Content of ileal EAAC1 and hepatic GLT-1 high-affinity glutamate transporters is increased in growing vs. nongrowing lambs, paralleling increased tissue D- and L-glutamate, plasma glutamine, and alanine concentrations

Effects of maternal protein supplementation and inclusion of rumen-protected fat in the finishing diet on nutrient digestibility and expression of intestinal genes in Nellore steers

The study aimed to evaluate nutrient digestibility and intestine gene expression in the progeny from cows supplemented during gestation and fed diets with or without rumen-protected fat (RPF) in the feedlot. Forty-eight Nellore steers, averaging 340 kg, were housed in individual pens and allotted in a completely randomized design using a 2 × 2 factorial arrangement (dams nutrition × RPF). Cows' supplementation started after 124 ± 21 days of gestation. The feedlot lasted 135 days and diets had the inclusion of zero or 6% of RPF. Digestibility was evaluated by total feces collection. Steers were slaughtered using the concussion technique and samples of pancreas and small intestine were collected immediately after the slaughter to analyze α-amylase activity, and the expression of SLC5A1, CD36, and CCK and villi morphometry. Feeding RPF increased nutrients digestibility (p < 0.01). There was no effect of maternal nutrition on digestibility and α-amylase activity in steers (p > 0.05). Duodenal expression of SLC5A1, CD36, and CCK increased in the progeny from restricted cows. In conclusion, protein restriction during mid to late gestation of dams has long-term effects on small-intestine length and on expression of membrane transporters genes in the duodenum of the progeny. However, maternal nutrition does not affect digestibility in the feedlot.

Glutathione content and expression of proteins involved with glutathione metabolism differs in longissimus dorsi, subcutaneous adipose, and liver tissues of finished vs. growing beef steers

Increased tissue redox state may result in sub-optimal growth. Our goal was to determine if glutathione (GSH) content and expression of proteins involved with GSH metabolism change in longissimus dorsi (LD), subcutaneous adipose (SA), and liver tissues of growing vs. finishing steer phenotypes. Tissues were taken from 16 Angus steers (BW = 209 ± 29.4 kg) randomly assigned (n = 8) to develop through Growing (final BW = 301 ± 7.06 kg) vs. Finished (final BW = 576 ± 36.9 kg) growth phases, and at slaughter had achieved different rib-eye area (REA) (53.2, 76.8 cm2), marbling scores (296, 668), and 12th rib adipose thickness (0.54, 1.73 cm), respectively (Amino Acids, doi:10.1007/s00726-018-2540-8). GSH content (mg/g wet tissue) was determined by a commercial assay and the relative content of target protein and mRNA in tissue homogenates was determined by Western blot and reverse-transcribed PCR analyses, respectively. The effect of growth phase (Finished vs. Growing) was assessed by ANOVA using the GLM procedure of SAS. The LD of Finished steers had more (P < 0.04) GSH (42%) and GSH synthesizing (GCLC, 61%; GCLM, 21%) and metabolizing (GPX1, 42%; GPX3, 73%; GGT1, 56%) enzymes, and less (P < 0.02) GPX2 (46%), EAAC1 (30%) and glutamine synthetase (GS) (28%), whereas GTRAP3-18 and ARL6IP1 did not differ (P > 0.57). Principal component analysis found that GSH content of LD was associated with REA and marbling score. The SA of Finished steers had less (P < 0.04) GSH (38%), GSH metabolizing (GPX4, 52%; GGT1, 71%) enzyme mRNA, and GTRAP3-18 (123%) and ARL6IP1 (43%), whereas the mRNA content of GSH-synthesizing enzymes and content of EAAC1 and GS did not differ (P > 0.32). The liver of Finished steers had less (P < 0.02) mRNA content of GSH synthesizing (GCLC, 39%; GSS 29%) and metabolizing (GPX1, 30%) enzymes, and more (P < 0.01) GSTM1 metabolizing enzyme (114%). The change in GSH content as steers fattened indicate an increased antioxidant capacity in the LD of Finished steers, and a decreased antioxidant capacity in SA, consistent with changes in enzyme and transporter expression. Changes in liver enzyme and transporter expression were consistent with no change in GSH content. The relationship of EAAC1 regulatory proteins (GTRAP3-18, ARL6IP1) to GSH, EAAC1, and GS content differs and changes as Growing steers develop into Finished phenotypes. These findings provide mechanistic insight into how antioxidant capacity occurs in tissues of economic and metabolic importance as cattle fatten.

Hepatic glutamate transport and glutamine synthesis capacities are decreased in finished vs. growing beef steers, concomitant with increased GTRAP3-18 content

Hepatic glutamate uptake and conversion to glutamine is critical for whole-body N metabolism, but how this process is regulated during growth is poorly described. The hepatic glutamate uptake activities, protein content of system [Formula: see text] transporters (EAAC1, GLT-1) and regulatory proteins (GTRAP3-18, ARL6IP1), glutamine synthetase (GS) activity and content, and glutathione (GSH) content, were compared in liver tissue of weaned Angus steers randomly assigned (n = 8) to predominantly lean (growing) or predominantly lipid (finished) growth regimens. Steers were fed a cotton seed hull-based diet to achieve final body weights of 301 or 576 kg, respectively, at a constant rate of growth. Liver tissue was collected at slaughter and hepatic membranes fractionated. Total (75%), Na+-dependent (90%), system [Formula: see text]-dependent (abolished) glutamate uptake activity, and EAAC1 content (36%) in canalicular membrane-enriched vesicles decreased as steers developed from growing (n = 6) to finished (n = 4) stages, whereas Na+-independent uptake did not change. In basolateral membrane-enriched vesicles, total (60%), Na+-dependent (60%), and Na+-independent (56%) activities decreased, whereas neither system [Formula: see text]-dependent uptake nor protein content changed. EAAC1 protein content in liver homogenates (n = 8) decreased in finished vs. growing steers, whereas GTRAP3-18 and ARL6IP1 content increased and GLT-1 content did not change. Concomitantly, hepatic GS activity decreased (32%) as steers fattened, whereas GS and GSH contents did not differ. We conclude that hepatic glutamate uptake and GS synthesis capacities are reduced in livers of finished versus growing beef steers, and that hepatic system [Formula: see text] transporter activity/EAAC1 content is inversely proportional to GTRAP3-18 content.

Expression of Glutamate Transporters in Mouse Liver, Kidney, and Intestine

Glutamate transport activities have been identified not only in the brain, but also in the liver, kidney, and intestine. Although glutamate transporter distributions in the central nervous system are fairly well known, there are still uncertainties with respect to the distribution of these transporters in peripheral organs. Quantitative information is mostly lacking, and few of the studies have included genetically modified animals as specificity controls. The present study provides validated qualitative and semi-quantitative data on the excitatory amino acid transporter (EAAT)1-3 subtypes in the mouse liver, kidney, and intestine. In agreement with the current view, we found high EAAT3 protein levels in the brush borders of both the distal small intestine and the renal proximal tubules. Neither EAAT1 nor EAAT2 was detected at significant levels in murine kidney or intestine. In contrast, the liver only expressed EAAT2 (but 2 C-terminal splice variants). EAAT2 was detected in the plasma membranes of perivenous hepatocytes. These cells also expressed glutamine synthetase. Conditional deletion of hepatic EAAT2 did neither lead to overt neurological disturbances nor development of fatty liver.

Gene expression profiling indicates an increased capacity for proline, serine, and ATP synthesis and mitochondrial mass by the liver of steers grazing high vs. low endophyte-infected tall fescue

Grazing -infected forages results in a variety of reduced animal performance parameters, collectively known as "fescue toxicosis." The initial, limited evaluations of hepatic mechanisms affected by fescue toxicosis have used transcriptomic expression profiling of experimental phenotypes developed by short-term feeding of concentrated ergot alkaloids or fescue seeds to rodents and steers. To assess the effects of fescue toxicosis in growing cattle using a commercially relevant phenotype, we induced fescue toxicosis in beef steers by summer-long grazing (89 to 105 d) of a single high toxic endophyte-infected tall fescue pasture (HE; 0.746 μg/g ergot alkaloids; 5.7 ha; = 10; BW = 267 ± 14.5 kg) vs. a low toxic endophyte tall fescue-mixed pasture (LE; 0.023 μg/g ergot alkaloids; 5.7 ha; = 9; BW = 266 ± 10.9 kg). High toxic endophyte tall fescue-mixed pasture steers had decreased BW (313 vs. 338 kg) and an increased potential for hepatic gluconeogenesis from AA-derived carbons. To gain a greater perspective into fescue toxicosis-induced hepatic metabolism and identify candidate regulatory mechanisms, the goal of the current research was to examine liver samples for changes in gene (mRNA) expression profiles using a Bovine Affymetrix microarray and selected reverse-transcription PCR and immunoblot analyses. The expression (false discovery rate < 10%; < 0.01) of 147 genes was increased (7 to 268%) and that of 227 was decreased (4 to 87%) in livers of HE vs. LE steers. The top (1) functional gene category was cell-mediated immune response (33 genes; ≤ 0.012), (2) canonical cell signaling pathway was primary immunodeficiency signaling (8 genes; ≤ 0.0003), and (3) canonical metabolic pathways were oxidative phosphorylation (5 genes; ≤ 0.016) and purine metabolism (8 genes; ≤ 0.029). High toxic endophyte tall fescue-mixed pasture steers had increased ( ≤ 0.022) expression of genes critical for increased (1) Pro () and Ser () synthesis, (2) shunting of AA carbons into pyruvate () and ATP synthesis (, , , COX4, , and ), and (3) mitochondrial mass (COX4). Targeted reverse-transcribed PCR or immunoblot assays corroborated ( ≤ 0.035) these latter microarray findings for , , , , and COX4. Moreover, network analysis identified glucocorticoid receptor-mediated signaling as the most probable mechanism to coordinate the above findings. These results greatly extend our knowledge of the consequences of summer-long grazing of endophyte-infected tall fescue to the hepatic metabolism of growing steers.

Glutamine synthetase and alanine transaminase expression are decreased in livers of aged vs. young beef cows and GS can be upregulated by 17β-estradiol implants

Aged beef cows (≥ 8 yr of age) produce calves with lower birth and weaning weights. In mammals, aging is associated with reduced hepatic expression of glutamine synthetase (GS) and alanine transaminase (ALT), thus impaired hepatic Gln-Glu cycle function. To determine if the relative protein content of GS, ALT, aspartate transaminase (AST), glutamate transporters (EAAC1, GLT-1), and their regulating protein (GTRAP3-18) differed in biopsied liver tissue of (a) aged vs. young (3 to 4 yr old) nonlactating, nongestating Angus cows (Exp. 1 and 2) and (b) aged mixed-breed cows with and without COMPUDOSE (17β-estradiol) ear implants (Exp. 3), Western blot analyses were performed. In Exp. 1, 12 young (3.62 ± 0.01 yr) and 13 aged (10.08 ± 0.42 yr) cows grazed the same mixed forage for 42 d (August-October). In Exp. 2, 12 young (3.36 ± 0.01 yr) and 12 aged (10.38 ± 0.47 yr) cows were individually fed (1.03% of BW) a corn-silage-based diet to maintain BW for 20 d. For both Exp. 1 and 2, the effect of cow age was assessed by ANOVA using the MIXED procedure of SAS. Cow BW did not change ( ≥ 0.17). Hepatic ALT (78% and 61%) and GS (52% and 71%) protein content (Exp. 1 and 2, respectively) was decreased ( ≤ 0.01), whereas GTRAP3-18 (an inhibitor of EAAC1 activity) increased ( ≤ 0.01; 170% and 136%) and AST, GLT-1, and EAAC1 contents did not differ ( ≥ 0.17) in aged vs. young cows. In Exp. 2, free concentrations (nmol/g) of Glu, Ala, Gln, Arg, and Orn in liver homogenates were determined. Aged cows tended to have less ( = 0.10) free Gln (15.0%) than young cows, whereas other AA concentrations did not differ ( 0.26). In Exp. 3, 14 aged (> 10 yr) cows were randomly allotted ( = 7) to sham or COMPUDOSE (25.7 mg of 17β-estradiol) implant treatment (TRT), and had ad libitum access to alfalfa hay for 28 d. Blood and liver biopsies were collected 14 and 28 d after implant treatment. Treatment, time after implant (DAY), and TRT × DAY effects were assessed by ANOVA using the MIXED procedure of SAS. Cow BW was not affected ( ≥ 0.96). Implant increased ( ≤ 0.02) total plasma estradiol by 220% (5.07 vs. 1.58 pg/mL) and GS protein by 300%, whereas the relative content of other proteins was not altered ( ≥ 0.16). We conclude that hepatic expression of ALT and GS are reduced in aged vs. young cows, and administration of 17β-estradiol to aged cows increases plasma estradiol and hepatic GS, but not that of other proteins that support hepatic Glu metabolism.

Effect of moderate dietary restriction on visceral organ weight, hepatic oxygen consumption, and metabolic proteins associated with energy balance in mature pregnant beef cows

Twenty-two nonlactating multiparous pregnant beef cows (639 ± 68 kg) were used to investigate the effect of dietary restriction on the abundance of selected proteins regulating cellular energy metabolism. Cows were fed at either 85% (n = 11; LOW) or 140% (n = 11; HIGH) of total NE requirements. The diet consisted of a haylage-based total mixed ration containing 20% wheat straw. Cows were slaughtered by block (predicted date of parturition), beginning 83 d after the initiation of dietary treatments and every week thereafter for 6 wk, such that each block was slaughtered at approximately 250 d of gestation. Tissue samples from liver, kidney, sternomandibularis muscle, ruminal papilli (ventral sac), pancreas, and small intestinal muscosa were collected at slaughter and snap frozen in liquid N2. Western blots were conducted to quantify abundance of proliferating cell nuclear antigen (PCNA), ATP synthase, ubiquitin, and Na/K+ ATPase for all tissues; PPARγ, PPARγ coactivator 1 α (PGC-1α), and 5´-adenosine monophosphate-activated protein kinase (AMPK) and the activated form phosphorylated-AMPK (pAMPK) for liver, muscle, and rumen; phosphoenolpyruvate carboxykinase (PEPCK) for liver and kidney; and uncoupling protein 2 (UCP2) for liver. Statistical analysis was conducted using Proc Mixed in SAS and included the fixed effects of dietary treatment, cow age, block, and the random effect of pen. Dietary treatments resulted in cows fed HIGH having greater (P ≤ 0.04) ADG and final BW than cows fed LOW. Abundance of ubiquitin in muscle was greater (P = 0.009) in cows fed LOW, and PCG-1 α in liver was greater (P = 0.03) in cows fed HIGH. Hepatic O2 consumption was greater in HIGH (P ≤ 0.04). Feed intake can influence the abundance of important metabolic proteins and suggest that protein degradation may increase in muscle from moderately nutrient restricted cows and that energy metabolism in liver increases in cows fed above NE requirements.

Ergopeptines bromocriptine and ergovaline and the dopamine type-2 receptor inhibitor domperidone inhibit bovine equilibrative nucleoside transporter 1-like activity

Neotyphodium coenophialum-infected tall fescue contains ergopeptines. Except for interactions with biogenic amine receptors (e.g., dopamine type-2 receptor, D2R), little is known about how ergopeptines affect animal metabolism. The effect of ergopeptines on bovine nucleoside transporters (NT) was evaluated using Madin-Darby bovine kidney (MDBK) cells. Equilibrative NT1 (ENT1)-like activity accounted for 94% of total NT activity. Inhibitory competition (IC(50)) experiments found that this activity was inhibited by both bromocriptine (a synthetic model ergopeptine and D2R agonist) and ergovaline (a predominant ergopeptine of tall fescue). Kinetic inhibition analysis indicated that bromocriptine inhibited ENT1-like activity through a competitive and noncompetitive mechanism. Domperidone (a D2R antagonist) inhibited ENT1 activity more in the presence than in the absence of bromocriptine and displayed an IC(50) value lower than that of bromocriptine or ergovaline, suggesting that inhibition was not through D2R-mediated events. These novel mechanistic findings imply that cattle consuming endophyte-infected tall fescue have reduced ENT1 activity and, thus, impaired nucleoside metabolism.

Dietary supplementation of selenium in inorganic and organic forms differentially and commonly alters blood and liver selenium concentrations and liver gene expression profiles of growing beef heifers

In geographic regions where selenium (Se) soil concentrations are naturally low, the addition of Se to animal feed is necessary. Even though it is known that Se in grass and forage crops is primarily present in organic forms (especially as L-selenomethionine, L-selenocystine, and L-selenocystathionine), the feeding of Se in the naturally occurring organic selenium (OSe) compounds produces higher blood and tissue Se levels than the inorganic Se (ISe) salts, and that animal metabolism of OSe and ISe is fundamentally different. Se is commonly added in inorganic form as sodium selenite to cattle feeds because it is a less expensive source of supplemental Se then are OSe forms. A trial was conducted with growing cattle to determine if the addition of OSe versus ISe forms of Se in beef cattle feed produces differences in hepatic gene expression, thereby gaining insight into the metabolic consequence of feeding OSe versus ISe. Thirty maturing Angus heifers (261 ± 6 days) were fed a corn silage-based diet with no Se supplementation for 75 days. Heifers (body weight = 393 ± 9 kg) then were randomly assigned (n = 10) and fed Se supplements that contained none (control) or 3 mg Se/day in ISe (sodium selenite) or OSe (Sel-Plex®) form and enough of a common cracked corn/cottonseed hull-based diet (0.48 mg Se/day) to support 0.5 kg/day growth for 105 or 106 days. More Se was found in jugular whole blood and red blood cells and biopsied liver tissue of ISe and OSe treatment animals than control animals, and OSe animals contained more Se in these tissues than did ISe. Microarray and bioinformatic analyses of liver tissue gene expression revealed that the content of at least 80 mRNA were affected by ISe or OSe treatments, including mRNA associated with nutrient metabolism; cellular growth, proliferation, and immune response; cell communication or signaling; and tissue/organ development and function. Overall, three Se supplement-dependent gene groups were identified: ISe-dependent, OSe-dependent, and Se form-independent. More specifically, both forms of supplementation appeared to upregulate mitochondrial gene expression capacity, whereas gene expression of a protein involved in antiviral capacity was downregulated in ISe-supplemented animals, and OSe-supplemented animals had reduced levels of mRNA encoding proteins known to be upregulated during oxidative stress and cancerous states.

Metabolic fate and function of dietary glutamate in the gut

Glutamate is a main constituent of dietary protein and is also consumed in many prepared foods as an additive in the form of monosodium glutamate. Evidence from human and animal studies indicates that glutamate is a major oxidative fuel for the gut and that dietary glutamate is extensively metabolized in first pass by the intestine. Glutamate also is an important precursor for bioactive molecules, including glutathione, and functions as a key neurotransmitter. The dominant role of glutamate as an oxidative fuel may have therapeutic potential for improving function of the infant gut, which exhibits a high rate of epithelial cell turnover. Our recent studies in infant pigs show that when glutamate is fed at higher (4-fold) than normal dietary quantities, most glutamate molecules are either oxidized or metabolized by the mucosa into other nonessential amino acids. Glutamate is not considered to be a dietary essential, but recent studies suggest that the level of glutamate in the diet can affect the oxidation of some essential amino acids, namely leucine. Given that substantial oxidation of leucine occurs in the gut, ongoing studies are investigating whether dietary glutamate affects the oxidation of leucine in the intestinal epithelial cells. Our studies also suggest that at high dietary intakes, free glutamate may be absorbed by the stomach as well as the small intestine, thus implicating the gastric mucosa in the metabolism of dietary glutamate. Glutamate is a key excitatory amino acid, and metabolism and neural sensing of dietary glutamate in the developing gastric mucosa, which is poorly developed in premature infants, may play a functional role in gastric emptying. These and other recent reports raise the question as to the metabolic role of glutamate in gastric function. The physiologic significance of glutamate as an oxidative fuel and its potential role in gastric function during infancy are discussed.

Ruminal and abomasal starch hydrolysate infusions selectively decrease the expression of cationic amino acid transporter mRNA by small intestinal epithelia of forage-fed beef steers

Although cationic amino acids (CAA) are considered essential to maximize optimal growth of cattle, transporters responsible for CAA absorption by bovine small intestinal epithelia have not been described. This study was conducted to test 2 hypotheses: 1) the duodenal, jejunal, and ileal epithelia of beef cattle differentially express 7 mRNA associated with 4 mammalian amino acid (AA) transport activities: y(+) (CAT1), B(0,+) (ATB(0,+)), b(0,+) (b(0,+)AT and rBAT), and y(+)L (y(+)LAT1, y(+)LAT2, and 4F2hc), and 2) the expression of these mRNA is responsive to small intestinal luminal supply of AA substrates (derived from ruminal microbes) or glucose-derived energy (from starch hydrolysate, SH), or both. Eighteen ruminally and abomasally catheterized Angus steers (body weight = 260 +/- 17 kg) fed an alfalfa cube-based diet at 1.33 x net energy for maintenance requirement were assigned to 3 treatments (n = 6): ruminal and abomasal water infusion (control); ruminal SH and abomasal water infusion; and ruminal water and abomasal SH infusion. The dosage of SH infusion amounted to 20% of metabolizable energy intake. After 14 or 16 d of infusion, steers were slaughtered, duodenal, jejunal, and ileal epithelia were harvested, and total RNA was extracted. The relative amounts of mRNA expressed by epithelia were quantified using real-time reverse transcription-PCR. All 7 mRNA species were expressed by the epithelium from each region, but their abundance differed among the regions. Specifically, duodenal expression of CAT1 and ATB(0,+) mRNA was greater than jejunal or ileal expression; ileal expression of b(0,+)AT, rBAT, and y(+)LAT1 mRNA was greater than jejunal or duodenal expression, whereas the expression of y(+)LAT2 and 4F2hc mRNA did not differ among the 3 epithelia. With regard to SH infusion effect, ruminal infusion down-regulated or tended to down-regulate the jejunal expression of CAT1, rBAT, y(+)LAT2, and 4F2hc mRNA. Abomasal infusion down-regulated the jejunal expression of y(+)LAT2 mRNA and tended to down-regulate the jejunal expression of 4F2hc mRNA. This study characterized the pattern of CAA transporter mRNA expressed by growing beef cattle fed an alfalfa-based diet. Moreover, this study demonstrated that increasing the luminal supply of microbe-derived AA (by ruminal supplementation of SH) results in a reduced capacity of apical and basolateral membrane to transport of CAA, whereas increasing luminal glucose supply (by abomasal supplementation of SH) reduces only the basolateral transport capacity, assuming that CAA transporter mRNA content represents functional capacity.

Influence of glutamine infusion on ubiquitin, caspase-3, cathepsins L and B, and m-calpain expression in sheep with nutritionally induced metabolic acidosis

Provision of AA has shown success in attenuating proteolytic activity in monogastrics suffering from metabolic acidosis. However, it is unknown whether AA supplementation can provide any beneficial effects to ruminants with nutritionally induced metabolic acidosis. The objective of the current study was to examine the effects of glutamine infusion on various protein degradation components across several tissues in sheep with induced metabolic acidosis. Sheep were assigned to a randomized complete block design with 2 x 2 factorial arrangement of treatments (n = 6 sheep/treatment) consisting of a control or acidosis diet, and receiving a saline or L-glutamine infusion. Sheep were fed diets for 10 d and slaughtered on d 11. Liver, kidney, and muscle samples were collected at slaughter and examined for relative messenger RNA (mRNA) expression of ubiquitin, C8, E2, cathepsin L, cathepsin B, caspase-3, and m-calpain, as well as protein expression of ubiquitin. Relative mRNA expression of C8 (P = 0.02), E2 (P = 0.06), and ubiquitin (P = 0.07) was less in kidney in acidotic vs. control sheep. Additionally, mRNA expression of m-calpain in kidney was greater (P = 0.01) as a result of glutamine infusion. There were no significant alterations (P > 0.10) in mRNA of any component as a result of acidosis in the liver or muscle. This study demonstrates the inability of metabolic acidosis to increase expression of the ubiquitin-mediated proteolytic pathway in skeletal muscle; however, downregulation of renal mRNA expression of these components is apparent during the induction of metabolic acidosis.

Basal expression of nucleoside transporter mRNA differs among small intestinal epithelia of beef steers and is differentially altered by ruminal or abomasal infusion of starch hydrolysate

In ruminants, microbial-derived nucleic acids are a major source of N and are absorbed as nucleosides by small intestinal epithelia. Although the biochemical activities of 2 nucleoside transport systems have been described for cattle, little is known regarding the regulation of their gene expression. This study was conducted to test 2 hypotheses: (1) the small intestinal epithelia of beef cattle differentially express mRNA for 3 concentrative (CNT1, 2, 3) and 2 equilibrative (ENT1, 2) nucleoside transporters (NT), and (2) expression of these NT is responsive to small intestine luminal supply of rumen-derived microbes (hence, nucleosides), energy (cornstarch hydrolysate, SH), or both. Eighteen ruminally and abomasally catheterized Angus steers (260 +/- 17 kg of BW) were fed an alfalfa cube-based diet at 1.33x NE(m) requirement. Six steers in each of 3 periods were blocked by BW (heavy vs. light). Within each block, 3 steers were randomly assigned to 3 treatments (n = 6): ruminal and abomasal water infusion (control), ruminal SH infusion/abomasal water infusion, or ruminal water infusion/abomasal SH infusion. The dosage of SH infusion amounted to 20% of ME intake. After a 14-or 16-d infusion period, steers were slaughtered, and duodenal, jejunal, and ileal epithelia were harvested for total RNA extraction and the relative amounts of mRNA expressed were determined using real-time RT-PCR quantification methodologies. All 5 NT mRNA were found expressed by each epithelium, but their abundance differed among epithelia. Specifically, jejunal expression of all 5 NT mRNA was higher than that by the ileum, whereas jejunal expression of CNT1, CNT3, and ENT1 mRNA was higher, or tended to be higher, than duodenal expression. Duodenal expression of CNT2, CNT3, and ENT2 mRNA was higher than ileal expression. With regard to SH infusion treatments, ruminal infusion increased duodenal expression of CNT3 (67%), ENT1 (51%), and ENT2 (39%) mRNA and ileal expression of CNT3 (210%) and ENT2 (65%) mRNA. Abomasal infusion increased (54%) ileal expression of ENT2 mRNA and tended to increase (50%) jejunal ENT2 mRNA expression. This study has uniquely characterized the pattern of NT mRNA expression by growing beef cattle and found that the mRNA abundance for CNT3, ENT1, and ENT2 in small intestinal epithelia can be increased by increasing the luminal supply of nucleotides (CNT3, ENT1, ENT2) or glucose (ENT2).

Extensive gut metabolism limits the intestinal absorption of excessive supplemental dietary glutamate loads in infant pigs

Glutamate (Glu) is a major intestinal oxidative fuel, key neurotransmitter, and may be a useful dietary supplement to augment health of the infant gut. We quantified the metabolic fate of various supplemental dietary Glu intakes in young pigs surgically implanted with vascular, intraduodenal (ID), or intragastric (IG) catheters and a portal blood flow probe. Piglets were acutely fed a range of dietary Glu intakes using a basal milk formula (100%) supplemented with varying amounts of monosodium Glu (up to 400%) via ID or IG routes. We quantified the gastrointestinal metabolic fate of dietary Glu using [U-(13)C] Glu tracer. The Glu net absorption in the basal 100% group was low in both ID and IG groups, ranging from 13 to 17% of intake. Enteral Glu supplementation significantly increased the absolute absorption rate and arterial concentration of Glu. In both the ID and IG groups, enteral [(13)C]Glu absorption was limited (<5% tracer input) at the basal Glu intake (100%) but increased nearly 4-fold ( approximately 20% input) in the 300% intake group. A substantial fraction (33-50%) of the enteral [(13)C]Glu input was oxidized by the gut to (13)CO(2) in both the 100 and 300% intake groups. We conclude that extensive gut metabolism limits the absorption of supplemental dietary Glu even at excessive intakes.

Cellular distribution of glutamate transporters in the gastrointestinal tract of mice. An immunohistochemical and in situ hybridization approach

L-Glutamate transport by intestinal epithelial cells is an initial step of the entire glutamate metabolism pathway in the gut mucosa. The present study examined the cellular distribution of glutamate transporters in the digestive tract of adult mice using immunohistochemistry and in situ hybridization technique. Expression of EAAC1 mRNA was more intense in the ileum, where the epithelium in crypts and the basal half of intestinal villi showed high levels of transcripts, suggesting an essential role of EAAC1 in differentiating or premature epithelial cells. Electron-microscopically, EAAC1 immunoreactivity was predominantly localized in the striated border of enterocytes. Immunoreactivity for GLT-1 was found in the lateral membrane of epithelial cells at the bottom of gastric glands and at the intestinal crypts, and also in the lateral membrane of secretory cells at the duodenal gland. GLAST immunoreactivity was restricted to the fundic and pyloric glands, and was especially intense in the neck portion of both glands. However, in situ hybridization analysis failed to confirm the expression of GLT-1 and GLAST at the mRNA level, possibly due to limited sensitivity. The strong and specific luminal localization of EAAC1 in the intestinal epithelium suggests that EAAC1 is a predominant transporter of glutamate, at least in the lower part of the small intestine.

Expression of apical membrane L-glutamate transporters in neonatal porcine epithelial cells along the small intestinal crypt-villus axis

Enteral l-glutamate is extensively utilized as an oxidative fuel by the gut mucosa in the neonate. To identify major uptake pathways and to understand uptake regulation, we examined transport kinetics and molecular identities of apical membrane l-glutamate transporters in epithelial cells sequentially isolated along the small intestinal crypt-villus axis from milk protein-fed, 16-day-old pigs. The distended intestinal sac method was used to isolate 12 sequential cell fractions from the tip villus to the bottom crypt. Initial rates and kinetics of l-glutamate uptake were measured with l-[G-(3)H]glutamate by fast filtration in apical membrane vesicles prepared by Mg(2+) precipitation and differential centrifugation, with membrane potential clamped by SCN(-). Initial l-glutamate uptake results suggested the presence of B(o) and X(AG)(-) transport systems, but the X(AG)(-) system was predominant for uptake across the apical membrane. Kinetic data suggested that l-glutamate uptake through the X(AG)(-) system was associated with higher maximal transport activity but lower transporter affinity in crypt than in villus cells. Molecular identity of the X(AG)(-) glutamate transporter, based on immunoblot and RT-PCR analysis, was primarily the defined excitatory amino acid carrier (EAAC)-1. EAAC-1 expression was increased with cell differentiation and regulated at transcription and translation levels from crypt to upper villus cells. In conclusion, efficiency and capacity of luminal l-glutamate uptake across the apical membrane are regulated by changing expression of the X(AG)(-) system transporter gene EAAC-1 at transcription and translation levels as well as maximal uptake activity and transporter affinity along the intestinal crypt-villus axis in the neonate.