Sodium-dependent and -independent transport of L-glutamate in the rat intestinal crypt-like cell line IEC-17

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.

Glutamine transporter in crypts compensates for loss of villus absorption in bovine cryptosporidiosis

Cryptosporidium parvum infection represents a significant cause of diarrhea in humans and animals. We studied the effect of luminally applied glutamine and the PG synthesis inhibitor indomethacin on NaCl absorption from infected calf ileum in Ussing chambers. Infected ileum displayed a decrease in both mucosal surface area and NaCl absorption. Indomethacin and glutamine or its stable derivative alanyl-glutamine increased the net absorption of Na(+) in infected tissue in an additive manner and to a greater degree than in controls. Immunohistochemical and Western blot studies showed that in control animals neutral amino acid transport system ASC was present in villus and crypts, whereas in infected animals, ASC was strongly present only on the apical border of crypts. These results are consistent with PGs mediating the altered NaCl and water absorption in this infection. Our findings further illustrate that the combined use of a PG synthesis inhibitor and glutamine can fully stimulate Na(+) and Cl(-) absorption despite the severe villous atrophy, an effect associated with increased expression of a Na(+)-dependent amino acid transporter in infected crypts.

EAAT1 is involved in transport of L-glutamate during differentiation of the Caco-2 cell line

Little is known concerning the expression of amino acid transporters during intestinal epithelial cell differentiation. The transport mechanism of L-glutamate and its regulation during the differentiation process were investigated using the human intestinal Caco-2 cell line. Kinetic studies demonstrated the presence of a single, high-affinity, D-aspartate-sensitive L-glutamate transport system in both confluent and fully differentiated Caco-2 cells. This transport was clearly Na(+) dependent, with a Hill coefficient of 2. 9 +/- 0.3, suggesting a 3 Na(+)-to-1 glutamate stoichiometry and corresponding to the well-characterized X(A,G)(-) system. The excitatory amino acid transporter (EAAT)1 transcript was consistently expressed in the Caco-2 cell line, whereas the epithelial and neuronal EAAT3 transporter was barely detected. In contrast with systems B(0) and y(+), which have previously been reported to be downregulated when Caco-2 cells stop proliferating, L-glutamate transport capacity was found to increase steadily between day 8 and day 17. This increase was correlated with the level of EAAT1 mRNA, which might reflect an increase in EAAT1 gene transcription and/or stabilization of the EAAT1 transcript.

Glutamate in enteral nutrition: can glutamate replace glutamine in supplementation to enteral nutrition in burned rats?

BACKGROUND

Glutamine (GLN) plays many important roles for the enterocytes in health and disease, but no liquid enteral products contain GLN because of its instability. We hypothesized that glutamate (GLU) may replace GLN in supplementation to an enteral diet, and compared the metabolic effect of GLU and GLN on the gut to each other.

METHODS

Rats suffering from a 30% burn received an enteral diet containing 30% GLU (m/w to total amino acids; GLU group), 30% GLN (GLN group), or a standard amino acid formula (CTR group). After a 64-hour feeding period, the small intestine and the portal and arterial blood were harvested to observe portal and arterial amino acid levels, and glutaminase activity and glutathione in the jejunal mucosa. In another study, 3H uptake into the mucosal protein was examined after a massive dose injection of 3H-phenylalanine.

RESULTS

Alanine, a product of GLN or GLU catabolism, significantly increased in the portal blood of the GLU group compared with the GLN group. In the gut mucosa of the GLU group, 3H uptake into protein and total glutathione were higher than those of other two groups. GLN did not elevate the glutaminase activity. Arterial GLU levels increased in the GLU group, however remained within safety limits.

CONCLUSIONS

Enterally delivered GLU may be a preferable fuel for the enterocytes and enhance the mucosal protein synthesis. GLU probably can substitute for GLN in supplementation to an enteral diet regarding many roles GLN plays in the intestinal mucosa under stress situations.

Effects of pH changes on systems ASC and B in rabbit ileum

Influx of D-aspartate (D-Asp), L-glutamate (L-Glu), and serine (Ser) across the brush-border membrane of the intact mucosa from rabbit ileum has been examined. L-Glu influx is chloride independent and completely sodium dependent. D-Asp and L-Glu share a transport system with a maximum transport rate of 1 micromol. cm-2. h-1 and an apparent affinity constant (K1/2) of approximately 0.3 mM. The function of this transport system is pH insensitive between pH 5.65 and 8.2, and bipolar amino acids do not affect the way in which the transport system handles D-Asp and L-Glu. The characteristics of this transport system match those of system X-AG. L-Glu and Ser share a transporter for which the inhibitor constant (Ki) of L-Glu against Ser decreases from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum rate of transport remains unaffected at approximately 10 micromol. cm-2. h-1. The Ki values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive contribution to Ser influx (0.8 micromol. cm-2. h-1 at 1 mM Ser) are the same at both pH values. The L-Glu-sensitive transport of Ser together with the contribution of system bo,+ account for approximately 50% of Ser influx at pH 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by system B is reduced by >95% at pH 5.65. At pH 7. 2 Ki of Ser against transport of leucine (Leu) by system B is 18 mM and Ki of Leu against transport of Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitive transport of Ser are performed by an equivalent of system ASC. Supplementary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small intestine.