedures described in accordance with the Guide for the Care and Use of Laboratory Animals.
Experimental model. After a 24-h food restriction, surgical plane was achieved using ketamine (87 mg/kg im) and xylazine (13 mg/kg im) anesthesia, and a laparotomy was performed. Procedures were performed at the same time each day to account for diurnal variations of SGLT-1 activity (24). Two centimeters distal to the ligament of Treitz, three 8-cm in situ jejunal loops were cannulated with flexible Silastic tubing (3.2 mm OD × 2.0 mm ID; Fisher Scientific, Itasca, IL) and secured with 4-0 silk suture (Ethicon, Somerville, NJ). After the preparation of jejunal loops, rats were randomized to one of two groups for 1 h: 1) hypoxia (created by clamping the superior mesenteric artery with a microbulldog clip (Roboz Surgical, Rockville, MD) or 2) normoxia control (with unrestricted small bowel perfusion). Sterile instruments and aseptic technique were used at all times.
Within each animal, in situ jejunal loops were randomized to receive luminal perfusions with one of three nutrients: 1) mannitol (an osmotic control), 2) glucose (absorbed by active transport via SGLT-1), or 3) glutamine (actively transported via Bo). All of the nutrients were perfused for 60 min at a concentration of 120 mM in a modified Krebs solution [(in mM) 141 Na, 117.6 Cl, 26 HCO3, 1.2 Mg, 1.2 Ca, 5.2 K, 2.4 HPO4, and 0.4 H2PO4, pH 7.4] maintained at 38°C throughout the experiment. Fresh nutrient infusion solutions were continuously perfused during the 60 min. Body temperature was maintained at 38°C with a heating element. At the end of the experimental protocol, anesthetized animals were euthanized by cardiac puncture.Tissue preparation.
After 60 min of luminal nutrient perfusions, the perfused segments were
rapidly excised and the mesentery was removed. Two 2-cm sections were
removed for electrophysiological analysis in modified Ussing chambers.
A 1-cm section was snap-frozen and stored at 80°C for RNA isolation
and determination of relative mRNA abundance of SGLT-1 and
Bo. A 1-cm section was snap frozen and stored at
20°C
for determination of ATP, lactate, pyruvate, and protein concentration.
A 1.5-cm section was frozen for quantification of total cellular and
brush-border SGLT-1 protein abundance.
Ion and nutrient transport. Techniques assessing gastrointestinal function through measurement of ion flux in modified Ussing chambers have been previously described (3, 7, 14, 19). Jejunal sections were cut longitudinally along the mesentery and mounted in modified Ussing chambers (Physiologic Instruments, San Diego, CA) to expose 0.5 cm2 of the brush border and serosal sides. The tissue was bathed in 8 ml of oxygenated (95% O2-5% CO2) modified Krebs buffer solution maintained at 37°C with a circulating water bath (Fisher Scientific). Basal transmural short-circuit current, resistance, and potential difference were measured using established techniques after a 20- to 30-min equilibration period. Sodium-dependent nutrient transport was determined by measuring the changes in short-circuit current induced by addition of either 10 mM glucose or glutamine to the medium in the mucosal reservoir. The modified Ussing chambers were connected to dual-channel voltage/current clamps (VCC MC2, Physiologic Instruments) with a computer interface allowing for real-time data acquisition and analysis (Acquire & Analyze software, Physiologic Instruments).
Jejunal ATP/ADP ratio. To ensure that jejunal mucosa exhibited metabolic indicators of oxygen deprivation, jejunal ATP/ADP ratio was measured. Frozen tissue sections were homogenized (model PCU-11, Brinkman Instruments, Westbury, NY) and sonicated for 30 s at setting 3 (model 450, Branson Sonifier, Danbury, CT) in a modified Krebs buffer for determination of jejunal ATP levels using the luciferin/luciferase method (30). One hundred microliters (20 mg/ml) of luciferin/luciferase (Sigma Chemical, St. Louis, MO) were added to 100 µl of sample and counted for 30 s on a scintillation counter (Beckman LS 6500, Fullerton, CA) using chemiluminescence parameters. Sample ATP concentrations were then calculated based on an ATP standard curve (Sigma Chemical). Additionally, ADP concentration was measured on a fluorometer (F-2000; Hitachi Instruments, Chicago, IL) by reduction of NADH and read at excitation of 340 nm and emission of 460 nm. Samples and standards were placed in a buffer of NADH (20 µM) and phosphoenolpyruvate (40 µM), and ADP reduction occurred following addition of pyruvate kinase (0.30 U/ml) (20). The sample ADP concentration was calculated based on an ADP standard curve.
Jejunal lactate concentration. As a marker of anaerobic metabolism, jejunal lactate concentration was measured in the prepared tissue homogenate using the Sigma Diagnostics Lactate kit (procedure no. 735) at 540 nm using an Elx800 plate reader (Bio-Tek, Winooski, VT). Sample lactate concentrations were determined using a lactate standard curve (Sigma Chemical). To normalize for differences in glycolytic substrate available between treatment groups, data were expressed as (micromoles lactate/micromoles pyruvate)/milligram protein. From the prepared homogenate, jejunal pyruvate concentration was quantified on a fluorometer (F-2000) based on a pyruvate standard curve. Ten microliters of each sample were placed in 1 ml NADH buffer (0.95 µM NADH; 0.5 M NaH2PO4; 0.5 M K2HPO4) read at excitation of 340 nm and emission of 460 nm. The sample was then reduced following addition of 5 U of lactate dehydrogenase (LDH) and read at excitation of 340 nm and emission of 460 nm. The differences in sample emission readings (initial reading minus reading following LDH addition) were determined and compared with a pyruvate standard curve (Sigma Chemical) (20, 30).
Relative RT-PCR mRNA abundance of nutrient transporters.
Total cellular RNA was isolated from the snap-frozen jejunal samples
using the guanidium isothiocyanate phenol-chloroform method of
Chomczynski (8). Total RNA was quantified by using a
spectrophotometer (U-2000, Hitachi) at OD260. RT-PCR was
performed in a Gene Mate Genius thermocycler (ISC Biosexpress,
Kaysville, UT) in a 20-µl total volume containing 3 µg RNA, 2.5 µM random decamers, 0.5 mM deoxynucleotide triphosphate
(dNTP) mix of the four dNTPs, 1× first strand buffer, 10 mM DTT, and
200 U Superscript II RT (GIBCO-BRL, Rockville, MD; Ambion, Austin,
Texas). Samples were then stored at 20°C.
SGLT-1 protein abundance. SGLT-1 protein abundance was determined in tissue homogenate in modified Krebs solution and brush border isolated by the procedure described by Tappenden and McBurney (29) and confirmed by ensuring sucrase-isomaltase enrichment with the brush-border fraction. Total and brush-border protein concentrations were determined using the Bio-Rad Protein Assay (Bio-Rad, Hercules, CA) with a BSA standard (1). Next, the protein was denatured by boiling for 4 min, and proteins were separated by size using 12.5% SDS-PAGE and were transferred to polyvinylidene difluoride (PVDF) membranes (Bio-Rad) using a semidry transfer apparatus (Bio-Rad). Before analyzing experimental samples, a linear range from 0 to 30 µg was established as the appropriate amount of protein per well to ensure that the bands produced fell within the linear range of the colorimetric detection system. Western blot analysis for SGLT-1 protein was performed using polyclonal antibody with known rat reactivity (Chemicon, Temecula, CA). In addition, actin monoclonal antibody with known rat reactivity (Chemicon) was added to the membrane to allow for SGLT-1 normalization to a constitutively expressed protein. PVDF membranes were developed using the Opti-4CN kit (Bio-Rad). According to the method established by Tappenden and McBurney (29), PVDF membranes were blocked using 5% nonfat dry milk in Tris-buffered saline (20 mM Tris, 137 mM NaCl, pH 7.6, 0.1% Tween) for 3 h at room temperature on a metabolic shaker. The membrane was washed for 5 min in phosphate-buffered saline with Tween 20 (PBST; 4.3 mM Na2PO4, 1.4 mM KH2PO4, 2.7 mM KCl, 137 mM NaCl, 0.1% Tween 20, pH 7.3) at room temperature. The SGLT-1 primary and actin primary antibodies were diluted 1:5,000 and 1:2,000, respectively, in PBST and 0.01% BSA. The primary antibodies were coincubated for 4.5 h at room temperature and washed at room temperature (3 × 10 min). Secondary antibodies with horseradish peroxidase conjugated to goat anti-rabbit IgG (for the SGLT-1 primary antibody) and goat anti-mouse IgG (for the actin primary antibody) were added at room temperature for 30 min (1:10,000 dilution in PBST and 0.01% BSA; Bio-Rad) and washed with PBST (3 × 10 min). Colorimetric detection followed addition of Opti-4CN diluent solution. Photographs of gels were taken using the FOTO/Analyst image-analysis system. Densitometry of SGLT-1 protein abundance was performed using Collage image-analysis software 4.0.
Statistical analysis.
The effects of hypoxia and nutrient perfusions on the outcome
parameters were determined using a two-way ANOVA. The sources of
variation were hypoxia (h = 2), nutrient perfused (p = 3), and hypoxia interacted with nutrient perfused. For analysis of relative
transporter mRNA and protein abundance, the effect of gel was
calculated using a randomized block ANOVA and was significant due to
expected experimental variation. Therefore, a completely randomized
block ANOVA was performed for these analyses (block = gel; main
effects = hypoxia and nutrient perfused; interaction = hypoxia × nutrient perfused). When a significant effect existed, comparisons were completed using Tukey's post hoc analysis.
Computations were performed using SAS (Version 8.1, SAS Institute,
Cary, NC). Statistical significance was defined as P 0.05.
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RESULTS |
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Indicators of jejunal hypoxia.
A pilot trial established the presence of hypoxia by determining a
reduction of the partial pressure of oxygen in the mesenteric venous
effluent using the protocol described herein [normoxia (n = 3; PO2 = 75.6 ± 1.5 mmHg; %O2 saturation = 96.0 ± 1.7); hypoxia rats (n = 3; PO2 = 44.0 ± 10.9 mmHg; %O2 saturation = 79.6 ± 7.6)]. In addition, indirect physiological responses to hypoxia were evaluated (basal transmural resistance, lactate concentration, and
ATP/ADP). Consistent with previous reports (19), basal
transmural resistance was significantly lower (P = 0.005) in the hypoxia groups, indicating increased permeability (Fig.
1). Additionally, glucose perfusion
resulted in significantly higher resistance than glutamine perfusion
(P = 0.002), regardless of hypoxia. However, jejunal
lactate concentration was higher (P < 0.001) during
hypoxia with glucose perfusion (Fig. 2).
This outcome was normalized to jejunal pyruvate concentration to
account for flux through the glycolytic pathway and therefore indicated
that luminal glucose alone increases the level of anaerobic metabolism.
The ATP/ADP ratio was measured as a metabolic indicator of energy
stores. ATP/ADP ratio was significantly lower (P < 0.001) in the hypoxia groups compared with the normoxia control groups
(Fig. 3). Furthermore, glucose perfusion
significantly increased ATP/ADP ratio compared with mannitol
(P = 0.01) and glutamine (P = 0.055)
both in normoxia and hypoxia groups.
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Jejunal sodium-dependent nutrient transport.
The effect of hypoxia on sodium-dependent glucose and glutamine
transport activities was assessed. Compared with normoxia controls,
glucose transport was significantly impaired (P < 0.001) in the hypoxia groups (Fig. 4).
However, glucose transport activity did not vary among nutrient
perfused (Fig. 4). In contrast, glutamine transport activity was not
affected by hypoxia (Fig. 5) and was significantly higher following luminal perfusion of glutamine compared
with glucose or mannitol (P = 0.02), regardless of
oxygenation state (Fig. 5).
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Jejunal sodium-dependent nutrient transporter mRNA abundance.
Relative RT-PCR was performed for SGLT-1 and Bo to
discern whether observed functional alterations were regulated at the
level of mRNA abundance. The relative mRNA abundance of Bo
did not differ following hypoxia (Fig.
6), consistent with the functional data
(Fig. 5). However, the changes in Bo function following
glutamine perfusion were not observed at the mRNA level, indicating
that the level of nutrient regulation for this transporter may be
posttranscriptional. SGLT-1 mRNA abundance did not differ following
hypoxia or nutrient perfused (Fig. 7). However, SGLT-1 activity was functionally impaired following hypoxia. These data necessitated further investigation into the regulation of
SGLT-1 at the level of protein abundance.
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Jejunal sodium-dependent glucose cotransporter protein abundance.
To discern the location of the SGLT-1 protein during hypoxia, SGLT-1
protein abundance was determined in both brush-border membrane and
total cellular fractions. Brush-border SGLT-1 protein abundance was
significantly lower during hypoxia (P = 0.039) compared with normoxia controls, regardless of nutrient perfused (Fig. 8). Consistent with the functional
observations, these data indicate that there is indeed less SGLT-1
protein with its functional location at the brush-border membrane
following hypoxia. Total cellular SGLT-1 protein abundance was not
altered by either hypoxia or nutrient perfusion (Fig.
9), indicating possible translocation of
the SGLT-1 protein into intracellular pools during hypoxia rather than
a targeted proteolytic degradation of this protein.
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DISCUSSION |
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Multiple studies have demonstrated the importance of minimal enteral nutrition to improve outcomes in traumatically injured adults (16, 19, 22). However, information regarding the appropriate nutrient composition for these patients is needed, because relatively few data exist regarding the functional and cellular responses to the provision of specific luminal nutrients into the hypoperfused small intestine. The results from this study support the hypothesis that, during hypoxia, glucose and glutamine transport are differentially regulated regardless of the luminal nutrient present. Kles et al. (15) previously reported that, although glucose transport is impaired, glutamine transport is maintained during hypoxia regardless of luminal carbohydrate present. However, it is possible that glutamine transport was maintained in this previous trial, because the jejunum had not been exposed to luminal glutamine. Therefore, the current study was designed with luminal perfusions of both glucose and glutamine to determine whether or not, during hypoxia, the provision of a specific nutrient inhibits the brush-border transport capacity of that particular nutrient. The results of the current study indicate that, during hypoxia, SGLT-1 activity is impaired but there is no effect on glutamine transport. This differential effect of hypoxia on nutrient transport activity was not impacted by the luminal nutrient present.
The hypoxia model used in the current study was associated with changes in three physiological indexes: transmural resistance, lactate, and ATP/ADP ratio. Regardless of nutrient provision, hypoperfusion reduced transmural resistance. Decreased resistance in this hypoxia model mimics that of a traumatic injury with increased septic morbidity, which may play a pathogenic role in the development of nonocclusive small bowel necrosis. (16) In addition, lactate concentration was increased in the current study due to anaerobic metabolism of glucose, which further increased lactate concentration beyond mannitol and glutamine levels during hypoxia. These results support the hypothesis that carbohydrates, such as glucose, could exacerbate intestinal injury during hypoperfusion. A decrease in ATP may represent decreased synthesis or increased utilization of ATP during hypoxia. Glucose perfusion significantly increased ATP/ADP ratio compared with mannitol and glutamine, indicating that there is increased synthesis of ATP/ADP during glucose perfusion regardless of oxygenation. In conclusion, decreased ATP/ADP ratio, increased lactate concentration, and decreased resistance occurred in response to the reduced jejunal oxygenation obtained with this model.
Evidence suggests that, when glucose and glutamine are provided to the small intestine, glutamine is preferentially oxidized (10). Therefore, we hypothesized that perhaps this specific alteration in brush-border nutrient transport during hypoxia, a condition of considerable stress, represents strategic sampling of luminal nutrients based on specific metabolic preferences and/or cellular requirements. In addition to the maintenance of glutamine transport during hypoxia, glutamine perfusion increases glutamine transport regardless of oxygenation status. This observation is consistent with those of other nutrient transporters, wherein the provision of the nutrient specific to the transporter will increase that transporter's maximal transport rate (27). These data illustrate a differential regulation of both glucose and glutamine transporters during hypoxia. Previously, nutrient transport was investigated in an intriguing study in which rats underwent atmospheric hypoxia, not superior mesenteric artery occlusion, in which labeled carbohydrate and amino acid uptake was decreased (18). The authors suggested that nutrient transport is depressed due to decreased Na+-K+-ATPase activity. However, Iannoli and et al. (13) measured both glucose and glutamine uptake in rabbits following superceliac aortic occlusion and did not report a statistically significant effect of hypoxia. Another study investigated the enzymatic analysis of glucose and 3-O-methylglucose uptake into tissue; the authors demonstrated decreased uptake as PO2 decreased in everted sacs (2). Although this previous report used semistarved and fed rats, it supports the hypothesis that decreasing levels of oxygen lead to lower glucose transport.
The impairment of glucose but not glutamine transport activity warrants further investigation to determine the differential regulation of transporters during hypoxia. The regulation of Bo appears to be posttranscriptional. However, a commercial antibody is not available to obtain data related to protein abundance. The regulation of SGLT-1 activity was measured at three levels: relative mRNA expression, brush-border protein abundance, and total cellular protein abundance. In our study, the relative mRNA abundance of SGLT-1 indicated that the SGLT-1 activity is not regulated at the level of mRNA abundance. Therefore, we measured SGLT-1 protein abundance to determine whether hypoxia downregulates this protein posttranslationally. Previous reports indicate that SGLT-1 protein is posttranslationally regulated (11, 17). One mechanism of posttranslational modification is that inactive stores of SGLT-1 protein are located within intracellular pools (4-6, 9). The data obtained in this study indicate that the rapid decline in SGLT-1 activity during hypoxia may be due to trafficking of functional SGLT-1 protein from the brush-border membrane to intracellular pools.
Additional information regarding the regulation of nutrient transport into the hypoperfused intestine is critical so that the composition of enteral nutrients provided to at-risk patient populations can be optimized to correspond with gastrointestinal function. Although relative SGLT-1 mRNA abundance and total cellular SGLT-1 protein abundance did not differ with hypoxia or nutrient perfusion, brush-border SGLT-1 protein abundance was impaired during hypoxia. Further investigation is necessary, because these results indicate that caution should be used in administering glucose to the hypoperfused jejunum due to impaired brush-border SGLT-1 activity.
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FOOTNOTES |
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Address for reprint requests and other correspondence: K. A. Tappenden, 443 Bevier Hall, 905 South Goodwin Ave., Dept. of Food Science and Human Nutrition, Univ. of Illinois at Urbana-Champaign, Urbana, IL 61801 (E-mail: tappende{at}uiuc.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
July 25, 2002;10.1152/ajpgi.00055.2002
Received 24 May 2002; accepted in final form 12 August 2002.
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