Metabolic Role of GABA in the Secretory Function of Pancreatic β-Cells: Its Hypothetical Implication in β-Cell Degradation in Type 2 Diabetes

The stimulus-secretion coupling of a glucose-induced release is generally attributed to the metabolism of the hexose in the β-cells in the glycolytic pathway and the citric acid cycle. Glucose metabolism generates an increased cytosolic concentration of ATP and of the ATP/ADP ratio that closes the ATP-dependent K⁺-channel at the plasma membrane. The resultant depolarization of the β-cells opens voltage-dependent Ca²⁺-channels at the plasma membrane that triggers the exocytosis of insulin secretory granules. The secretory response is biphasic with a first and transient peak followed by a sustained phase. The first phase is reproduced by a depolarization of the β-cells with high extracellular KCl maintaining the KATP-channels open with diazoxide (triggering phase); the sustained phase (amplifying phase) depends on the participation of metabolic signals that remain to be determined. Our group has been investigating for several years the participation of the β-cell GABA metabolism in the stimulation of insulin secretion by three different secretagogues (glucose, a mixture of L-leucine plus L-glutamine, and some branched chain alpha-ketoacids, BCKAs). They stimulate a biphasic secretion of insulin accompanied by a strong suppression of the intracellular islet content of gamma-aminobutyric acid (GABA). As the islet GABA release simultaneously decreased, it was concluded that this resulted from an increased GABA shunt metabolism. The entrance of GABA into the shunt is catalyzed by GABA transaminase (GABAT) that transfers an amino group between GABA and alpha-ketoglutarate, resulting in succinic acid semialdehyde (SSA) and L-glutamate. SSA is oxidized to succinic acid that is further oxidized in the citric acid cycle. Inhibitors of GABAT (gamma-vinyl GABA, gabaculine) or glutamic acid decarboxylating activity (GAD), allylglycine, partially suppress the secretory response as well as GABA metabolism and islet ATP content and the ATP/ADP ratio. It is concluded that the GABA shunt metabolism contributes together with the own metabolism of metabolic secretagogues to increase islet mitochondrial oxidative phosphorylation. These experimental findings emphasize that the GABA shunt metabolism is a previously unrecognized anaplerotic mitochondrial pathway feeding the citric acid cycle with a β-cell endogenous substrate. It is therefore a postulated alternative to the proposed mitochondrial cataplerotic pathway(s) responsible for the amplification phase of insulin secretion. It is concluded the new postulated alternative suggests a possible new mechanism of β-cell degradation in type 2 (perhaps also in type 1) diabetes.

Fumarate Hydratase Deletion in Pancreatic β Cells Leads to Progressive Diabetes

We explored the role of the Krebs cycle enzyme fumarate hydratase (FH) in glucose-stimulated insulin secretion (GSIS). Mice lacking Fh1 in pancreatic β cells (Fh1βKO mice) appear normal for 6–8 weeks but then develop progressive glucose intolerance and diabetes. Glucose tolerance is rescued by expression of mitochondrial or cytosolic FH but not by deletion of Hif1α or Nrf2. Progressive hyperglycemia in Fh1βKO mice led to dysregulated metabolism in β cells, a decrease in glucose-induced ATP production, electrical activity, cytoplasmic [Ca²⁺]_i elevation, and GSIS. Fh1 loss resulted in elevated intracellular fumarate, promoting succination of critical cysteines in GAPDH, GMPR, and PARK 7/DJ-1 and cytoplasmic acidification. Intracellular fumarate levels were increased in islets exposed to high glucose and in islets from human donors with type 2 diabetes (T2D). The impaired GSIS in islets from diabetic Fh1βKO mice was ameliorated after culture under normoglycemic conditions. These studies highlight the role of FH and dysregulated mitochondrial metabolism in T2D.

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Fh1 loss in β cells causes progressive Hif1α-independent diabetes

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Fh1 loss in β cells impairs ATP generation, electrical activity, and GSIS

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Elevated fumarate is a feature of diabetic murine and human islets

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“Normoglycemia” restores GSIS in Fh1βKO islets

Direct Stimulation of Islet Insulin Secretion by Glycolytic and Mitochondrial Metabolites in KCl-Depolarized Islets

We have previously demonstrated that islet depolarization with 70 mM KCl opens Cx36 hemichannels and allows diffusion of small metabolites and cofactors through the β-cell plasma membrane. We have investigated in this islet “permeabilized” model whether glycolytic and citric acid cycle intermediates stimulate insulin secretion and how it correlates with ATP production (islet content plus extracellular nucleotide accumulation). Glycolytic intermediates (10 mM) stimulated insulin secretion and ATP production similarly. However, they showed differential sensitivities to respiratory chain or enzyme inhibitors. Pyruvate showed a lower secretory capacity and less ATP production than phosphoenolpyruvate, implicating an important role for glycolytic generation of ATP. ATP production by glucose-6-phosphate was not sensitive to a pyruvate kinase inhibitor that effectively suppressed the phosphoenolpyruvate-induced secretory response and islet ATP rise. Strong suppression of both insulin secretion and ATP production induced by glucose-6-phosphate was caused by 10 μM antimycin A, implicating an important role for the glycerophosphate shuttle in transferring reducing equivalents to the mitochondria. Five citric acid cycle intermediates were investigated for their secretory and ATP production capacity (succinate, fumarate, malate, isocitrate and α-ketoglutarate at 5 mM, together with ADP and/or NADP⁺ to feed the NADPH re-oxidation cycles). The magnitude of the secretory response was very similar among the different mitochondrial metabolites but α-ketoglutarate showed a more sustained second phase of secretion. Gabaculine (1 mM, a GABA-transaminase inhibitor) suppressed the second phase of secretion and the ATP-production stimulated by α-ketoglutarate, supporting a role for the GABA shuttle in the control of glucose-induced insulin secretion. None of the other citric acid intermediates essayed showed any suppression of both insulin secretion or ATP-production by the presence of gabaculine. We propose that endogenous GABA metabolism in the “GABA-shunt” facilitates ATP production in the citric acid cycle for an optimal insulin secretion.

KCl -Permeabilized Pancreatic Islets: An Experimental Model to Explore the Messenger Role of ATP in the Mechanism of Insulin Secretion

Our previous work has demonstrated that islet depolarization with KCl opens connexin36 hemichannels in β-cells of mouse pancreatic islets allowing the exchange of small metabolites with the extracellular medium. In this study, the opening of these hemichannels has been further characterized in rat islets and INS-1 cells. Taking advantage of hemicannels'opening, the uptake of extracellular ATP and its effect on insulin release were investigated. 70 mM KCl stimulated light emission by luciferin in dispersed rat islets cells transduced with the fire-fly luciferase gene: it was suppressed by 20 mM glucose and 50 μM mefloquine, a specific connexin36 inhibitor. Extracellular ATP was taken up or released by islets depolarized with 70 mM KCl at 5 mM glucose, depending on the external ATP concentration. 1 mM ATP restored the loss of ATP induced by the depolarization itself. ATP concentrations above 5 mM increased islet ATP content and the ATP/ADP ratio. No ATP uptake occurred in non-depolarized or KCl-depolarized islets simultaneously incubated with 50 μM mefloquine or 20 mM glucose. Extracellular ATP potentiated the secretory response induced by 70 mM KCl at 5 mM glucose in perifused rat islets: 5 mM ATP triggered a second phase of insulin release after the initial peak triggered by KCl-depolarization itself; at 10 mM, it increased both the initial, KCl-dependent, peak and stimulated a greater second phase of secretion than at 5 mM. These stimulatory effects of extracellular ATP were almost completely suppressed by 50 μM mefloquine. The magnitude of the second phase of insulin release due to 5 mM extracellular ATP was decreased by addition of 5 mM ADP (extracellular ATP/ADP ratio = 1). ATP acts independently of KATP channels closure and its intracellular concentration and its ATP/ADP ratio seems to regulate the magnitude of both the first (triggering) and second (amplifying) phases of glucose-induced insulin secretion.

Delineation of glutamate pathways and secretory responses in pancreatic islets with β-cell-specific abrogation of the glutamate dehydrogenase

The amino acid profile and the secretory responses of glutamate dehydrogenase (GDH)-deficient β-cells are characterized. This study shows that GDH is essential for both insulin release and net glutamate synthesis evoked by glucose. Adding cellular glutamate restored the full development of glucose-stimulated insulin secretion, showing the requirement for permissive glutamate levels.

In pancreatic β-cells, glutamate dehydrogenase (GDH) modulates insulin secretion, although its function regarding specific secretagogues is unclear. This study investigated the role of GDH using a β-cell–specific GDH knockout mouse model, called βGlud1^−/−. The absence of GDH in islets isolated from βGlud1^–/– mice resulted in abrogation of insulin release evoked by glutamine combined with 2-aminobicyclo[2.2.1]heptane-2-carboxylic acid or l-leucine. Reintroduction of GDH in βGlud1^–/– islets fully restored the secretory response. Regarding glucose stimulation, insulin secretion in islets isolated from βGlud1^–/– mice exhibited half of the response measured in control islets. The amplifying pathway, tested at stimulatory glucose concentrations in the presence of KCl and diazoxide, was markedly inhibited in βGlud1^–/– islets. On glucose stimulation, net synthesis of glutamate from α-ketoglutarate was impaired in GDH-deficient islets. Accordingly, glucose-induced elevation of glutamate levels observed in control islets was absent in βGlud1^–/– islets. Parallel biochemical pathways, namely alanine and aspartate aminotransferases, could not compensate for the lack of GDH. However, the secretory response to glucose was fully restored by the provision of cellular glutamate when βGlud1^–/– islets were exposed to dimethyl glutamate. This shows that permissive levels of glutamate are required for the full development of glucose-stimulated insulin secretion and that GDH plays an indispensable role in this process.

Reduction of plasma membrane glutamate transport potentiates insulin but not glucagon secretion in pancreatic islet cells

Glutamate is generated during nutrient stimulation of pancreatic islets and has been proposed to act both as an intra- and extra-cellular messenger molecule. We demonstrate that glutamate is not co-secreted with the hormones from intact islets or purified α- and β-cells. Fractional glutamate release was 5-50 times higher than hormone secretion. Furthermore, various hormone secretagogues did not elicit glutamate efflux. Interestingly, epinephrine even decreased glutamate release while increasing glucagon secretion. Rather than being co-secreted with hormones, we show that glutamate is mainly released via plasma membrane excitatory amino acid transporters (EAAT) by uptake reversal. Transcripts for EAAT1, 2 and 3 were present in both rat α- and β-cells. Inhibition of EAATs by L-trans-pyrrolidine-2,4-dicarboxylate augmented intra-cellular glutamate and α-ketoglutarate contents and potentiated glucose-stimulated insulin secretion from islets and purified β-cells without affecting glucagon secretion from α-cells. In conclusion, intra-cellular glutamate-derived metabolite pools are linked to glucose-stimulated insulin but not glucagon secretion.

Tacrolimus-induced diabetes in rats courses with suppressed insulin gene expression in pancreatic islets

An animal model of post-transplant diabetes was induced in rats by treating them daily with 0.1 mg/kg body weight of tacrolimus (FK506) in two i.p. injections. Rats developed hyperglycaemia and glucose intolerance after 9 days of treatment. Pancreatic islets, isolated from treated rats on different days, showed a decreased capacity to secrete insulin in response to 20 mM glucose at days 7 and 14. This suppression of insulin secretion was preceded by a reduction of the islet insulin content on day 5 that was progressively decreasing until the end of the treatment (day 14). Islet content of insulin mRNAs, transcribed from rat insulin genes 1 and 2, was strongly suppressed, similar to the insulin content, at days 7 and 14. Islet mass was not strikingly modified by tacrolimus treatment: the DNA content was slightly decreased at the end (day 14) and the rate of islet cell apoptosis slightly increased. Tacrolimus-induced diabetes in the rat seems to be mainly provoked by a decreased insulin gene transcription with little or no alteration of islet mass. This explains that the observed suppression of all the islet and animal parameters studied was completely reversed 2 weeks after interrupting tacrolimus treatment.

Oxo-4-methylpentanoic acid directs the metabolism of GABA into the Krebs cycle in rat pancreatic islets

OMP (oxo-4-methylpentanoic acid) stimulates by itself a biphasic secretion of insulin whereas L-leucine requires the presence of L-glutamine. L-Glutamine is predominantly converted into GABA (gamma-aminobutyric acid) in rat islets and L-leucine seems to promote its metabolism in the 'GABA shunt' [Fernández-Pascual, Mukala-Nsengu-Tshibangu, Martín del Río and Tamarit-Rodríguez (2004) Biochem. J. 379, 721-729]. In the present study, we have investigated how 10 mM OMP affects L-glutamine metabolism to uncover possible differences with L-leucine that might help to elucidate whether they share a common mechanism of stimulation of insulin secretion. In contrast with L-leucine, OMP alone stimulated a biphasic insulin secretion in rat perifused islets and decreased the islet content of GABA without modifying its extracellular release irrespective of the concentration of L-glutamine in the medium. GABA was transaminated to L-leucine whose intracellular concentration did not change because it was efficiently transported out of the islet cells. The L-[U-14C]-Glutamine (at 0.5 and 10.0 mM) conversion to 14CO2 was enhanced by 10 mM OMP within 30% and 70% respectively. Gabaculine (250 microM), a GABA transaminase inhibitor, suppressed OMP-induced oxygen consumption but not L-leucine- or glucose-stimulated respiration. It also suppressed the OMP-induced decrease in islet GABA content and the OMP-induced increase in insulin release. These results support the view that OMP promotes islet metabolism in the 'GABA shunt' generating 2-oxo-glutarate, in the branched-chain alpha-amino acid transaminase reaction, which would in turn trigger GABA deamination by GABA transaminase. OMP, but not L-leucine, suppressed islet semialdehyde succinic acid reductase activity and this might shift the metabolic flux of the 'GABA shunt' from gamma-hydroxybutyrate to succinic acid production.

Effect of maternal low-protein diet and taurine on the vulnerability of adult Wistar rat islets to cytokines

AIMS/HYPOTHESIS

A maternal low-protein diet has been shown to induce an increased susceptibility of fetal islets to cytokines, but this effect can be avoided by maternal taurine supplementation. Here, we question whether these effects persist until adulthood in the offspring, despite the animal having a normal diet after weaning.

METHODS

Pregnant Wistar rats received a diet of either 20% or 8% protein (control [C group] and recuperated [R group] respectively), which was or was not supplemented with taurine (control treated with taurine [CT group] and recuperated treated with taurine [RT group] respectively) during gestation and lactation. When the female offspring reached adulthood, an OGTT was performed. In a second stage, islets were isolated from these offspring, then pretreated or not with taurine, and subsequently treated with cytokines.

RESULTS

Fasting glycaemia was higher (p<0.05) and insulinaemia was lower (p<0.01) in the R group than in the C group. Taurine supplementation decreased insulinaemia in the CT group and tended to increase it in the RT group. After the OGTT, glycaemia in R animals was not different from that in the C group, despite a blunted insulin response (p<0.05) which was restored by taurine. Supplementation in C-group mothers led to a weak glucose intolerance. In vitro, more apoptotic cells were observed in R islets after cytokines treatment (p<0.01). The addition of taurine to the culture medium in the R and C groups protected the islets from the cytokines (p<0.01). Maternal taurine supplementation decreased the sensitivity of islets in the RT group (p<0.01), but increased sensitivity in the CT group (p<0.01).

CONCLUSIONS/INTERPRETATION

The increased vulnerability of islets to cytokines due to a restriction of protein during fetal development was still evident when the offspring reached adulthood. The low-protein diet also induced hyperglycaemia in the presence of lower insulinaemia. Taurine supplementation protected adult islets of the R group from cytokine toxicity and restored the insulinaemia. However, unnecessary supplementation of taurine could have detrimental effects.

Similar effects of succinic acid dimethyl ester and glucose on islet calcium oscillations and insulin release

The relative contribution of glycolysis vs. oxidative metabolism to the stimulus secretion coupling mechanism of beta-cells was investigated in isolated islets. For that purpose, the secretory and intracellular calcium responses of islets to both glucose and succinic acid dimethyl ester (SAD) were compared. After 45 min of rat islet perifusion in the absence of substrates, the maximum secretory responses to glucose (20 mmol/L) and SAD (10 mmol/L) were qualitatively and quantitatively indistinguishable. Malonic acid dimethyl ester (a permeable citric acid cycle inhibitor) suppressed the insulin secretory response to both 20 mmol/L glucose and 10 mmol/L SAD (-70% on average). The inhibitor decreased within 70% the rate of 14CO2-production from 10 mmol/L [2-(14)C]pyruvate without affecting the rate of 20 mmol/L D-[5-(3)H]glucose utilization. Both, 11.1 mmol/L glucose and 10 mmol/L SAD, elevated the intracellular calcium concentration and induced a similar pattern of oscillations that were rapidly ablated by 20 mmol/L malonic acid dimethyl ester. However, the intracellular concentration of calcium declined to basal values several minutes after the introduction of the inhibitor in the presence of SAD whereas it remained elevated in the case of glucose.

IN CONCLUSION

(1) An exclusive increase of mitochondrial metabolism in pancreatic islets was sufficient to mimic the effects of glucose on intracellular calcium and insulin secretion. (2) Islet glycolysis and/or the re-oxidation of cytoplasmic NADH allowed the maintenance of an elevated, though non-oscillating, intracellular calcium concentration, but a reduced response to glucose.

An osmotic-sensitive taurine pool is localized in rat pancreatic islet cells containing glucagon and somatostatin

Previous reports have dealt with the hypoglycemic properties of taurine and its effects on insulin secretion by adult and fetal isolated islets. We have studied the presence and cellular distribution of taurine in rat islets, the conditions to evoke its release, and its possible modulatory action on insulin secretion. We localized taurine by techniques of double immunolabeling in most glucagon-positive cells and in some somatostatin-positive cells, whereas insulin-positive cells were not labeled with the taurine antibody. Although high-glucose stimulation did not evoke any taurine release, a hyposmotic solution (17% osmolarity reduction) induced a specific phasic release of taurine and GABA (34 and 52% increase on their basal release rate). On the other hand, taurine (10 mmol/l) application slightly reduced the second phase of insulin secretion induced by glucose stimulation. In conclusion, taurine is highly concentrated in glucagon-containing cells of the islet periphery. It is not liberated by glucose stimulation but is strongly released under hyposmotic conditions. All of these data suggest that taurine plays an osmoregulatory role in alpha-cells.

Lactate production in pancreatic islets

Lactate production, glucose utilization, glucose oxidation, and insulin release were studied in islets from rat and ob/ob mice. Lactate was determined with a highly sensitive method, based on esterification, subsequent separation, and quantitation with high-performance liquid chromatography. There was a significant lactate production in the absence of glucose, which increased with glucose concentrations up to 3 mmol/l, reaching its half-maximal rate in the presence of 0.2-1.0 mmol/l glucose in both species. Glucose utilization displayed a wider glucose concentration dependence, with a K0.5 value between 3 and 10 mmol/l glucose. The rates of glucose utilization and lactate production were similar at 3 mmol/l glucose in rat islets and at about 6 mmol/l glucose in ob/ob mice islets. Saturation of lactate production at low glucose concentrations is probably contributing to the observed preferential stimulation of oxidative metabolism at higher concentrations. D-Mannoheptulose caused a marked inhibition of glucose utilization and glucose oxidation at 20 mmol/l glucose in islets from rat or ob/ob mice, as would be expected from a competitive inhibition of glucokinase. By contrast, D-mannoheptulose reduced only marginally the islet metabolism at 3 mmol/l glucose, which is consistent with an effective mannoheptulose-induced inhibition of the glucokinase-dependent, minor part of glucose phosphorylation at this low glucose concentration.

Stimulation of islet protein kinase C translocation by palmitate requires metabolism of the fatty acid

The secretory, metabolic, and signaling aspects of glucose/palmitate interaction on beta-cell function have been studied on rat islets. Palmitate potentiated the glucose-induced insulin response of perifused islets at suprathreshold (>3 mmol/l) sugar concentrations. This potentiating effect could be suppressed by 8-bromo-cGMP, which also blocks palmitate metabolism. Palmitate did not modify glucose utilization, but it slightly reduced glucose oxidation and concomitantly increased lactate production. The very low rate of palmitate oxidation (80-fold lower than that of 20 mmol/l glucose) might explain its lack of effect on glycolysis and hence that the glucose/fatty acid cycle is inoperative in islet cells. However, glucose determines the metabolic fate of exogenous palmitate, which is mainly diverted toward lipid synthesis at high sugar concentrations and might then generate lipid messengers for cell signaling. Palmitate did not increase glucose-induced production of inositol-1,4,5-trisphosphate, but it stimulated the translocation of protein kinase C activity from a cytosolic to a particulate fraction at 20 but not at 3 mmol/l glucose. This increased translocation was partially or completely blocked by hydroxycitrate or 8-bromo-cGMP, respectively, which are agents interfering with palmitate metabolism (inhibiting lipid synthesis). The metabolic interaction between glucose and palmitate might generate lipid messengers (diacylglycerol, phosphatidylserine) necessary for the activation of islet protein kinase C, which would in turn result in a potentiation of glucose-induced insulin secretion.

Low lactate dehydrogenase and high mitochondrial glycerol phosphate dehydrogenase in pancreatic beta-cells. Potential role in nutrient sensing

Nutrient metabolism was examined with regard to insulin secretion in purified rat islet beta- and non-beta-cells, beta-cell lines, and hepatocytes. Lactate dehydrogenase (LDH) activity (nanomoles.min-1.mg protein-1) was remarkably low in the glucose-sensitive INS-1 cell line (15.7) and in beta-cells (22.3). Thus, beta-cell LDH was respectively 8-, 122-, 17-, and 136-fold lower than in islet non-beta, liver, HIT-T15, and RINm5F cells. Plasma membrane lactate transport activity was 3-10-fold lower in beta- or INS-1 cells than in the other cell types. Conversely, mitochondrial glycerol phosphate dehydrogenase was strongly expressed only in beta- and INS-1 cells. The significance of these findings to nutrient recognition was explored using INS-1 cells as a model of native beta-cells. Glucose-stimulated lactate output and glucose utilization were, respectively, 12- and 5-fold lower in INS-1 than in RINm5F cells. Each process was entirely blocked by respiratory chain inhibitors in INS-1 cells, whereas glucose utilization was barely affected and lactate output stimulated in RINm5F cells. Glucose oxidation represented 73% of total utilization in INS-1 cells, but only 9% in RINm5F cells. Absolute rates of glucose oxidation, and the extent of mitochondrial NAD(P) reduction, were similar in the two cell types, and glucose stimulated insulin secretion 1.9-fold in INS-1 and 1.4-fold in RINm5F cells. The mitochondrial substrates, monomethyl succinate, pyruvate, and leucine, each triggered secretion in INS-1 cells. The balance of LDH, plasma membrane lactate transport, and mitochondrial glycerol phosphate dehydrogenase activities therefore appear to be important in beta- and INS-1 cell glucose recognition to ensure that mitochondrial oxidation is the principle fate of pyruvate and NADH produced by glycolysis. The resultant close coupling of glycolysis with mitochondrial oxidation explains the absence in beta-cells of Crabtree and Pasteur effects.

Does cyclic guanosine monophosphate mediate noradrenaline-induced inhibition of islet insulin secretion stimulated by glucose and palmitate?

Noradrenaline inhibits in rat islets the stimulation of insulin secretion induced by glucose and its potentiation by palmitate, but the signalling system responsible remains unknown. We have tested the hypothesis that noradrenaline-induced inhibition is mediated by an elevation of cyclic GMP (cGMP) levels. The analogue 8-Br-cGMP decreases dose-dependently the potentiation by palmitate of glucose-induced insulin secretion, whereas it only slightly affects the proper effect of glucose. Similarly, it abolishes palmitate acceleration of glucose-induced 45Ca2+ uptake without modifying the sugar effect. Finally, 8-Br-cGMP completely inhibits the stimulation of the lipid synthesis de novo induced by palmitate, but not that caused by glucose alone. On the other hand, noradrenaline increases dose-dependently islet cGMP content, with alpha 2-adrenergic specificity. As noradrenaline-induced elevation of cGMP is sensitive to pertussis toxin, it probably results from alpha 2-adrenoceptor activation of islet guanylate cyclase through a guanine nucleotide regulatory protein. It is concluded that the elevated cGMP levels mediate noradrenaline inhibition of lipid synthesis de novo, and hence of acceleration by palmitate of 45Ca2+ uptake and insulin secretion in the presence of glucose.

Insulin, glucagon, somatostatin, and thyrotropin-releasing hormone content and secretion by perifused fetal rat islets during culture

In the neonatal period of the rat, pancreatic thyrotropin-releasing hormone content decreases and the sensitivity of insulin secretion to glucose increases. In adult rat islets, TRH inhibits glucose-induced insulin release. The aim of this study was to investigate whether a high TRH content and release can be part of the explanation for the functional immaturity of neonatal islets. For that purpose, we have measured the tissue content and the secretion of immunoreactive insulin, glucagon, somatostatin and TRH in islets from 21.5-day-old rat fetuses cultured for up to one week. Insulin, glucagon and somatostatin content increased during one week of culture in the presence of 11.1 mmol/l glucose. The TRH content decreased during culture, but did not equal adult values. Insulin, glucagon and somatostatin responses to glucose were present after one week of culture. Glucose had no effect on TRH release in cultured fetal islets, but inhibited TRH release in adult islets. We conclude that glucose can stimulate insulin secretion without inhibiting TRH release, but that a decrease in islet TRH content and a sensitization of TRH secretion to glucose may be important in the full maturation of fetal pancreatic islets.

Norepinephrine inhibits islet lipid metabolism, 45Ca2+ uptake, and insulin secretion

We have previously shown that palmitate potentiates, in isolated islets, glucose-induced stimulation of insulin release, "de novo" lipid synthesis, and 45Ca2+ turnover in a correlative manner. Norepinephrine, a known inhibitor of the secretory response, has now been used to further investigate the relationships among the three phenomena. The amine decreased insulin secretion dose dependently in response to glucose and palmitate with alpha 2-adrenergic specificity. It also reduced similarly the oxidation of 1 mmol/l [U-14C]palmitate as well as the incorporation of 20 mmol/l D-[U-14C]glucose into islet phospholipids and neutral lipids through an alpha 2-adrenergic mechanism. These results indirectly suggest that alpha 2-adrenoceptor stimulation inhibits in islets both palmitate oxidation and esterification through an inactivation of long-chain acyl-CoA synthetase and other enzymes of glycerolipid synthesis. Islet uptake of 45Ca2+ was also decreased by norepinephrine with a similar sensitivity to that shown by insulin release and de novo lipid synthesis. Therefore, it is suggested that alpha 2-adrenoceptor-mediated reduction of the potentiation by palmitate of the secretory response to glucose depends on the inhibition of fatty acid metabolism and the resulting impairment of de novo lipid synthesis and 45Ca2+ turnover.

Effects of L-leucine on palmitate metabolism and insulin release by isolated islets of fed and starved rats

The occurrence of lipid metabolic changes associated with L-leucine (10 mM) stimulation of insulin release was investigated in isolated islets from either fed or starved rats. L-Leucine-stimulated secretion was potentiated by 3 mM glucose and/or 0.5 mM palmitate and was unaffected by 48 h of starvation. Islet palmitate oxidation showed a maximum rate at 3 mM glucose, and starvation increased it almost 2-fold. Regardless of the nutritional state, L-leucine strongly reduced the oxidation of palmitate and increased its incorporation into islet triacylglycerols and phospholipids at 3 mM glucose. This shift of fatty acid metabolism toward esterification might play a role in the mechanism of potentiation of the islet secretory response to L-leucine by glucose and palmitate.

Glucose stimulation of insulin secretion in islets of fed and starved rats and its dependence on lipid metabolism

The influence of a physiologic range of palmitate concentrations (0, 0.25, 0.5, and 1.0 mmol/L) on glucose ability to modify insulin secretion, (U-14C) palmitate oxidation, and (U-14C) glucose incorporation into lipids has been studied in islets isolated from either fed or 48-hour starved rats. Palmitate potentiated the insulin response of fed islets to glucose in a particular dose-related manner. Glucose stimulated secretion was accompanied by a decreased palmitate oxidation and an increased (U-14C) glucose incorporation into di-, tri-acylglycerols, and predominantly into phospholipids. These metabolic parameters showed also a positive dependence on palmitate concentration. Starvation increased islet capacity to oxidize palmitate, rendered it insensitive to glucose inhibition, and inhibited both (U-14C) glucose incorporation into all lipid fractions and sugar induced insulin release. The stimulation of islet lipid synthesis by glucose seems to be limited by the exogenous supply of fatty acids and their rate of oxidation. As judged from (U-14C) glucose incorporation data, the rate of phospholipid biosynthesis showed a significant and positive correlation with insulin secretion. This metabolic pathway might provide islet cells with some lipid intermediates (diacylglycerol and/or specific phospholipids) that have been considered as possible mediators of the calcium messenger system.

Antigenic specificity of a new and potent somatostatin antiserum

The specificity of a new somatostatin antiserum (Ab 6) has been investigated using eight different peptide analogues. For this purpose, somatostatin and all the analogues were tested, in an equimolar concentration range, for their ability to displace 125I-tyr1-somatostatin from its binding to antibodies. Analysis of the displacement curves shows that antiserum Ab 6 predominantly recognizes changes at the central aminoacid sequence of the somatostatin molecule. Its ligand specificity seems to be similar to that of other somatostatin antisera previously described (R-141 and A-101).

Starvation-induced secretory changes of insulin, somatostatin, and glucagon and their modification by 2-bromostearate

The hypothesis was made of an increased oxidation of fatty acids (FFA) and a decrease of their esterification rate contributing to the islet secretory defect during starvation. 2-Bromostearate (BrS), a FFA-oxidation inhibitor, was therefore tested on the islet secretion of insulin, glucagon and somatostatin stimulated by glucose or palmitate under fasted or fed conditions. Starvation for 48 h blocked both the glucose-induced stimulation and inhibition of insulin and somatostatin and the glucagon secretion. BrS completely restored the insulin response and stimulated both somatostatin and glucagon-basal release, the latter inhibition by glucose being partially recovered. Palmitate transient stimulation of insulin and somatostatin and inhibition of glucagon release was turned into a sustained increase in all three cases by addition of BrS. The potentiation by BrS of palmitate secretory effects in "fed" islets and of hormone release in "fasted" islets, apparently suggest that inhibition of FFA-oxidation may play a role in the regulation of islet secretion.

Different effects of the ionophore A-23187 and D-glucose on 45Ca2+ fluxes in isolated islets of ob/ob-mice

Fluxes of 45Ca2+ were studied in beta-cell rich islets of non-inbred ob/ob-mice, using LaCl3 to wash out extra-cellular and superficially bound 45Ca2+. The ionophore A-23187 (10 microM) increased the 45Ca2+ uptake in islets both at 3 and 20 mM D-glucose, the effect being more pronounced after 10 min than after 120 min of incubation. In incubations for 120 min, 20 mM D-glucose induced a higher uptake of 45Ca2+ than did A-23187. The ionophore enhanced the unidirectional efflux of 45Ca2+ from preloaded islets. Pretreatment of islets with 20 mM D-glucose in non-radioactive medium inhibited the subsequent D-glucose-induced 45Ca2+ uptake. Similar pretreatment with A-23187 increased the subsequent ionophore-induced 45Ca2+ uptake. The results suggest that A-23187 acts by catalyzing Ca2+ fluxes across the beta-cell plasma membrane. The different effects of D-glucose and A-23187 on 45Ca2+ fluxes suggest that the two agents act through different mechanisms in the beta-cells.

Effects of valinomycin on Rb+ fluxes, ATP content and insulin release in pancreatic islets

Valinomycin, 0.5-500 nM, was tested for its effects on pancreatic islets microdissected fron non-inbred ob/ob-mice. Valinomycin decreased the islet accumulation Rb+ and the content of ATP in a dose-dependent manner; efflux of Rb+ from pre-loaded islets was not noticeably changed. Rb+ accumulation and ATP content correlated markedly; on the model of linear regression, less than 10% of the change Rb+ accumulation in valinomycin-treated islets was statistically attributable to factors other than ATP. Valinomycin did not cause a prompt inhibition of glucose-stimulated insulin release that could reflect hyperpolarization due to increased K+ permeability. The following conclusions are drawn: 1) The plasma membranes of beta-cells resemble those of neurons in having such a high ion permeability as to be relatively little influenced by valinomycin; 2) Islet accumulation of Rb+ is due to a vectorial catalyst in theplasma membrane rather than to uptake by mitochondria; 3) Rb+ accumulation in islets is ATP-dependent.

Metabolic characteristics of pancreatic beta-cells exposed to calcium-transporting ionophores

The effects of the ionophores A-23187 and X-537 A on glucose metabolism, ATP content and sucrose permeability in pancreatic islets microdissected from obese-hyperglycemic mice were studied. The formation of 14CO2 from 10 mM D-[U-14C] GLUCOSE WAS INHIBITED BY OMISSION OF Ca2+ from the medium. A-23187 (10 muM) induced a further decrease of 14CO2 formation whereas X-537 A (10 muM) had no effect. At 20 mM glucose both A-23187 (48 muM) and X-537 A (43 muM) decreased the 14CO2 formation in the absence of Ca2+ whereas only X-537 A inhibited in the presence of Ca2+. X-537 A (43 muM) also decreased the formation of 3H2O from 20 mM D-[5-3H] glucose. The islet content of ATP was not changed after incubation in media deficient in either Mg2+ or Ca2+. However, omission of both Mg2+ and Ca2+ resulted in about 50% decrease of the ATP content. A-23187 and X-537 A induced dose-dependent decreases of the islet ATP content. X-537 A was much more potent than A-23187. Both ionophores induced stronger depression of the ATP content when Ca2+ was omitted. X-537 A (43 muM) but not A-23187 (48 muM) increased the beta-cell membrane permeability as indicated by an increased sucrose space in relation to the urea space of islets. Such an effect was not obtained with X-537 A at 1 muM or by omission of Ca2+. It is suggested that the marked metabolic effects of the ionophores reflect an impaired mitochondrial metabolism. These metabolic changes should be considered in interpretations of ionophore action on insulin secretion.