Am J Physiol Endocrinol Metab 290: E494-E499, 2006.
First published October 25, 2005; doi:10.1152/ajpendo.00304.2005
0193-1849/06 $8.00
Glucose inhibits GABA release by pancreatic 
-cells through an increase in GABA shunt activity
Chen Wang,1
Karen Kerckhofs,1
Mark Van de Casteele,1
Ilse Smolders,2
Daniel Pipeleers,1 and
Zhidong Ling1
1Diabetes Research Center, Brussels Free University-Vrije Universiteit Brussel, and Juvenile Diabetes Research Foundation Center for Beta Cell Therapy; and 2Department of Pharmaceutical Chemistry and Drug Analysis, Brussels Free University-VUB, Brussels, Belgium
Submitted 5 July 2005
; accepted in final form 17 October 2005
 |
ABSTRACT
|
|---|
GABA is the major inhibitory neurotransmitter in the nervous system. It is also released by the insulin-producing 
-cells, providing them with a potential paracrine regulator. Because glucose was found to inhibit GABA release, we investigated whether extracellular GABA can serve as a marker for glucose-induced mitochondrial activity and thus for the functional state of -cells. GABA release by rat and human -cells was shown to reflect net GABA production, varying with the functional state of the cells. Net GABA production is the result of GABA formation through glutamate decarboxylase (GAD) and GABA catabolism involving a GABA-transferase (GABA-T)-mediated shunt to the TCA cycle. GABA-T exhibits Km values for GABA (1.25 mM) and for 
-ketoglutarate (-KG; 0.49 mM) that are, respectively, similar to and lower than those in brain. The GABA-T inhibitor 
-vinyl GABA was used to assess the relative contribution of GABA formation and catabolism to net production and release. The nutrient status of the -cells was found to regulate both processes. Glutamine dose-dependently increased GAD-mediated formation of GABA, whereas glucose metabolism shunts part of this GABA to mitochondrial catabolism, involving -KG-induced activation of GABA-T. In absence of extracellular glutamine, glucose also contributed to GABA formation through aminotransferase generation of glutamate from -KG; this stimulatory effect increased GABA release only when GABA-T activity was suppressed. We conclude that GABA release from -cells is regulated by glutamine and glucose. Glucose inhibits glutamine-driven GABA formation and release through increasing GABA-T shunt activity by its cellular metabolism. Our data indicate that GABA release by -cells can be used to monitor their metabolic responsiveness to glucose irrespective of their insulin-secretory activity.
glutamine; -aminobutyric acid; -aminobutyric acid transferase
PANCREATIC -CELLS are one of the few cell types outside the brain that express glutamate decarboxylase (GAD) with the capacity of producing biologically relevant amounts of -aminobutyric acid (GABA) (5, 17, 22). Nutrients influence GABA release, as shown by its glutamine dependence and the inhibitory effect of glucose (20). The present study investigates the mechanisms involved in this glucose-induced suppression. It was undertaken in the perspective that extracellular GABA concentrations can reflect the functional state of the -cells, in particular a glucose-regulated function that is not necessarily expressed by the rates of insulin release. Previous work had indeed indicated that GABA release proceeds independently of insulin release (20, 26). We were thus interested in further examining the potential significance of extracellular GABA as a functional marker for -cells (23) rather than investigating the membrane events underlying GABA release (2, 24).
GABA release by -cells is stimulated by extracellular glutamine, which provides the glutamate substrate for GAD. It is expected to be influenced by mitochondrial GABA-transaminase (GABA-T), which forms succinic semialdehyde in a transamination reaction with -ketoglutarate (6, 11). GABA catabolism can thus influence the TCA cycle through formation of succinate from succinate semialdehyde. Conversely, glucose-induced changes in mitochondrial activity could then affect the rate of GABA catabolism and subsequent release.
Although the glucose-induced inhibition of GABA release has been a consistent finding in primary -cells, it is presently unclear to what extent this mechanism influences the activity of neighboring cells. Although GABA is known as the main inhibitory neurotransmitter in brain (1, 10), a physiological role in the pancreas has not yet been clearly identified. The compound has been implicated in inhibitory actions on local immune and inflammatory responses (22). It has also been proposed as a -cell signal that mediates an inhibitory effect on neighboring -cells (8, 24) and thus participates in the coordination of insulin and glucagon release. The observation that GABA production and release by -cells is regulated by glucose (7, 20, 26), can be seen as further suggestive support to a paracrine role on neighboring cells. However, the reported effects of glucose on GABA release by -cells seem controversial (4, 7, 9, 20, 26): in our study on primary rat -cells, glucose induced a dose-dependent decrease in cellular GABA content and release (20, 26), which is in agreement with the study of Hayashi et al. in rat islet cells (9) but not with experiments on a TC6 cell line (7), in which GABA release was increased after exposure to high glucose. Such a discrepancy might be caused by differences in the metabolic and functional state of the cells or in the different experimental conditions used for primary or tumoral cells. Because the present study intends to identify a marker function of extracellular GABA, it may also help explain differences among preparations.
|
MATERIALS AND METHODS
|
|---|
Preparation and culture of rat pancreatic -cells.
Adult male Wistar rats were bred according to Belgian regulations of animal welfare. Isolated islets and purified -cells were prepared as previously described (18). Freshly isolated -cells were reaggregated for 2 h in a rotatory shaking incubator at 37°C (13) and then statically cultured for 16 h in Ham's F-10 medium (GIBCO, Paisley, UK) supplemented with 2 mM glutamine (Cambrex, Verviers, Belgium), 6.1 mM glucose, 0.5% (wt/vol) charcoal-extracted bovine serum albumin (BSA type V, RIA grade; Boehringer Mannheim, Mannheim, Germany), 0.075 g/l penicillin, and 0.1 g/l streptomycin. After this overnight culture period, -cell aggregates were cultured for another 24 h in Ham's F-10 medium containing 0.5% charcoal-extracted BSA, 0.075 g/l penicillin, and 0.1 g/l streptomycin supplement with different concentrations of glucose, glutamine, glutamic acid -methyl ester (Sigma), and -vinyl-GABA (GVG, Sigma). At the end of culture, cells and media were collected for GABA and insulin determinations and, in one series of experiments, for measurement of GAD and GABA-transferase (GABA-T) activity.
Preparation and culture of human pancreatic -cell preparations.
Human pancreata were obtained from organ donors at European hospitals affiliated with Eurotransplant Foundation (Leiden, The Netherlands). They were processed by the Beta-Cell Bank of the Juvenile Diabetes Research Foundation Center for Beta Cell Therapy in Brussels, with the purpose of preparing islet cell grafts for a clinical trial in diabetic patients. During this procedure, the endocrine-enriched fraction is separated from the exocrine-enriched fraction. When isolated islet cell fractions did not fulfill the quality control criteria for transplantation, they could be made available to approved research projects when fulfilling the guidelines of Eurotransplant and of the Ethics Committee of the Brussels Free University-Vrije Universiteit Brussel. Islet-enriched fractions were precultured for 1–2 days in Ham's F-10 medium containing 0.075 g/l penicillin, 0.1 g/l streptomycin, 6.1 mM glucose, 2 mM glutamine, 0.5% BSA, 2% human serum, and 2 mM nicotinamide (Biowhittaker, Verviers, Belgium) and then for 4–11 days in the above medium but without serum and nicotinamide (12). After this preculture period, islet cell preparations (40–70% -cells, 10–25% -cells) were cultured for 24 h in Ham's F-10 medium containing 2 mM glutamine, 0.5% charcoal-extracted BSA, 0.075 g/l penicillin, and 0.1 g/l streptomycin at different glucose concentrations in the presence or absence of GVG, after which cells and media were collected for GABA and insulin assays.
GABA, glutamate, and insulin assays.
GABA/glutamate were measured by isocratic elution and electrochemical detection/gradient elution and fluorescence detection after precolumn derivatization with o-phthaldialdehyde and microbore liquid chromatography separation (21) in filtered media and cell extracts following sonication in PBS with 0.1% BSA and centrifugation. Insulin was assayed in culture media and in acetic acid extracts of cells (18).
GABA-T and GAD activity assay.
GABA-T activity in cell extracts (40 x 103 -cells) was measured as the formation of glutamate (19), in 100-µl assay buffer [50 mM potassium phosphate, pH 8.0, 0.25 mM DTT (Sigma), and 0.1 mM pyridoxal phosphate (PLP; ICN Biomedicals) (25)] with or without GABA (5 mM) and -ketoglutarate (-KG, 2 mM). The reaction was performed at 37°C for 60 min and stopped by heating at 97°C for 7 min. The glutamate content was then measured by microbore liquid chromatography (21). GABA-T activity was calculated by subtracting the values measured in the absence of GABA and expressed as picomoles per 103 cells per hour. GAD activity in -cell extracts was measured as the formation of GABA at saturated concentration of glutamate (30 mM) according to Winnock et al. (26), with some modification. GABA was measured at the start and after a 1-h incubation at 37°C; the net increase was taken as an index of GAD activity and expressed as picomoles per 103 cells per hour.
Data expression and statistical analysis.
Insulin and GABA were expressed as picomoles per 103 cells at the start of the experiments. Data represent means ± SE of n independent experiments. Statistical significance of differences was determined using either the Student's t-test or ANOVA.
|
RESULTS
|
|---|
GABA-T activity in -cell extracts.
GABA-T activity in rat -cell extracts was low (0.18 pmol·10–3 cells·h–1) when measured in the absence of -KG and GABA but markedly increased, up to 132.5 pmol·10–3 cells·h–1, in the presence of both substrates, which reached their maximal effect at, respectively, 2 and 5 mM (Fig. 1). Km values, as derived from Hanes plots, were 0.49 mM for -KG and 1.25 mM for GABA. Changes in PLP concentration exerted little effect upon GABA-T activity (maximal stimulation of 19 ± 9% at 0.1 mM), whereas the same concentration stimulated GAD activity twofold (Fig. 2). GVG inhibited GABA-T activity dose dependently with a 75% decrease at 0.04 mg/ml and >95% at 1 mg/ml; in this concentration range, GVG exerted no effect on GAD activity (data not shown), irrespective of the PLP concentration.
Effect of GABA-T inhibition on GABA production by -cells.
Addition of GVG (0.04 to 1 mg/ml) to 72-h cultures of rat -cells did not influence cell survival (data not shown). The agent was found to increase GABA content in both medium and cells (Fig. 3), indicating that inhibition of GABA-T results in a higher GABA net production by -cells. This effect is dose dependent with a maximum at GVG-0.2 mg/ml. In the presence of GVG, high glucose (20 mM) did not suppress GABA release and net production, suggesting that this earlier reported glucose effect involves GABA-T activity (Fig. 3). On the other hand, GVG-induced inhibition of GABA-T did not block glucose stimulation of insulin release and the concomitant degranulation of -cells (Fig. 3); an inhibitory effect was seen at a GVG concentration (1 mg/ml) that was fivefold higher than that with maximal effect on GABA production and therefore not directly related to the latter.
Similar effects were noticed in human -cells: inhibition of GABA-T activity by GVG (0.2 mg/ml) 1) increased medium and cellular GABA content during 24-h culture and 2) interfered with the glucose-induced suppression of GABA production but not with glucose-induced insulin release (Table 1). The glucose-suppressive effect on GABA production also disappears when glucose phosphorylation is blocked by mannoheptulose (Table 2) but is then associated with a suppression of glucose-induced insulin release (Table 2).
Effect of glucose on GABA-T and GAD activities in -cells.
No differences in GABA-T activity were measured after rat -cells had been cultured for 24 h at either 3 or 20 mM glucose (Fig. 4). The presence of GVG in the culture medium reduced subsequently measured GABA-T by 80–90% without eliciting a difference between both glucose concentrations (Fig. 4). As previously reported, culture at 20 mM glucose did not affect GAD activity in rat -cells (26); this was also the case in human -cell preparations (Fig. 5).
View larger version (16K):
[in this window]
[in a new window]
|
Fig. 4. Effect of GVG on GABA-T activity in rat -cells following 24-h culture at 3 or 20 mM glucose in the presence or absence of GVG (0.2 mg/ml). Enzyme activity was determined in cell extracts prepared at the end of culture. Data represent means ± SE of 4–5 independent experiments. Statistical significance of difference between conditions with and without GVG: *P < 0.05.
|
|
View larger version (38K):
[in this window]
[in a new window]
|
Fig. 5. Effect of glucose on glutamate decarboxylase (GAD65) protein expression (A) and activity (B) in human -cells following 24-h culture at 3, 6, or 20 mM glucose. Data represent mean ± SE of 3–5 independent experiments.
|
|
Effect of glucose on substrate provision for GAD.
Because culture media are routinely supplemented with glutamine, and because glutamine can provide glutamate substrate for GAD activity and subsequent GABA production, we examined whether variations in extracellular glutamine concentration influenced the glucose-mediated suppression of GABA production. When extracellular glutamine was increased from 2 mM (control) to 5 and 10 mM, medium GABA content also increased, but the suppressive effect of high glucose remained present (Table 3). Addition of the methyl ester of glutamate also increased GABA production without preventing the glucose effect (data not shown).
In the absence of extracellular glutamine, medium GABA levels at 3 and 20 mM glucose were, respectively, 30 to 70% lower than at 2 mM glutamine. They remained clearly detectable over 24 h, suggesting endogenous provision of the glutamate substrate for GAD. They increased twofold when GABA-T was inhibited (GVG 0.2 mg/ml), indicating that 50% of this endogenously driven GABA production is catabolized through mitochondrial activity. When this GABA-T-mediated catabolism is blocked by GVG, higher GABA levels were measured at 20 mM glucose than at 3 mM, suggesting that glucose increases endogenous provision of glutamate in the absence of an extracellular source (Table 4). Addition of aminooxyacetate (AOA, 0.5 to 2 mg/ml), an aminotransferase inhibitor, dose-dependently suppressed GABA release without altering insulin release (Table 5). This observation is indicative for the formation of glutamate from -KG through aminotransferase activity. Glucose-induced generation of -KG may thus increase glutamate formation as well as GABA catabolism. The provision of glutamate from another source and the activity of GABA-T will thus determine how glucose will influence the amount of GABA released in the medium (Table 5 and Fig. 6).
|
DISCUSSION
|
|---|
We recently reported that medium and cellular GABA serve as cell-specific markers for living rat and human -cells during culture (23). Medium GABA content reflects net GABA production by the cells during 2- to 24-h incubations (23). The present study confirms that elevated glucose levels reduce net GABA production by rat -cells (20, 26) and extrapolates this finding to human -cells. This glucose effect requires metabolism of the sugar as it is blocked by mannoheptulose, an inhibitor of glucose phosphorylation, but is transmitted through another signal than glucose-induced insulin release, since both effects can vary independently of each other. Medium GABA levels are thus not only an extracellular marker for the mass of living -cells but also for their glucose responsiveness.
The present study further demonstrated that the glucose-suppressive effect on net GABA production is not mediated through an inhibition of GAD expression or activity. In fact, both GAD protein expression and enzyme activity were slightly increased by high glucose. A major determinant of GAD activity in intact -cells appears to be the provision of substrate as shown by the dose-dependent increase in GABA release induced by extracellular glutamine. It is unlikely that glucose reduces the availability of this substrate, since an increase in glutamine concentration or addition of the methyl ester of glutamate did not neutralize or reduce the glucose-suppressive effect. An additional argument is that cellular and medium glutamate content were not decreased after culture at high glucose (Ref. 15 and our unpublished observations).
Our data indicate that the glucose-induced reduction in net GABA formation results from an increase in GABA metabolism to succinic semialdehyde involving GABA-T (Fig. 6). This enzyme was now demonstrated in extracts of rat -cells, with a Km for GABA (1.25 mM) that is similar to that in rat brain (16) and a Km for -KG (0.49 mM) that is lower (16); in contrast to the brain enzyme and to GAD, its activity was virtually not influenced by PLP. In -cell extracts, GABA-T, but not GAD, was blocked by GVG, which is known to covalently bind to the enzyme and thus inhibit access to and catabolism of GABA (14). This GVG effect was also achieved in intact -cells, as indicated by the marked increase in medium and cellular GABA content following culture with this compound. Presence of GVG interfered with the glucose-induced suppression of GABA release but not with glucose-induced insulin release. It can thus be concluded that 1) the observed effects of GVG are not caused by inhibition of glucose metabolism, and 2) the suppressive effect of glucose is mediated through GABA-T activity. In view of the dependence of GABA-T on its substrate -KG, it is likely that a glucose-induced increase in this metabolite results in an increased GABA-T activity by providing its substrate and thus in an increased transition of GABA to succinic semialdehyde (Fig. 6).
The glucose-induced GABA catabolism was seen in conditions where glutamine was added to the culture medium, thus providing an exogenous source for the glutamate substrate of the GABA formation. In the absence of glutamine, GABA formation was markedly lower but still proceeded at 
25% of the rate measured at 2 mM glutamine. In this condition, GABA release was found 1) to be markedly suppressed by an aminotransferase inhibitor, suggesting its dependence on glutamate formation from -KG, and 2) to be elevated by GVG, indicating also its dependence on GABA catabolism. Culture at high glucose should then stimulate both pathways through an increase in -KG (Fig. 6), which is supported by the absence of an effect on GABA release unless GABA-T is blocked. These observations indicate that, in the absence of exogenous glutamine, GABA formation uses glutamate that is formed from -KG through aminotransferase activities. They are consistent with previous work in which glucose was shown to generate glutamate through aminotransferase activities (3). Interestingly, the latter study was also undertaken on cells incubated without extracellular glutamine. In the presence of exogenous glutamine, the aminotransferase-mediated formation of glutamate might be negligible, so that the glucose effect is primarily occurring through a GABA-T mediated increase in GABA metabolism (Fig. 6).
In conclusion, the amount of GABA released by -cells depends on the rate of GAD-mediated GABA formation and GABA-T-mediated GABA catabolism. In both rat and human -cells, the activities of both GAD and GABA-T are highly dependent on their substrates and, hence, on the nutrient and metabolic state of the cells. Extracellular glutamine increases GABA formation dose dependently, whereas the associated GABA release is inhibited by glucose-activated catabolism of formed GABA. In the absence of extracellular glutamine, glutamate provision for GABA formation seems dependent on aminotransferases that use -KG as substrate. In this condition, glucose-induced -KG is thought to increase GABA formation as well as GABA catabolism, which explains the elevated GABA release when GABA-T is blocked at high glucose. Glucose can thus inhibit or stimulate GABA release from -cells depending on whether the GAD substrate is provided by extracellular glutamine or by mitochondrially generated -KG and whether the mitochondrial GABA-T activity is activated or not. Our observations provide an explanation for apparent discrepancies in reports on glucose effects on GABA release (7, 9, 20, 26). They extend the notion that extracellular GABA is a marker for living -cells and for their glucose responsiveness (20, 23, 26) and now define in vitro conditions in which the glucose-induced changes in GABA release can be used to assess the mitochondrial activity of the cells. This finding may add a potentially useful marker to quality control tests of islet preparations that are intended for transplantation.
|
GRANTS
|
|---|
This study was supported by grants from the Belgian Science Policy (Interuniversity Attraction Pole P5/17), the Juvenile Diabetes Research Foundation (Grant 4-2001-434), and by grants from the Belgian Fund Scientific Research-FWO (Grant G0375.00). K. Kerckhofs is a research fellow (aspirant) of the Belgian Fund for Scientific Research Flanders.
|
ACKNOWLEDGMENTS
|
|---|
We thank An Van Hemelrijck and Ria Berckmans for their support to the microbore liquid chromatography determination, and Frederic Winnock and Rene De Proft for technical assistance. Part of this work was presented at the Islet Study Group Meeting 2003, Brussels, Belgium.
|
FOOTNOTES
|
|---|
Address for reprint requests and other correspondence: Z. Ling, Diabetes Research Center, Brussels Free University-VUB, Laarbeeklaan 103, B-1090 Brussels, Belgium (e-mail: Zhidong.Ling{at}vub.ac.be)
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.
|
REFERENCES
|
|---|
- Awapara J, Landua AJ, Fuerst R, and Seale B. Free gamma-aminobutyric acid in brain. J Biol Chem 187: 35–39, 1950.[Free Full Text]
- Braun M, Wendt A, Buschard K, Salehi A, Sewing S, Gromada J, and Rorsman P. GABAB receptor activation inhibits exocytosis in rat pancreatic beta-cells by G-protein-dependent activation of calcineurin. J Physiol 559: 397–409, 2004.[Abstract/Free Full Text]
- Brennan L, Corless M, Hewage C, Malthouse JP, McClenaghan NH, Flatt PR, and Newsholme P. 13C NMR analysis reveals a link between L-glutamine metabolism, D-glucose metabolism and gamma-glutamyl cycle activity in a clonal pancreatic beta-cell line. Diabetologia 46: 1512–1521, 2003.[CrossRef][ISI][Medline]
- Franklin IK and Wollheim CB. GABA in the endocrine pancreas: its putative role as an islet cell paracrine-signalling molecule. J Gen Physiol 123: 185–190, 2004.[Free Full Text]
- Gammelsaeter R, Froyland M, Aragon C, Danbolt NC, Fortin D, Storm-Mathisen J, Davanger S, and Gundersen V. Glycine, GABA and their transporters in pancreatic islets of Langerhans: evidence for a paracrine transmitter interplay. J Cell Sci 117: 3749–3758, 2004.[Abstract/Free Full Text]
- Garry DJ, Coulter HD, McIntee TJ, Wu JY, and Sorenson RL. Immunoreactive GABA transaminase within the pancreatic islet is localized in mitochondria of the B-cell. J Histochem Cytochem 35: 831–836, 1987.[Abstract]
- Gaskins HR, Baldeon ME, Selassie L, and Beverly JL. Glucose modulates gamma-aminobutyric acid release from the pancreatic beta TC6 cell line. J Biol Chem 270: 30286–30289, 1995.[Abstract/Free Full Text]
- Gilon P, Bertrand G, Loubatieres-Mariani MM, Remacle C, and Henquin JC. The influence of gamma-aminobutyric acid on hormone release by the mouse and rat endocrine pancreas. Endocrinology 129: 2521–2529, 1991.[Abstract]
- Hayashi M, Yamada H, Uehara S, Morimoto R, Muroyama A, Yatsushiro S, Takeda J, Yamamoto A, and Moriyama Y. Secretory granule-mediated co-secretion of L-glutamate and glucagon triggers glutamatergic signal transmission in islets of Langerhans. J Biol Chem 278: 1966–1974, 2003.[Abstract/Free Full Text]
- Krnjevic K. Role of GABA in cerebral cortex. Can J Physiol Pharmacol 75: 439–451, 1997.[CrossRef][ISI][Medline]
- Kugler P. In situ measurements of enzyme activities in the brain. Histochem J 25: 329–338, 1993.[CrossRef][ISI][Medline]
- Ling Z and Pipeleers DG. Prolonged exposure of human beta cells to elevated glucose levels results in sustained cellular activation leading to a loss of glucose regulation. J Clin Invest 98: 2805–2812, 1996.[ISI][Medline]
- Ling Z, Van de Casteele M, Eizirik DL, and Pipeleers DG. Interleukin-1beta-induced alteration in a beta-cell phenotype can reduce cellular sensitivity to conditions that cause necrosis but not to cytokine-induced apoptosis. Diabetes 49: 340–345, 2000.[Abstract]
- Lippert B, Metcalf BW, Jung MJ, and Casara P. 4-Amino-hex-5-enoic acid, a selective catalytic inhibitor of 4-aminobutyric-acid aminotransferase in mammalian brain. Eur J Biochem 74: 441–445, 1977.[ISI][Medline]
- MacDonald MJ and Fahien LA. Glutamate is not a messenger in insulin secretion. J Biol Chem 275: 34025–34027, 2000.[Abstract/Free Full Text]
- Maitre M, Ciesielski L, Cash C, and Mandel P. Purification and studies on some properties of the 4-aminobutyrate:2-oxoglutarate transaminase from rat brain. Eur J Biochem 52: 157–169, 1975.[CrossRef][ISI][Medline]
- Okada Y, Taniguchi H, and Schimada C. High concentration of GABA and high glutamate decarboxylase activity in rat pancreatic islets and human insulinoma. Science 194: 620–622, 1976.[Abstract/Free Full Text]
- Pipeleers DG, in't Veld PA, Van de Winkel M, Maes E, Schuit FC, and Gepts W. A new in vitro model for the study of pancreatic A and B cells. Endocrinology 117: 806–816, 1985.[Abstract]
- Schor DS, Struys EA, Hogema BM, Gibson KM, and Jakobs C. Development of a stable-isotope dilution assay for gamma-aminobutyric acid (GABA) transaminase in isolated leukocytes and evidence that GABA and beta-alanine transaminases are identical. Clin Chem 47: 525–531, 2001.[Abstract/Free Full Text]
- Smismans A, Schuit F, and Pipeleers D. Nutrient regulation of gamma-aminobutyric acid release from islet beta cells. Diabetologia 40: 1411–1415, 1997.[CrossRef][ISI][Medline]
- Smolders I, Sarre S, Michotte Y, and Ebinger G. The analysis of excitatory, inhibitory and other amino acids in rat brain microdialysates using microbore liquid chromatography. J Neurosci Methods 57: 47–53, 1995.[CrossRef][ISI][Medline]
- Tian J, Lu Y, Zhang H, Chau CH, Dang HN, and Kaufman DL. Gamma-aminobutyric acid inhibits T cell autoimmunity and the development of inflammatory responses in a mouse type 1 diabetes model. J Immunol 173: 5298–5304, 2004.[Abstract/Free Full Text]
- Wang C, Ling Z, and Pipeleers D. Comparison of cellular and medium insulin and GABA content as markers for living -cells. Am J Physiol Endocrinol Metab 288: E307–E313, 2005.[Abstract/Free Full Text]
- Wendt A, Birnir B, Buschard K, Gromada J, Salehi A, Sewing S, Rorsman P, and Braun M. Glucose inhibition of glucagon secretion from rat alpha-cells is mediated by GABA released from neighboring beta-cells. Diabetes 53: 1038–1045, 2004.[Abstract/Free Full Text]
- White HL. 4-Aminobutyrate:2-oxoglutarate aminotransferase in blood platelets. Science 205: 696–698, 1979.[Abstract/Free Full Text]
- Winnock F, Ling Z, De Proft R, Dejonghe S, Schuit F, Gorus F, and Pipeleers D. Correlation between GABA release from rat islet -cells and their metabolic state. Am J Physiol Endocrinol Metab 282: E937–E942, 2002.[Abstract/Free Full Text]
This article has been cited by other articles:
|
|
|
|
 
P. Bansal and Q. Wang
Insulin as a physiological modulator of glucagon secretion
Am J Physiol Endocrinol Metab,
October 1, 2008;
295(4):
E751 - E761.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
M. M. Bonaventura, P. N. Catalano, A. Chamson-Reig, E. Arany, D. Hill, B. Bettler, F. Saravia, C. Libertun, and V. A. Lux-Lantos
GABAB receptors and glucose homeostasis: evaluation in GABAB receptor knockout mice
Am J Physiol Endocrinol Metab,
January 1, 2008;
294(1):
E157 - E167.
[Abstract]
[Full Text]
[PDF]
|
|
|
|
|
|
|
C. Wang, R. Mao, M. Van De Casteele, D. Pipeleers, and Z. Ling
Glucagon-like peptide-1 stimulates GABA formation by pancreatic beta-cells at the level of glutamate decarboxylase
Am J Physiol Endocrinol Metab,
April 1, 2007;
292(4):
E1201 - E1206.
[Abstract]
[Full Text]
[PDF]
|
|
|
Copyright © 2006 by the American Physiological Society.
TOP
ABSTRACT
MATERIALS AND METHODS
), 1 mM (
), 2 mM (
), and 5 mM (
). GABA concentrations were varied at 
), 0.8 mM (
), 1 mM (
), and 5 mM (
). Bottom: ratio of substrate concentration over enzyme activity as a function of substrate concentration (Hanes plots).
