Relationships between the Na(+)/K(+) pump and ATP and ADP content in mouse pancreatic islets: effects of meglitinide and glibenclamide

Chronic exposure to KATP channel openers results in attenuated glucose sensing in hypothalamic GT1-7 neurons

Individuals with Type 1 diabetes (T1D) are often exposed to recurrent episodes of hypoglycaemia. This reduces hormonal and behavioural responses that normally counteract low glucose in order to maintain glucose homeostasis, with altered responsiveness of glucose sensing hypothalamic neurons implicated. Although the molecular mechanisms are unknown, pharmacological studies implicate hypothalamic ATP-sensitive potassium channel (K_ATP) activity, with K_ATP openers (KCOs) amplifying, through cell hyperpolarization, the response to hypoglycaemia. Although initial findings, using acute hypothalamic KCO delivery, in rats were promising, chronic exposure to the KCO NN414 worsened the responses to subsequent hypoglycaemic challenge. To investigate this further we used GT1-7 cells to explore how NN414 affected glucose-sensing behaviour, the metabolic response of cells to hypoglycaemia and K_ATP activity. GT1-7 cells exposed to 3 or 24 h NN414 exhibited an attenuated hyperpolarization to subsequent hypoglycaemic challenge or NN414, which correlated with diminished K_ATP activity. The reduced sensitivity to hypoglycaemia was apparent 24 h after NN414 removal, even though intrinsic K_ATP activity recovered. The NN414-modified glucose responsiveness was not associated with adaptations in glucose uptake, metabolism or oxidation. K_ATP inactivation by NN414 was prevented by the concurrent presence of tolbutamide, which maintains K_ATP closure. Single channel recordings indicate that NN414 alters K_ATP intrinsic gating inducing a stable closed or inactivated state. These data indicate that exposure of hypothalamic glucose sensing cells to chronic NN414 drives a sustained conformational change to K_ATP, probably by binding to SUR1, that results in loss of channel sensitivity to intrinsic metabolic factors such as MgADP and small molecule agonists.

Prolonged L-alanine exposure induces changes in metabolism, Ca2+ handling and desensitization of insulin secretion in clonal pancreatic β-cells

Acute insulin-releasing actions of amino acids have been studied in detail, but comparatively little is known about the β-cell effects of long-term exposure to amino acids. The present study examined the effects of prolonged exposure of β-cells to the metabolizable amino acid L-alanine. Basal insulin release or cellular insulin content were not significantly altered by alanine culture, but acute alanine-induced insulin secretion was suppressed by 74% (P<0.001). Acute stimulation of insulin secretion with glucose, KCl or KIC (2-oxoisocaproic acid) following alanine culture was not affected. Acute alanine exposure evoked strong cellular depolarization after control culture, whereas AUC (area under the curve) analysis revealed significant (P<0.01) suppression of this action after culture with alanine. Compared with control cells, prior exposure to alanine also markedly decreased (P<0.01) the acute elevation of [Ca2+]i (intracellular [Ca2+]) induced by acute alanine exposure. These diminished stimulatory responses were partially restored after 18 h of culture in the absence of alanine, indicating reversible amino-acid-induced desensitization. 13C NMR spectra revealed that alanine culture increased glutamate labelling at position C4 (by 60%; P<0.01), as a result of an increase in the singlet peak, indicating increased flux through pyruvate dehydrogenase. Consistent with this, protein expression of the pyruvate dehydrogenase kinases PDK2 and PDK4 was significantly reduced. This was accompanied by a decrease in cellular ATP (P<0.05), consistent with diminished insulin-releasing actions of this amino acid. Collectively, these results illustrate the phenomenon of β-cell desensitization by amino acids, indicating that prolonged exposure to alanine can induce reversible alterations to metabolic flux, Ca2+ handling and insulin secretion.

Elevation in intracellular long-chain acyl-coenzyme A esters lead to reduced beta-cell excitability via activation of adenosine 5'-triphosphate-sensitive potassium channels

Closure of pancreatic beta-cell ATP-sensitive potassium (K(ATP)) channels links glucose metabolism to electrical activity and insulin secretion. It is now known that saturated, but not polyunsaturated, long-chain acyl-coenyzme A esters (acyl-CoAs) can potently activate K(ATP) channels when superfused directly across excised membrane patches, suggesting a plausible mechanism to account for reduced beta-cell excitability and insulin secretion observed in obesity and type 2 diabetes. However, reduced beta-cell excitability due to elevation of endogenous saturated acyl-CoAs has not been confirmed in intact pancreatic beta-cells. To test this notion directly, endogenous acyl-CoA levels were elevated within primary mouse beta-cells using virally delivered overexpression of long-chain acyl-CoA synthetase-1 (AdACSL-1), and the effects on beta-cell K(ATP) channel activity and cell excitability was assessed using the perforated whole-cell and cell-attached patch-clamp technique. Data indicated a significant increase in K(ATP) channel activity in AdACSL-1-infected beta-cells cultured in medium supplemented with palmitate/oleate but not with the polyunsaturated fat linoleate. No changes in the ATP/ADP ratio were observed in any of the groups. Furthermore, AdACSL-1-infected beta-cells (with palmitate/oleate) showed a significant decrease in electrical responsiveness to glucose and tolbutamide and a hyperpolarized resting membrane potential at 5 mm glucose. These results suggest a direct link between intracellular fatty ester accumulation and K(ATP) channel activation, which may contribute to beta-cell dysfunction in type 2 diabetes.

Interaction of Aqueous Extract of Pleurotus pulmonarius (Fr.) Quel-Champ. with Glyburide in Alloxan Induced Diabetic Mice

Mushrooms are low calorie food with very little fat and are highly suitable for obese persons. With no starch and very low sugars, they are the 'delight of the diabetics'. Combination of herbal drugs (or isolated phytochemicals) is found to be beneficial in certain diseases when given along with conventional drugs. The aim of the present study was to evaluate the effects of aqueous extract of Pleurotus pulmonarius (Lentinaceae) (called as PP-aqu) and its interaction with glyburide in alloxan induced diabetic mice. The diabetic mice treated were with PP-aqu (500 mg/kg, p.o.) alone or combination with glyburide (10 mg/kg, p.o.) for 28 days. Blood samples were collected by orbital sinus puncture using heparinized capillary glass tubes and were analyzed for serum glucose on 0, 7th, 14th, 21st and 28th days. Body weights and mortality were noted during the study period. In oral glucose tolerance test (OGTT), glucose (2.5 g/kg, p.o.) was administered with either vehicle, PP-aqu alone or in combination with glyburide and serum glucose level analyzed at 0, 30, 60 and 120 min after drug administration. Administration of PP-aqu (500 mg/kg) and its combination with glyburide (10 mg/kg) significantly (P < 0.001) decreased serum glucose level in diabetic mice. In OGTT, glyburide or PP-aqu treatment alone or their combination produced significant (P < 0.001) increase in glucose threshold. Thus we suggest that P. pulmonarius showed potent and synergistic antihyperglycemic effect in combination with glyburide.

Fuel-induced amplification of insulin secretion in mouse pancreatic islets exposed to a high sulfonylurea concentration: role of the NADPH/NADP+ ratio

AIMS/HYPOTHESIS

The aim of this study was to examine whether the cytosolic NADPH/NADP+ ratio of beta cells serves as an amplifying signal in fuel-induced insulin secretion and whether such a function is mediated by cytosolic alpha-ketoglutarate.

METHODS

Pancreatic islets and islet cells were isolated from albino mice by collagenase digestion. Insulin secretion of incubated or perifused islets was measured by ELISA. The NADPH and NADP+ content of incubated islets was determined by enzymatic cycling. The cytosolic Ca2+ concentration ([Ca2+]c) in islets was measured by microfluorimetry and the activity of ATP-sensitive K+ channels in islet cells by patch-clamping.

RESULTS

Both 30 mmol/l glucose and 10 mmol/l alpha-ketoisocaproate stimulated insulin secretion and elevated the NADPH/NADP+ ratio of islets preincubated in the absence of fuel. The increase in the NADPH/NADP+ ratio was abolished in the presence of 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels). However, alpha-ketoisocaproate, but not glucose, still stimulated insulin secretion. That glipizide did not inhibit alpha-ketoisocaproate-induced insulin secretion was not the result of elevated [Ca2+]c, as glucose caused a more marked [Ca2+]c increase. Insulin release triggered by glipizide alone was moderately amplified by dimethyl alpha-ketoglutarate (which is cleaved to produce cytosolic alpha-ketoglutarate), but there was no indication of a signal function of cytosolic alpha-ketoglutarate.

CONCLUSIONS/INTERPRETATION

The results strongly suggest that the NADPH/NADP+ ratio in the beta cell cytosol does not serve as an amplifying signal in fuel-induced insulin release. The study supports the view that amplification results from the intramitochondrial production of citrate by citrate synthase and from the associated export of citrate into the cytosol.

Investigation of the effects of sulfonylurea exposure on pancreatic beta cell metabolism

Prolonged exposure of pancreatic beta cells to the sulfonylureas glibencamide and tolbutamide induces subsequent desensitization to the actions of these drugs. The precise mechanisms underlying this desensitization remain unknown, prompting the present study, which investigated the impact of prolonged sulfonylurea exposure on glucose and energy metabolism using clonal pancreatic BRIN-BD11 beta cells. Following prolonged exposure to tolbutamide, BRIN-BD11 beta cells were incubated in the presence of [U-(13)C]glucose, and isotopomer analysis revealed that there was a change in the ratio of flux through pyruvate carboxylase (EC 6.4.1.1) and pyruvate dehydrogenase (EC 1.2.4.1, EC 2.3.1.12, EC 1.8.1.4). Energy status in intact BRIN-BD11 cells was determined using (31)P-NMR spectroscopy. Exposure to tolbutamide did not alter the nucleotide triphosphate levels. Collectively, data from the present study demonstrate that prolonged exposure of beta cells to tolbutamide results in changes in flux through key enzymes involved in glucose metabolism that, in turn, may impact on glucose-induced insulin secretion.

Selective loss of glucose-induced amplification of insulin secretion in mouse pancreatic islets pretreated with sulfonylurea in the absence of fuels

AIMS/HYPOTHESIS

The beta cell metabolism of glucose, and some other fuels, initiates insulin secretion by closure of ATP-sensitive K+ channels and amplifies the secretory response via unknown metabolic intermediates. The aim of this study was to further characterise the mechanism responsible for the metabolic amplification of insulin secretion.

MATERIALS AND METHODS

Pancreatic islets were isolated from albino mice by collagenase digestion. Insulin secretion in perifused islets was determined by ELISA. Bioluminometry was used to determine the ATP and ADP content of the incubated islets.

RESULTS

After perifusing islets for 60 min with 2.7 micromol/l glipizide (closing all ATP-sensitive K+ channels) in the absence of any fuel, perifusion with a test medium containing 2.7 micromol/l glipizide plus 30 mmol/l glucose did not enhance insulin secretion. However, test media supplemented with 2.7 micromol/l glipizide plus either 10 mmol/l alpha-ketoisocaproate or 10 mmol/l 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid amplified the glipizide-induced insulin secretion. In pancreatic islets preincubated for 60 min with 2.7 micromol/l glipizide in the absence of any fuel, 40 min incubations in the presence of 2.7 micromol/l glipizide plus 30 mmol/l glucose or plus 10 mmol/l alpha-ketoisocaproate produced an increase in the ATP content, no change in the ADP content and a rather small increase in the ATP:ADP ratio. The corresponding effects of glucose and alpha-ketoisocaproate were similar.

CONCLUSIONS/INTERPRETATION

These results suggest that metabolic amplification of fuel-induced insulin secretion is not mediated by changes in the beta cell content of ATP and ADP, but might be due to export of citrate cycle intermediates to the beta cell cytosol.

Phosphatidylinositol 3-kinase-mediated effects of glucose on vacuolar H+-ATPase assembly, translocation, and acidification of intracellular compartments in renal epithelial cells

Vacuolar H+-ATPases (V-ATPases) are a family of ATP-driven proton pumps. They maintain pH gradients between intracellular compartments and are required for proton secretion out of the cytoplasm. Mechanisms of extrinsic control of V-ATPase are poorly understood. Previous studies showed that glucose is an important regulator of V-ATPase assembly in Saccharomyces cerevisiae. Human V-ATPase directly interacts with aldolase, providing a coupling mechanism for glucose metabolism and V-ATPase function. Here we show that glucose is a crucial regulator of V-ATPase in renal epithelial cells and that the effect of glucose is mediated by phosphatidylinositol 3-kinase (PI3K). Glucose stimulates V-ATPase-dependent acidification of the intracellular compartments in human proximal tubular cells HK-2 and porcine renal epithelial cells LLC-PK1. Glucose induces rapid ATP-independent assembly of the V1 and Vo domains of V-ATPase and extensive translocation of the V-ATPase V1 and Vo domains between different membrane pools and between membranes and the cytoplasm. In HK-2 cells, glucose stimulates polarized translocation of V-ATPase to the apical plasma membrane. The effects of glucose on V-ATPase trafficking and assembly can be abolished by pretreatment with the PI3K inhibitor LY294002 and can be reproduced in glucose-deprived cells by adenoviral expression of the constitutively active catalytic subunit p110alpha of PI3K. Taken together these data provide evidence that, in renal epithelial cells, glucose plays an important role in the control of V-ATPase-dependent acidification of intracellular compartments and V-ATPase assembly and trafficking and that the effects of glucose are mediated by PI3K-dependent signaling.

Regulation of ATP/ADP in pancreatic islets

ATP and ADP levels are critical regulators of glucose-stimulated insulin secretion. In many aerobic cell types, the phosphorylation potential (ATP/ADP/P(i)) is controlled by sensing mechanisms inherent in mitochondrial metabolism that feed back and induce compensatory changes in electron transport. To determine whether such regulation may contribute to stimulus-secretion coupling in islet cells, we used a recently developed flow culture system to continuously and noninvasively measure cytochrome c redox state and oxygen consumption as indexes of electron transport in perifused isolated rat islets. Increasing substrate availability by increasing glucose increased cytochrome c reduction and oxygen consumption, whereas increasing metabolic demand with glibenclamide increased oxygen consumption but not cytochrome c reduction. The data were analyzed using a kinetic model of the dual control of electron transport and oxygen consumption by substrate availability and energy demand, and ATP/ADP/P(i) was estimated as a function of time. ATP/ADP/P(i) increased in response to glucose and decreased in response to glibenclamide, consistent with what is known about the effects of these agents on energy state. Therefore, a simple model representing the hypothesized role of mitochondrial coupling in governing phosphorylation potential correctly predicted the directional changes in ATP/ADP/P(i). Thus, the data support the notion that mitochondrial-coupling mechanisms, by virtue of their role in establishing ATP and ADP levels, may play a role in mediating nutrient-stimulated insulin secretion. Our results also offer a new method for continuous noninvasive measures of islet cell phosphorylation potential, a critical metabolic variable that controls insulin secretion by ATP-sensitive K(+)-dependent and -independent mechanisms.

Differential effects of glucose and glyburide on energetics and Na+ levels of betaHC9 cells: nuclear magnetic resonance spectroscopy and respirometry studies

In the present study, noninvasive (31)P and (23)Na(+)-nuclear magnetic resonance (NMR) technology and respirometry were used to compare the effect of high glucose (30 mmol/l) with the effect of the antidiabetic sulfonylurea (SU) compound glyburide (GLY) on energy metabolism, Na(+) flux, insulin, and cAMP release of continuously superfused beta-HC9 cells encapsulated in microscopic agarose beads. Both high glucose and GLY increased oxygen consumption in beta-HC9 cells (15-30%) with a maximal effect at 8 mmol/l for glucose and at 250 nmol/l for GLY. At the same time, insulin release from beta-cells increased by 15- and 25-fold with high glucose or GLY, respectively. The P-creatine (PCr) level was greatly increased and inorganic phosphate (P(i)) was decreased with 30 mmol/l glucose in contrast to the decreased level of PCr and increased P(i) with GLY. ATP levels remained unchanged during both interventions. Studies on isolated mitochondria of beta-HC9 cells showed that GLY added to mitochondria oxidizing glutamine or glutamate abolished the stimulation of respiration by ADP (state 3) meanwhile leaving state 3 respiration unchanged during oxidation of other substrates. Exposure of beta-HC9 cells to 5 mmol/l glucose decreased intracellular Na(+) levels monitored by (23)Na(+)-NMR spectroscopy and 30 mmol/l glucose resulted in a further decrease in cytosolic Na(+). In contrast, Na(+) increased when 1 micro mol/l GLY was added to the perfusate containing 5 mmol/l glucose. These data support the hypothesis that glucose activates the beta-cell through a "push mechanism" due to substrate pressure enhancing fuel flux, energy production, and extrusion of Na(+) from the cells in contrast to SU receptor (SUR)-1 inhibitors, which may modify intermediary and energy metabolism secondarily through a "pull mechanism" due to higher energy demand resulting from increased ion fluxes and the exocytotic work load.

Correlation between GABA release from rat islet beta-cells and their metabolic state

Pancreatic beta-cells express glutamate decarboxylase (GAD), which is responsible for the production and release of gamma-aminobutyric acid (GABA). Over a 24-h culture period, total GABA release by purified rat beta-cells is eightfold higher than the cellular GABA content and can thus be used as an index of cellular GAD activity. GABA release is 40% reduced by glucose (58 pmol/10(3) cells at 10 mM glucose vs. 94 pmol at 3 mM glucose, P < 0.05). This suppressive effect of glucose was not observed when glucose metabolism was blocked by mannoheptulose or 2,4-dinitrophenol; it was amplified when ATP-dependent beta-cell activities were inhibited by addition of diazoxide, verapamil, or cycloheximide or by reduction of extracellular calcium levels; it was counteracted when beta-cell functions were activated by nonmetabolized agents, such as glibenclamide, IBMX, glucagon, or glucacon-like peptide-1 (GLP-1), which are known to stimulate calcium-dependent activities, such as hormone release and calcium-dependent ATPases. These observations suggest that GABA release from beta-cells varies with the balance between ATP-producing and ATP-consuming activities in the cells. Less GABA is released in conditions of elevated glucose metabolism, and hence ATP production, but this effect is counteracted by ATP-dependent activities. The notion that increased cytoplasmic ATP levels can suppress GAD activity in beta-cells, and hence GABA production and release, is compatible with previous findings on ATP suppression of brain GAD activity.

Modulation of islet ATP content by inhibition or stimulation of the Na(+)/K(+) pump

High (30 mM) K(+), known to cause beta-cell membrane depolarisation, significantly decreased the islet total ATP content, supporting the view that beta-cell membrane depolarisation can activate the ATP-consuming Na(+)/K(+) pump. Ouabain (1 mM) did not change the islet ATP content after 5-15 min of incubation in the absence or presence of 3 mM glucose but reduced it after 30 min, and in the presence of 20 mM glucose, the reduction by ouabain occurred already after 15 min. Incubation of islets with ouabain for 60 min decreased the islet ATP content in the presence of 3, 10 or 20 mM glucose or 30 mM K(+). Also, the islet glucose oxidation rate was decreased by ouabain. When K(+) deficiency was used to inhibit the Na(+)/K(+) pump, no change in ATP content was observed irrespective of glucose concentration, although K(+) deficiency caused a slight inhibition of the glucose oxidation rate. Diazoxide reduced the islet glucose oxidation rate and increased the islet ATP content in the presence of 20 mM glucose. There may exist a feedback mechanism decreasing the flow of glucose metabolism in response to reduced ATP consumption by the Na(+)/K(+) pump.