Glycerol 3-phosphate-induced ATP production in intact mitochondria from pancreatic B-cells

The Importance of Calcium Ions for Determining Mitochondrial Glycerol-3-Phosphate Dehydrogenase Activity When Measuring Uncoupling Protein 1 (UCP1) Function in Mitochondria Isolated from Brown Adipose Tissue

Glycerol-3-phosphate is an excellent substrate for FAD-linked mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) in brown adipose tissue mitochondria and is regularly used as the primary substrate to measure oxygen consumption and reactive oxygen consumption by these mitochondria. mGPDH converts cytosolic glycerol-3-phosphate to dihydroxyacetone phosphate, feeding electrons directly from the cytosolic side of the mitochondrial inner membrane to the CoQ-pool within the inner membrane. mGPDH activity is allosterically activated by calcium, and when calcium chelators are present in the mitochondrial preparation medium and/or experimental incubation medium, calcium must be added to insure maximal mGPDH activity. It was demonstrated that in isolated brown adipose tissue mitochondria (1) mGPDH enzyme activity is maximal at free calcium ion concentrations in the 350 nM-1 μM range, (2) that ROS production also peaks in the 10-100 nM range in the presence of a UCP1 inhibitory ligand (GDP) but wanes with further increasing calcium concentration, and (3) that oxygen consumption rates peak in the 10-100 nM range with rates being maintained at higher calcium concentrations. This article provides easy-to-follow protocols to facilitate the measurement of mGPDH-dependent UCP1 activity in the presence of calcium for isolated brown adipose tissue mitochondria.

Targeting mitochondria with folic acid and vitamin B12 ameliorates nicotine mediated islet cell dysfunction

Nicotine, one of the well-known highly toxic components of cigarette smoke, causes a number of adverse health effects and diseases. Our previous study has shown that nicotine induces reactive oxygen species (ROS) in islet cell and disrupts islet cell mitochondrial membrane potential (ΔΨm). However, supplementation with folic acid and vitamin B12 were found effective against nicotine induced changes in pancreatic islet cells. But the toxicological effects and underlying mechanisms of nicotine-induced mitochondrial dysfunction is still unknown. In this study, nicotine exposure decreases mitochondrial enzymes (pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, aconitase, malate dehydrogenase) activities by increasing cytosolic Ca2+ level which may contribute to increased mitochondrial ROS production by raising its flow to mitochondria. This in turn produces malondialdehyde and nitric oxide (NO) with a concomitant decrease in the activities of antioxidative enzymes and glutathione levels leading to loss of ΔΨm. Simultaneously, nicotine induces pancreatic islet cell apoptosis by modulating ΔΨm via increased cytosolic Ca2+ level, altered Bcl-2, Bax, cytochrome c, caspase-9, PARP expressions which were prevented by the supplementation of folic acid and vitamin B12 . In conclusion, nicotine alters islet cell mitochondrial redox status, apoptotic machinery, and enzymes to cause disruption in the ΔΨm and supplementation of folic acid and vitamin B12 possibly blunted all these mitochondrial alterations. Therefore, this study may help to determine the pathophysiology of nicotine-mediated islet cell mitochondrial dysfunction.

Silencing of AQP3 induces apoptosis of gastric cancer cells via downregulation of glycerol intake and downstream inhibition of lipogenesis and autophagy

Gastric cancer (GC) has a poor prognosis and is a leading cause of cancer-related death. Optimal therapeutic targets have not been identified. AQP3 is capable of transporting glycerol across the cytomembrane. Previous studies have shown that AQP3 is involved in proliferation, invasion and migration by regulating glycerol and lipid metabolism in diverse cancer cell types. However, the potential roles of glycerol and lipid metabolism in AQP3-related cell apoptosis in GC remain unclear. In this study, we observed that AQP3 expression was upregulated in tumor tissues, and positively correlated with tumor size, lymph node metastasis and glycerol concentration in human GC samples. Silencing of AQP3 resulted in decreased glycerol intake and impaired lipid synthesis, which contributed to increased cell apoptosis. Furthermore, inhibition of autophagy induced by AQP3 knockdown promoted cell apoptosis. Administration of either glycerol or rapamycin restored cell viability, and overexpression of AQP3 increased cell viability by upregulating cellular glycerol metabolism and autophagy. Our study demonstrates that the increase in cell apoptosis of AQP3-deficient GC cells is a consequence of reduced glycerol uptake and lipogenesis and is associated with autophagy inhibition induced by AQP3 deficiency.

The Pancreatic β-Cell: A Bioenergetic Perspective

The pancreatic β-cell secretes insulin in response to elevated plasma glucose. This review applies an external bioenergetic critique to the central processes of glucose-stimulated insulin secretion, including glycolytic and mitochondrial metabolism, the cytosolic adenine nucleotide pool, and its interaction with plasma membrane ion channels. The control mechanisms responsible for the unique responsiveness of the cell to glucose availability are discussed from bioenergetic and metabolic control standpoints. The concept of coupling factor facilitation of secretion is critiqued, and an attempt is made to unravel the bioenergetic basis of the oscillatory mechanisms controlling secretion. The need to consider the physiological constraints operating in the intact cell is emphasized throughout. The aim is to provide a coherent pathway through an extensive, complex, and sometimes bewildering literature, particularly for those unfamiliar with the field.

High Ca2+ load promotes hydrogen peroxide generation via activation of α-glycerophosphate dehydrogenase in brain mitochondria

H(2)O(2) generation associated with α-glycerophosphate (α-GP) oxidation was addressed in guinea pig brain mitochondria challenged with high Ca(2+) load (10 μM). Exposure to 10 μM Ca(2+) induced an abrupt 2.5-fold increase in H(2)O(2) release compared to that measured in the presence of a physiological cytosolic Ca(2+) concentration (100 nM) from mitochondria respiring on 5 mM α-GP in the presence of ADP (2 mM). The Ca(2+)-induced stimulation of H(2)O(2) generation was reversible and unaltered by the uniporter blocker Ru 360, indicating that it did not require Ca(2+) uptake into mitochondria. Enhanced H(2)O(2) generation by Ca(2+) was also observed in the absence of ADP when mitochondria exhibited permeability transition pore opening with a decrease in the NAD(P)H level, dissipation of membrane potential, and mitochondrial swelling. Furthermore, mitochondria treated with the pore-forming peptide alamethicin also responded with an elevated H(2)O(2) generation to a challenge with 10 μM Ca(2+). Ca(2+)-induced promotion of H(2)O(2) formation was further enhanced by the complex III inhibitor myxothiazol. With 20 mM α-GP concentration, stimulation of H(2)O(2) formation by Ca(2+) was detected only in the presence, not in the absence, of ADP. It is concluded that α-glycerophosphate dehydrogenase, which is accessible to and could be activated by a rise in the level of cytosolic Ca(2+), makes a major contribution to Ca(2+)-stimulated H(2)O(2) generation. This work highlights a unique high-Ca(2+)-stimulated reactive oxygen species-forming mechanism in association with oxidation of α-GP, which is largely independent of the bioenergetic state and can proceed even in damaged, functionally incompetent mitochondria.

T cell activation is driven by an ADP-dependent glucokinase linking enhanced glycolysis with mitochondrial reactive oxygen species generation

Mitochondria-originating reactive oxygen species (ROS) control T cell receptor (TCR)-induced gene expression. Here, we show that TCR-triggered activation of ADP-dependent glucokinase (ADPGK), an alternative, glycolytic enzyme typical for Archaea, mediates generation of the oxidative signal. We also show that ADPGK is localized in the endoplasmic reticulum and suggest that its active site protrudes toward the cytosol. The ADPGK-driven increase in glycolytic metabolism coincides with TCR-induced glucose uptake, downregulation of mitochondrial respiration, and deviation of glycolysis toward mitochondrial glycerol-3-phosphate dehydrogenase(GPD) shuttle; i.e., a metabolic shift to aerobic glycolysis similar to the Warburg effect. The activation of respiratory-chain-associated GPD2 results in hyperreduction of ubiquinone and ROS release from mitochondria. In parallel, mitochondrial bioenergetics and ultrastructure are altered. Downregulation of ADPGK or GPD2 abundance inhibits oxidative signal generation and induction of NF-κB-dependent gene expression, whereas overexpression of ADPGK potentiates them.

The regulation of OXPHOS by extramitochondrial calcium

Despite extensive research, the regulation of mitochondrial function is still not understood completely. Ample evidence shows that cytosolic Ca2+ has a strategic task in co-ordinating the cellular work load and the regeneration of ATP by mitochondria. Currently, the paradigmatic view is that Cacyt2+ taken up by the Ca2+ uniporter activates the matrix enzymes pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase and isocitrate dehydrogenase. However, we have recently found that Ca2+ regulates the glutamate-dependent state 3 respiration by the supply of glutamate to mitochondria via aralar, a mitochondrial glutamate/aspartate carrier. Since this activation is not affected by ruthenium red, glutamate transport into mitochondria is controlled exclusively by extramitochondrial Ca2+. Therefore, this discovery shows that besides intramitochondrial also extramitochondrial Ca2+ regulates oxidative phosphorylation. This new mechanism acts as a mitochondrial "gas pedal", supplying the OXPHOS with substrate on demand. These results are in line with recent findings of Satrustegui and Palmieri showing that aralar as part of the malate-aspartate shuttle is involved in the Ca2+-dependent transport of reducing hydrogen equivalents (from NADH) into mitochondria. This review summarises results and evidence as well as hypothetical interpretations of data supporting the view that at the surface of mitochondria different regulatory Ca2+-binding sites exist and can contribute to cellular energy homeostasis. Moreover, on the basis of our own data, we propose that these surface Ca2+-binding sites may act as targets for neurotoxic proteins such as mutated huntingtin and others. The binding of these proteins to Ca2+-binding sites can impair the regulation by Ca2+, causing energetic depression and neurodegeneration.

Folic acid or combination of folic acid and vitamin B(12) prevents short-term arsenic trioxide-induced systemic and mitochondrial dysfunction and DNA damage

The effect of folic acid and folic acid + vitamin B(12) supplementation upon short-term arsenic-induced systemic and pancreatic islet cell mitochondria oxidative stress was investigated in male rats. Arsenic trioxide was administered orally at a dose of 3 mg kg body weight(-1) day(-1) for 30 days, and folic acid and vitamin B(12) were administered at a dose of 36 and 0.63 microg kg body weight(-1) day(-1), respectively, for 30 days. Compared to control, arsenic-treated group showed a significant increase in the levels of systemic oxidative markers, malondialdehyde (MDA), nitric oxide (NO), and hydroxyl radical (OH(-)) formation, which were found decreased significantly after supplementation either with folic acid or a combination of folic acid + vitamin B(12). Similar supplementations were found effective against arsenic-induced oxidative marker changes (MDA, NO, and OH(-)) in pancreatic islet cell mitochondria. Also, low activities of antioxidant defense enzymes such as superoxide dismutase and catalase, and level of antioxidant glutathione, all could regain significantly on supplementations both against systemic and islet cell mitochondria oxidative stress. Results of agarose-gel electrophoresis of DNA from lymphocytes and islet cells of arsenic-exposed rats showed DNA smearing, which could be reduced with simultaneous administration either with folic acid or a combination of folic acid + vitamin B(12). Significantly, similar supplementations were found effective in increasing the urinary clearance of arsenic. Together, these results indicate that folic acid and vitamin B(12) may be effective to reduce the arsenic-induced damage at molecular target level.

Prevention of skin tumorigenesis and impairment of epidermal cell proliferation by targeted aquaporin-3 gene disruption

Aquaporin-3 (AQP3) is a water/glycerol-transporting protein expressed strongly at the plasma membranes of basal epidermal cells in skin. We found that human skin squamous cell carcinoma strongly overexpresses AQP3. A novel role for AQP3 in skin tumorigenesis was discovered using mice with targeted AQP3 gene disruption. We found that AQP3-null mice were remarkably resistant to the development of skin tumors following exposure to a tumor initiator and phorbol ester promoter. Though tumor initiator challenge produced comparable apoptotic responses in wild-type and AQP3-null mice, promoter-induced cell proliferation was greatly impaired in the AQP3-null epidermis. Reductions of epidermal cell glycerol, its metabolite glycerol-3-phosphate, and ATP were found in AQP3 deficiency without impairment of mitochondrial function. Glycerol supplementation corrected the reduced proliferation and ATP content in AQP3 deficiency, with cellular glycerol, ATP, and proliferative ability being closely correlated. Our data suggest involvement of AQP3-facilitated glycerol transport in epidermal cell proliferation and tumorigenesis by a novel mechanism implicating cellular glycerol as a key determinant of cellular ATP energy. AQP3 may thus be an important determinant in skin tumorigenesis and hence a novel target for tumor prevention and therapy.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Functional MR microimaging of pancreatic beta-cell activation

The increasing incidence of diabetes and the need to further understand its cellular basis has resulted in the development of new diagnostic and therapeutic techniques. Nonetheless, the quest to noninvasively ascertain beta-cell mass and function has not been achieved. Manganese (Mn)-enhanced MRI is presented here as a tool to image beta-cell functionality in cell culture and isolated islets. Similar to calcium, extracellular Mn was taken up by glucose-activated beta-cells resulting in 200% increase in MRI contrast enhancement, versus nonactivated cells. Similarly, glucose-activated islets showed an increase in MRI contrast up to 45%. Although glucose-stimulated Ca influx was depressed in the presence of 100 microM Mn, no significant effect was seen at lower Mn concentrations. Moreover, islets exposed to Mn showed normal glucose sensitivity and insulin secretion. These results demonstrate a link between image contrast enhancement and beta-cell activation in vitro, and provide the basis for future noninvasive in vivo imaging of islet functionality and beta-cell mass.

Pancreatic beta cells from db/db mice show cell-specific [Ca2+]i and NADH responses to glucose but not to alpha-ketoisocaproic acid

OBJECTIVE

We recently showed that timing and magnitude of the glucose-induced cytoplasmic calcium [Ca2+]i response are reproducible and specific for the individual beta cell. We now wanted to identify which step(s) of stimulus-secretion coupling determine the cell specificity of the [Ca2+]i response and whether cell specificity is lost in beta-cells from diabetic animals. Besides glucose, we studied the effects of glyceraldehyde, a glycolytic intermediate, and alpha-ketoisocaproic acid (KIC), a mitochondrial substrate.

METHODS

Early [Ca2+]i changes were studied stimulations in fura-2-labeled dispersed beta cells from lean, ob/ob, and db/db mice. Lag time and peak height were compared during 2 consecutive stimulations with the same stimulator. Nicotinamide adenine dinucleotide (NADH) responses to glucose and KIC were studied as a measure of metabolic flux.

RESULTS

Both glyceraldehyde and KIC induced cell-specific temporal responses in lean mouse beta cells with a correlation between lag times for [Ca2+]i rise during the first and second stimulation. Beta cells from ob/ob and db/db mice showed cell-specific temporal [Ca2+]i responses to glucose and glyceraldehyde but not to KIC. Glucose induced cell-specific NADH responses in all 3 models, but KIC did so only in lean mouse [beta] cells.

CONCLUSIONS

A cell-specific response may be induced at several steps of beta-cell stimulus-secretion coupling. Mitochondrial metabolism generates a cell-specific response in normal beta cells but not in db/db and ob/ob mouse beta cells.

Chronic exposure to beta-hydroxybutyrate inhibits glucose-induced insulin release from pancreatic islets by decreasing NADH contents

To investigate the effects of chronic exposure to ketone bodies on glucose-induced insulin secretion, we evaluated insulin release, intracellular Ca2+ and metabolism, and Ca2+ efficacy of the exocytotic system in rat pancreatic islets. Fifteen-hour exposure to 5 mM d-beta-hydroxybutyrate (HB) reduced high glucose-induced insulin secretion and augmented basal insulin secretion. Augmentation of basal release was derived from promoting the Ca2+-independent and ATP-independent component of insulin release, which was suppressed by the GDP analog. Chronic exposure to HB affected mostly the second phase of glucose-induced biphasic secretion. Dynamic experiments showed that insulin release and NAD(P)H fluorescence were lower, although the intracellular Ca2+ concentration ([Ca2+](i)) was not affected 10 min after exposure to high glucose. Additionally, [Ca2+](i) efficacy in exocytotic system at clamped concentrations of ATP was not affected. NADH content, ATP content, and ATP-to-ADP ratio in the HB-cultured islets in the presence of high glucose were lower, whereas glucose utilization and oxidation were not affected. Mitochondrial ATP production shows that the respiratory chain downstream of complex II is not affected by chronic exposure to HB, and that the decrease in ATP production is due to decreased NADH content in the mitochondrial matrix. Chronic exposure to HB suppresses glucose-induced insulin secretion by lowering the ATP level, at least partly by inhibiting ATP production by reducing the supply of NADH to the respiratory chain. Glucose-induced insulin release in the presence of aminooxyacetate was not reduced, which implies that chronic exposure to HB affects the malate/aspartate shuttle and thus reduces NADH supply to mitochondria.

K-ATP channel independent effects of pinacidil on ATP production in isolated cardiomyocyte or pancreatic beta-cell mitochondria

Evidence has been presented that mitochondria contain ATP sensitive potassium channels (mK-ATP channels), which may confer tissue protection upon activation. It is, however, not known whether activation of mK-ATP channels has a direct effect on mitochondrial ATP production. This study was performed to define the effect of pinacidil (PIN) on ATP production by oxidative phosphorylation in isolated cardiomyocyte or pancreatic beta-cell mitochondria. Cardiomyocyte mitochondria produced seven times more ATP than beta-cell mitochondria in the presence of pyruvate/malate. PIN inhibited pyruvate/malate-induced mitochondrial ATP production with half maximal effect at 360 microM in both cell types. The inclusion of 5-hydroxydecanoate (5-HD) did not prevent this inhibition. Succinate induced a similar ATP production in cardiomyocyte or beta-cell mitochondria. In beta-cell mitochondria succinate-induced ATP production was inhibited by PIN with half maximal effects at 500 microM PIN. However, in cardiomyocyte mitochondria PIN stimulated succinate-induced ATP production 3-fold with half maximal effect at 100 microM and maximal effect at 200 microM. This PIN-dependent stimulation was mimicked by rotenone. The inclusion of 5-HD could not prevent these PIN effects. In conclusion, PIN may inhibit complex 1 of the respiratory chain without indications of opening mK-ATP channels. In cardiomyocytes with metabolically inhibited succinate dehydrogenase this results in a stimulation of ATP production conferring tissue protection. In beta-cells without a metabolically inhibited succinate dehydrogenase, there is no stimulation by PIN and tissue protection by PIN is not to be expected.

Measurement of ATP production and respiratory chain enzyme activities in mitochondria isolated from small muscle biopsy samples

A set of methods suitable for assessment of respiratory chain function in mitochondria isolated from 25mg of muscle is described. This set of methods includes determination of the mitochondrial ATP production rate (MAPR) and the activities of the respiratory chain complexes I, I+III, II+III, and IV and citrate synthase. MAPR is determined with an optimized version of a luminometric method previously described. The optimized method measures 50-220% higher activities than the original method. The highest MAPRs are recorded using the substrate combinations glutamate+succinate and N,N,N(1),N(1)-tetramethyl-1,4-phenyldiamine+ascorbate. The respiratory chain complex activities are determined with standard spectrophotometric methods, adapted to an automated photometer. The sensitivity in the determination of complex I, I+III, and II+III activities was increased considerably by pretreating the samples with saponin. The set of methods was evaluated on double biopsy samples from five healthy volunteers and showed coefficients of variation between 7 and 14% when citrate synthase was used as reference base. All of the various measures of mitochondrial function showed high correlation coefficients to each other (r=0.84-0.98; p<0.01). It is concluded that the set of methods is suitable for diagnosis of mitochondrial disorders in adults and small children.

Ouabain suppresses glucose-induced mitochondrial ATP production and insulin release by generating reactive oxygen species in pancreatic islets

We examined the effects of reduced Na(+)/K(+)-ATPase activity on mitochondrial ATP production and insulin release from rat islets. Ouabain, an inhibitor of Na(+)/K(+)-ATPase, augmented 16.7 mmol/l glucose-induced insulin release in the early period but suppressed it after a delay of 20-30 min. Unexpectedly, the ATP content in an islet decreases in the presence of 16.7 mmol/l glucose when Na(+)/K(+)-ATPase activity is diminished by ouabain, despite the reduced consumption of ATP by the enzyme. Ouabain also suppressed the increment of ATP content produced by glucose even in Ca(2+)-depleted or Na(+)-depleted conditions. That mitochondrial membrane hyperpolarization and O(2) consumption in islets exposed to 16.7 mmol/l glucose were suppressed by ouabain indicates that the glycoside inhibits mitochondrial respiration but does not produce uncoupling. Ouabain induced mitochondrial reactive oxygen species (ROS) production that was blocked by myxothiazol, an inhibitor of site III of the mitochondrial respiratory chain. An antioxidant, alpha-tocopherol, also blocked ouabain-induced ROS production as well as the suppressive effect of ouabain on ATP production and insulin release. However, ouabain did not directly affect the mitochondrial ATP production originating from succinate and ADP. These results indicate that ouabain suppresses mitochondrial ATP production by generating ROS via transduction, independently of the intracellular cationic alternation that may account in part for the suppressive effect on insulin secretion.

Metabolic oscillations in beta-cells

Whereas the mechanisms underlying oscillatory insulin secretion remain unknown, several models have been advanced to explain if they involve generation of metabolic oscillations in beta-cells. Evidence, including measurements of oxygen consumption, glucose consumption, NADH, and ATP/ADP ratio, has accumulated to support the hypothesis that energy metabolism in beta-cells can oscillate. Where simultaneous measurements have been made, these oscillations are well correlated with oscillations in intracellular [Ca(2+)] and insulin secretion. Considerable evidence has been accumulated to suggest that entry of Ca(2+) into cells can modulate metabolism both positively and negatively. The main positive effect of Ca(2+) is an increase in oxygen consumption, believed to involve activation of mitochondrial dehydrogenases. Negative feedback by Ca(2+) includes decreases in glucose consumption and decreases in the mitochondrial membrane potential. Ca(2+) also provides negative feedback by increasing consumption of ATP. The negative feedback provided by Ca(2+) provides a mechanism for generating oscillations based on a model in which glucose stimulates a rise in ATP/ADP ratio that closes ATP-sensitive K(+) (K(ATP)) channels, thus depolarizing the cell membrane and allowing Ca(2+) entry through voltage-sensitive channels. Ca(2+) entry reduces the ATP/ADP ratio and allows reopening of the K(ATP) channel.

Methyl pyruvate initiates membrane depolarization and insulin release by metabolic factors other than ATP

The role of mitochondria in stimulus-secretion coupling of pancreatic beta-cells was examined using methyl pyruvate (MP). MP stimulated insulin secretion in the absence of glucose, with maximal effect at 5 mM. K+ (30 mM) alone, or in combination with diazoxide (100 microM), failed to enhance MP-induced secretion. Diazoxide (100 microM) inhibited MP-induced insulin secretion. MP depolarized the beta-cell in a concentration-dependent manner (5-20 mM). The sustained depolarization induced by 20 mM MP was not influenced by 100 microM diazoxide, but the continuous spiking activity was suppressed by 500 microM diazoxide. Pyruvate failed to initiate insulin release (5-20 mM) or to depolarize the membrane potential. ATP production in isolated beta-cell mitochondria was detected as accumulation of ATP in the medium during incubation in the presence of malate or glutamate in combination with pyruvate or MP. There was no difference in ATP production induced by pyruvate/malate or MP/malate in isolated beta-cell mitochondria. ATP production by MP/glutamate was higher than that induced by pyruvate/glutamate, but it was much lower than that induced by alpha-ketoisocaproate/glutamate. Pyruvate (5 mM) or MP (5 mM) had no effect on the ATP/ADP ratio in whole islets, whereas glucose (20 mM) significantly increased the whole islet ATP/ADP ratio. It is concluded that MP-induced beta-cell membrane depolarization or insulin release does not relate directly to mitochondrial ATP production. Instead MP may exert a direct extramitochondrial effect, or it may stimulate beta-cell mitochondria to produce coupling factors different from ATP to initiate insulin release.

The oscillatory behavior of pancreatic islets from mice with mitochondrial glycerol-3-phosphate dehydrogenase knockout

Glucose stimulation of pancreatic beta cells induces oscillations of the membrane potential, cytosolic Ca(2+) ([Ca(2+)](i)), and insulin secretion. Each of these events depends on glucose metabolism. Both intrinsic oscillations of metabolism and repetitive activation of mitochondrial dehydrogenases by Ca(2+) have been suggested to be decisive for this oscillatory behavior. Among these dehydrogenases, mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH), the key enzyme of the glycerol phosphate NADH shuttle, is activated by cytosolic [Ca(2+)](i). In the present study, we compared different types of oscillations in beta cells from wild-type and mGPDH(-/-) mice. In clusters of 5-30 islet cells and in intact islets, 15 mM glucose induced an initial drop of [Ca(2+)](i), followed by an increase in three phases: a marked initial rise, a partial decrease with rapid oscillations and eventually large and slow oscillations. These changes, in particular the frequency of the oscillations and the magnitude of the [Ca(2+)] rise, were similar in wild-type and mGPDH(-/-) mice. Glucose-induced electrical activity (oscillations of the membrane potential with bursts of action potentials) was not altered in mGPDH(-/-) beta cells. In single islets from either type of mouse, insulin secretion strictly followed the changes in [Ca(2+)](i) during imposed oscillations induced by pulses of high K(+) or glucose and during the biphasic elevation induced by sustained stimulation with glucose. An imposed and controlled rise of [Ca(2+)](i) in beta cells similarly increased NAD(P)H fluorescence in control and mGDPH(-/-) islets. Inhibition of the malate-aspartate NADH shuttle with aminooxyacetate only had minor effects in control islets but abolished the electrical, [Ca(2+)](i) and secretory responses in mGPDH(-/-) islets. The results show that the two distinct NADH shuttles play an important but at least partially redundant role in glucose-induced insulin secretion. The oscillatory behavior of beta cells does not depend on the functioning of mGPDH and on metabolic oscillations that would be generated by cyclic activation of this enzyme by Ca(2+).

Parallel oscillations of intracellular calcium activity and mitochondrial membrane potential in mouse pancreatic B-cells

Insulin secretion in normal B-cells is pulsatile, a consequence of oscillations in the cell membrane potential (MP) and cytosolic calcium activity ([Ca(2+)](c)). We simultaneously monitored glucose-induced changes in [Ca(2+)](c) and in the mitochondrial membrane potential DeltaPsi, as a measure for ATP generation. Increasing the glucose concentration from 0.5 to 15 mM led to the well-known hyperpolarization of DeltaPsi and ATP-dependent lowering of [Ca(2+)](c). However, as soon as [Ca(2+)](c) rose due to the opening of voltage-dependent Ca(2+) channels, DeltaPsi depolarized and thereafter oscillations in [Ca(2+)](c) were parallel to oscillations in DeltaPsi. A depolarization or oscillations of DeltaPsi cannot be evoked by a substimulatory glucose concentration, but Ca(2+) influx provoked by 30 mM KCl was followed by a depolarization of DeltaPsi. The following feedback loop is suggested: Glucose metabolism via mitochondrial ATP production and closure of K(+)(ATP) channels induces an increase in [Ca(2+)](c). The rise in [Ca(2+)](c) in turn decreases ATP synthesis by depolarizing DeltaPsi, thus transiently terminating Ca(2+) influx.

Desensitization of mitochondrial Ca2+ and insulin secretion responses in the beta cell

The role of mitochondria in the desensitization of insulin secretion was investigated. In rat pancreatic beta cells, both insulin secretion and mitochondrial [Ca2+] increases were desensitized following two challenges with the mitochondrial substrate methyl succinate. In the beta cell line INS-1, similar results were observed when a 5-min interval separated two 5-min pulses. In contrast, ATP generation monitored in luciferase-expressing INS-1 cells was stimulated to the same extent during both exposures to methyl succinate. Succinate, like alpha-glycerophosphate, activates the electron transport chain at complex II. As a consequence, the mitochondrial membrane hyperpolarizes, promoting ATP synthesis and Ca2+ influx into the mitochondria through the uniporter. The mitochondrial desensitization was further studied in permeabilized INS-1 cells. Increasing extramitochondrial [Ca2+] from 100 to 500 nM enhanced succinate oxidation 4-fold. At 500 nM Ca2+, 1 mM succinate caused a blunted mitochondrial [Ca2+] increase upon the second, compared with the first, stimulation. These effects were mimicked by alpha-glycerophosphate, and there was cross-desensitization between the two compounds. Succinate hyperpolarized the mitochondrial membrane during both the first and second applications. This suggests that the uniporter itself, rather than the respiratory chain, is desensitized. These results emphasize the key role of the mitochondria not only in the stimulation of insulin secretion, but also in its desensitization.

Glucose metabolism in Goto-Kakizaki rat islets

Islets from Goto-Kakizaki (GK) rats from our colony, despite marked impairment of glucose-induced insulin release, used glucose and produced CO2 at a rate 3 times that of islets from control Wistar rats. Almost all glucose used was accounted for in CO2 and lactate production. The percentages of glucose carbon used collected in CO2 and lactate were similar for control and GK islets. GK islets also oxidized 40% more acetate and leucine to CO2 than did control islets. The fraction of carbon leaving the Krebs cycle relative to CO2 production was the same in GK and control islets. The capacities of mitochondria from GK islets to generate ATP from glutamate and malate were similar and that to generate ATP from succinate and rotenone was somewhat less from GK islets. The reason for the enhanced utilization of substrates by islets of the GK rat is not apparent. In conclusion, there is no decrease in islet glucose utilization, glucose oxidation, Krebs cycle function, or the electron transport system evident from these measurements to explain the impaired insulin release in islets from GK rats.

Mitochondrial activation directly triggers the exocytosis of insulin in permeabilized pancreatic beta-cells

In the pancreatic beta-cell, insulin secretion is stimulated by glucose metabolism resulting in membrane potential-dependent elevation of cytosolic Ca2+ ([Ca2+]c). This cascade involves the mitochondrial membrane potential (delta psi[m]) hyperpolarization and elevation of mitochondrial Ca2+ ([Ca2+]m) which activates the Ca(2+)-sensitive NADH-generating dehydrogenases. Metabolism-secretion coupling requires unidentified signals, other than [Ca2+]c, possibly generated by the mitochondria through the rise in [Ca2+]m. To test this paradigm, we have established an alpha-toxin permeabilized cell preparation permitting the simultaneous monitoring of [Ca2+] with mitochondrially targeted aequorin and insulin secretion under conditions of saturating [ATP] (10 mM) and of clamped [Ca2+]c at substimulatory levels (500 nM). The tricarboxylic acid (TCA) cycle intermediate succinate hyperpolarized delta psi(m), raised [Ca2+]m up to 1.5 microM and stimulated insulin secretion 20-fold, without changing [Ca2+]c. Blockade of the uniporter-mediated Ca2+ influx into the mitochondria abolished the secretory response. Moreover, glycerophosphate, which raises [Ca2+]m by hyperpolarizing delta psi(m) without supplying carbons to the TCA cycle, failed to stimulate exocytosis. Activation of the TCA cycle with citrate evoked secretion only when combined with glycerophosphate. Thus, mitochondrially driven insulin secretion at permissive [Ca2+]c requires both a substrate for the TCA cycle and a rise in [Ca2+]m. Therefore, mitochondrial metabolism generates factors distinct from Ca2+ and ATP capable of inducing insulin exocytosis.

The role of plasma membrane K+ and Ca2+ permeabilities for glucose induction of slow Ca2+ oscillations in pancreatic beta-cells

In individual pancreatic beta-cells the rise of the cytoplasmic Ca2+ concentration ([Ca2+]i), induced by 11 mM glucose, is manifested either as oscillations (0.2-0.5 min-1) or as a sustained elevation. The significance of the plasma membrane permeability of Ca2+ and K- for the establishment of these slow oscillations was investigated by dual wavelength microfluorometric measurements of [Ca2+]i in individual ob/ob mouse beta-cells loaded with fura-2. Increasing the extracellular Ca2+ to 10 mM or the addition of Ca2+ channel agonist BAY K 8644 (1 microM) or K+ channel blocker tetraethylammonium+ (TEA: 10-20 mM) caused steeper rises and higher peaks of the glucose-induced oscillations. However, when extracellular Ca2+ was lowered to 0.5 mM the oscillations were transformed into a sustained suprabasal level. When the beta-cells exhibited glucose-stimulated sustained elevation of [Ca2+]i in the presence of a physiological Ca2+ concentration (1.3 mM), it was possible to induce slow oscillations by promoting the entry of Ca2+ either by raising the extracellular Ca2+ concentration to 10 mM or adding TEA or BAY K 8644. The results indicate that glucose-induced slow oscillations of [Ca2+]i depend on the closure of ATP-regulated K+ channels and require that the rate of Ca2+ influx exceeds a critical level. Apart from an inherent periodicity in ATP production it is proposed that Ca(2+)-induced ATP consumption in the submembrane space contributes to the cyclic changes of the membrane potential determining periodic entry of Ca2+.

Firefly luciferase can use L-luciferin to produce light

L-Luciferin is a competitive inhibitor of firefly luciferase with a K1 between 3 and 4 microM. Furthermore L-luciferin can serve as an alternative substrate for light production. Catalysis of L-luciferin can be observed in the absence of, or at low concentrations of, D-luciferin. The light production from L-luciferin increases slowly (maximal half-time 8 min) to a stable plateau. At low concentrations of enzyme and L-luciferin, maximal light production is about half of that observed at corresponding D-luciferin concentrations. Increasing the concentration of enzyme or L-luciferin reduces the light production relative to that obtained by D-luciferin catalysis. In contrast to the catalysis of D-luciferin the light production from L-luciferin can be effectively stimulated by the addition of PP1 provided that luciferase is premixed with inorganic pyrophosphatase (PP1-ase). A flash is emitted if PP1 is injected into a mixture of luciferase, L-luciferin, ATP and PP1-ase. The system maintains its responsiveness and emits further flashes of about equal duration and intensity upon repeated additions of PP1. It is proposed that PP1 induces a racemization of enzyme-bound L-luciferyl adenylate. The potential usefulness of PP1-dependent intracellular ATP monitoring is discussed. The proposed activation of firefly luciferase by PP1 may be part of the regulation of in vivo flashing.