Alloxan effects on mitochondria: study of oxygen consumption, fluxes of Mg2+, Ca2+, K+ and adenine nucleotides, membrane potential and volume change in vitro

Mersalyl prevents the Tl+-induced permeability transition pore opening in the inner membrane of Ca2+-loaded rat liver mitochondria

It was earlier shown that the calcium load of rat liver mitochondria in medium containing TlNO₃ and KNO₃ resulted in the Tl⁺-induced mitochondrial permeability transition pore (MPTP) opening in the inner membrane. This opening was accompanied by an increase in swelling and membrane potential dissipation and a decrease in state 3, state 4, and 2,4-dinitrophenol-uncoupled respiration. This respiratory decrease was markedly leveled by mersalyl (MSL), the phosphate symporter (PiC) inhibitor which poorly stimulated the calcium-induced swelling, but further increased the potential dissipation. All of these effects of Ca²⁺ and MSL were visibly reduced in the presence of the MPTP inhibitors (ADP, N-ethylmaleimide, and cyclosporine A). High MSL concentrations attenuated the ability of ADP to inhibit the MPTP. Our data suggest that the PiC can participate in the Tl⁺-induced MPTP opening in the inner membrane of Ca²⁺-loaded rat liver mitochondria.

Early hyperglycemia following alloxan administration in vivo is not associated with altered hepatic mitochondrial function: acceptable model for type 1 diabetes?

Alloxan and oxidative stress, which have been detected in livers of laboratory animals shortly after in vivo alloxan administration, cause in vitro mitochondrial dysfunction, thus questioning alloxan diabetes as an acceptable model for type 1 diabetes, a model that cannot legitimately be used to investigate mitochondrial metabolism in a diabetic state. In the current study, the blood glucose concentration increased in the drug-treated group of Sprague–Dawley rats (compared with the placebo group) 45 or 60 min after alloxan treatment, whereas at 30 min the blood glucose concentration was unchanged. State 4, state 3, respiratory control, efficiency of oxidative phosphorylation, and mitochondrial ATP synthase activity, assayed using glutamate plus malate, pyruvate plus malate, or succinate as a substrate, were not negatively altered during the entire study. These results indicated that early increases of blood glucose concentration, after in vivo alloxan administration, did not lead to liver mitochondrial dysfunction, suggesting that alloxan diabetes can be used for the study of liver mitochondrial respiration in a diabetic state.

Alloxan decreases intracellular potassium content of the isolated frog skin epithelium

Alloxan has been widely used to provoke diabetes mellitus. This compound induces necrosis of the beta-pancreatic cells and the renal tubules. However, the mechanism of this action has not been fully established. There is some evidence that this drug may act by an alteration of several ionic transport mechanisms. Nevertheless, there is scant information on the effect of alloxan on these ionic transport mechanisms of the membrane in epithelial cells. We reported that this drug induces a decrease in sodium transport in the frog skin. In order to obtain information about the mechanism involved in the sodium transport diminution provoked by alloxan, in this study the function of Na+-K+ ATPase enzyme on transepithelial sodium transport altered by alloxan is explored. We measured changes in the short circuit current and in the intracellular content of sodium and potassium under conditions of maximally stimulated enzyme activity. Short circuit current was not modified by the treatment with alloxan during the period of highest activity of the enzyme, suggesting a site of action independent of this ATPase. Cell potassium was reduced in alloxan-treated epithelia, without significant changes in Na+ content. This finding points out the existence of an alteration induced by alloxan of some modulator mechanisms of the intracellular K+ concentration. The treatment of the frog skin with cesium chloride, a K+ channel blocker, prevented the decrease of Na+ transport produced by alloxan. This result suggests an action of this diabetogenic drug on the K+ channels of the frog skin epithelium.

Apoptosis and mitochondrial damage in INS-1 cells treated with alloxan

To evaluate the participation of mitochondrial damage, oxygen radicals and cell death in diabetes mellitus, we designed a way to investigate INS-1 cells, rat pancreatic beta-cell line, to die by treatment with alloxan which generate reactive oxygen species (ROS). Incubation of INS-1 cells with alloxan for 24 h resulted in a decrease in viability of cells as well as inhibition of glucose-stimulated insulin release; this could be prevented by antioxidants, vitamin E and butylated hydroxyanisol (BHA). The formation of a DNA ladder and the distribution of phosphatidylserine at the external surface of plasma membrane were observed as indicators of apoptosis in the cells treated with alloxan at concentrations below 0.5 mM. The formation of DNA ladder was prevented by vitamin E, BHA and catalase, suggesting that the ROS is involved in the process of apoptosis in INS-1 cells treated with alloxan. Lower levels of intracellular ATP, collapse of mitochondrial membrane potential and release of cytochrome c from mitochondria were also observed in INS-1 cells treated with alloxan, suggesting that alloxan caused the damage of mitochondria in cells and was related to the process of apoptosis. In contrast, rat liver RLC-18 cells treated with alloxan were not observed in the decrease of viability. It follows from the present study that mitochondrial damages by ROS generated from alloxan is linked to apoptosis in INS-1 cells.

Alloxan-induced mitochondrial permeability transition triggered by calcium, thiol oxidation, and matrix ATP

In addition to their critical function in energy metabolism, mitochondria contain a permeability transition pore, which is regulated by adenine nucleotides. We investigated conditions required for ATP to induce a permeability transition in mammalian mitochondria. Mitochondrial swelling associated with mitochondria permeability transition (MPT) was initiated by adding succinate to a rat liver mitochondrial suspension containing alloxan, a diabetogenic agent. If alloxan was added immediately with or 5 min after adding succinate, MPT was strikingly decreased. MPT induced by alloxan was inhibited by EGTA and several agents causing thiol oxidation, suggesting that alloxan leads to permeability transition through a mechanism dependent on Ca(2+) uptake and sulfhydryl oxidation. Antimycin A and cyanide, inhibitors of electron transfer, carbonyl cyanide m-chlorophenylhydrazone, and oligomycin all inhibited MPT. During incubation with succinate, alloxan depleted ATP in mitochondria after an initial transient increase. However, in a mitochondrial suspension containing EGTA, ATP significantly increased in the presence of alloxan to a level greater than that of the control. These results suggest the involvement of energized transport of Ca(2+) in the MPT initiation. Addition of exogenous ATP, however, did not trigger MPT in the presence of alloxan and had no effect on MPT induced by alloxan. We conclude that alloxan-induced MPT requires mitochondrial energization, oxidation of protein thiols, and matrix ATP to promote energized uptake of Ca(2+).

Oxygen microsensor and its application to single cells and mouse pancreatic islets

An oxygen microsensor with a < 3-micron tip diameter was developed for monitoring oxygen levels at single cells and mouse pancreatic islets. The sensor was fabricated by electrochemically recessing an etched Pt wire inside a pulled glass micropipet and then coating with cellulose acetate. This fabrication process was found to be simpler than previous oxygen electrode designs of comparable size. The microsensors had a average sensitivity of 0.59 +/- 0.29 pA/mmHg (mean +/- SD, n = 42), signals that were minimally perturbed by convection, and response times of < 1 s. The electrode was used to measure the oxygen gradient around and inside single mouse islets. The measurements demonstrate that oxygen levels within even the largest islets at maximal glucose stimulation are 67 +/- 1.6 mmHg (mean +/- SD, n = 5), indicating that islets have adequate oxygen supplies by diffusion under tissue culture conditions to support insulin secretion. The electrode was also used to record the dynamics of oxygen level at single islets as a function of glucose concentration. As glucose level was changed from 3 to 10 mM, oxygen level decreased by 15.8 +/- 2.3 mmHg (mean +/- SEM, n = 6) and oscillations with a period of 3.3 +/- 0.6 min (mean +/- SEM, n = 6) appeared in the oxygen level. In islets bathed in quiescent solutions containing 10 mM glucose, similar oscillations could be observed. In addition, in the quiet solutions it was possible to detect faster oscillations with a period of 12.1 +/- 1.7 s (mean +/- SEM, n = 6) superimposed on the slower oscillations. Oxygen consumption could also be observed at single insulinoma cells using the electrode. Individual cells also showed oscillations in oxygen consumption with a period of a few seconds. The results demonstrate that the electrode can be used for dynamic oxygen level recordings in biological microenvironments.

Free radical modulation of insulin release in INS-1 cells exposed to alloxan

Generation of free radicals is thought to mediate the cytotoxic action of alloxan on the pancreatic beta-cell. In this investigation, the early effects of alloxan on cell function were studied. When INS-1D insulinoma cells were exposed to alloxan (1 mM) for 45 min followed by a 3-hr recovery period, the drug increased basal insulin release while abolishing the effect of glucose in static incubations. This was associated with impaired stimulation of cellular metabolism by glucose and reduced viability, both monitored colorimetrically with 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT). These alterations were largely counteracted by the antioxidant butylated hydroxyanisol (BHA). Similar changes occurred when glucose was added directly after 5 min of alloxan treatment, whereas KCl-induced secretion was only partially inhibited. In perifusion, alloxan caused transient insulin secretion to 50% of the rates obtained with glucose 30 min later. Under these conditions, epinephrine abolished the stimulation due to both agents. Membrane potential and cytosolic calcium concentrations ([Ca2+]i) were recorded to clarify the action of alloxan. Alloxan-induced insulin release correlated with depolarization of INS-1D cells and a rise in [Ca2+]i. Alloxan did not augment [Ca2+]i in the presence of BHA or the absence of extracellular calcium. Nickel chloride blocked the effect of alloxan on [Ca2+]i, whereas verapamil was ineffective. This suggests that alloxan promotes Ca2+ influx through channels distinct from L-type channels, perhaps through non-selective cation channels. Thus, alloxan causes changes in INS-1D cells prevented by antioxidant treatment, suggesting that free radicals may modulate the ionic permeability leading to functional activation.

Hydrogen peroxide induces intracellular calcium overload by activation of a non-selective cation channel in an insulin-secreting cell line

Fura-2 fluorescence was used to investigate the effects of H2O2 on [Ca2+]i in the insulin-secreting cell line CRI-G1. H2O2 (1-10 mM) caused a biphasic increase in free [Ca2+]i, an initial rise observed within 3 min and a second, much larger rise following a 30-min exposure. Extracellular calcium removal blocked the late, but not the initial, rise in [Ca2+]i. Thapsigargin did not affect either response to H2O2, but activated capacitive calcium entry, an action abolished by 10 microM La3+. Simultaneous recordings of membrane potential and [Ca2+]i demonstrated the same biphasic [Ca2+]i response to H2O2 and showed that the late increase in [Ca2+]i coincided temporally with cell membrane potential collapse. Buffering Ca2+i to low nanomolar levels prevented both phases of increased [Ca2+]i and the H2O2-induced depolarization. The H2O2-induced late rise in [Ca2+]i was prevented by extracellular application of 100 microM La3+. La3+ (100 microM) inhibited the H2O2-induced cation current and NAD-activated cation (NSNAD) channel activity in these cells. H2O2 increased the NAD/NADH ratio in intact CRI-G1 cells, consistent with increased cellular [NAD]. These data suggest that H2O2 increases [NAD], which, coupled with increased [Ca2+]i, activates NSNAD channels, causing unregulated Ca2+ entry and consequent cell death.

Elevation of intracellular calcium ions is essential for the H2O2-induced activation of SAPK/JNK but not for that of p38 and ERK in Chinese hamster V79 cells

The mitogen-activated protein kinases (MAPK), including stress-activated protein kinase/c-Jun N-terminal kinase (SAPK/JNK), p38, and extracellular signal-related kinase (ERK), are believed to be important biomolecules in cell proliferation, survival, and apoptosis induced by extracellular stimuli. In Chinese hamster V79 cells exposed to hydrogen peroxide (H2O2), we recently demonstrated that SAPK/JNK was activated by tyrosine kinase and intracellular Ca2+ ([Ca2+]i). In this study, we report that [Ca2+]i release from intracellular stores is important in the activation of SAPK/JNK but not p38 and ERK. H2O2-induced elevation of [Ca2+]i was observed in Ca2+-free medium. Pretreatment with thapsigargin, a Ca2+-ATPase inhibition of endoplasmic reticulum (ER), did not influence H2O2-induced elevation of [Ca2+]i in the absence of external Ca2+. An intracellular Ca2+ chelator (BAPTA-AM) inhibited H2O2-induced phosphorylation of SAPK/JNK, but an extracellular Ca2+ chelator (EDTA) or a Ca2+ entry blocker (NiCl2) did not. Activation of p38 and ERK in V79 cells exposed to H2O2 was observed in the presence of these inhibitors. These results suggest that [Ca2+]i release from intracellular stores such as mitochondria or nuclei but not ER, occurred after H2O2 treatment and Ca2+-dependent tyrosine kinase-induced activation of SAPK/JNK, although [Ca2+]i was unnecessary for the H2O2-induced activation of p38 and ERK.

Effects of alloxan and ninhydrin on mitochondrial Ca2+ transport

Alloxan at millimolar concentrations slightly inhibited the velocity of Ca2+ uptake by isolated rat liver mitochondria irrespective of the free Ca2+ concentration between 1 and 10 microM and was an effective concentration-dependent stimulator of mitochondrial Ca2+ efflux. Ninhydrin also slightly inhibited the velocity of mitochondrial Ca2+ uptake but only at free Ca2+ concentrations above 5 microM. However, ninhydrin was a strong stimulator of mitochondrial Ca2+ efflux even at micromolar concentrations, 10-50 times more potent than alloxan. The mitochondrial membrane potential was reduced 10-20% at most by alloxan and ninhydrin. Alloxan and ninhydrin also stimulated Ca2+ efflux from isolated permeabilized liver cells. When isolated intact liver cells had been pre-incubated with alloxan or ninhydrin before permeabilization of the cells the ability of spermine to induce mitochondrial Ca2+ uptake was abolished. Glucose provided the typical protection against the effects of alloxan on mitochondrial Ca2+ transport only in experiments with intact cells but not in experiments with permeabilized cells or isolated mitochondria. Therefore glucose protection is apparently due to inhibition of alloxan uptake into the cell. Glucose provided no protection against effects of ninhydrin under any of the experimental conditions. Thus both alloxan and ninhydrin are potent stimulators of Ca2+ efflux by isolated mitochondria but very weak inhibitors of the velocity of mitochondrial Ca2+ uptake. The direct effects of ninhydrin on mitochondrial Ca2+ efflux may contribute to the cytotoxic action of this agent whereas the direct effects of alloxan on mitochondrial Ca2+ transport require concentrations which are too high to be of relevance for the induction of the typical pancreatic B-cell toxic effects of alloxan. However, the effects on mitochondrial Ca2+ transport during incubation of intact cells which may result from the generation of cytotoxic intermediates during alloxan xenobiotic metabolism may well contribute to the pancreatic B-cell toxic effect of alloxan.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

Effects of sodium nitroprusside on blood glucose concentration, B-cell morphology and islet glutamate dehydrogenase activity in mice

Mice injected with sodium nitroprusside (NaNP) exhibited a marked, transient hyperglycemia and enhanced activity of glutamate dehydrogenase in the pancreatic islets. Ultrastructurally, the islet B-cells of NaNP-treated mice showed expanded granular endoplasmic reticulum, prominent Golgi complex, increased amount of secretory granules, mitochondrial enlargement and vacuolation, and mitochondrion-secretory granule complexes. Stereological analyses disclosed increased volume of endoplasmic reticulum, Golgi complex and mitochondria, and increased number of secretory granules in the B-cells 1 h after the injection of NaNP. Isolated mouse islets exposed to NaNP showed stimulated activity of glutamate dehydrogenase both in the presence of 2 and 18 mM glucose, whereas the release of insulin was stimulated at 2 mM glucose, but inhibited at 18 mM glucose. The observations demonstrate that NaNP induces transiently altered structure and function in mouse islet B-cells.

Cerebral mitochondrial respiration in diabetic and chronically hypoglycemic rats

The respiratory function of cerebral mitochondria harvested from genetically diabetic (BB/W) and streptozotocin-diabetic rats deprived of insulin for 3-4 weeks was found to be unchanged from control values. Furthermore, insulin-deprived BB/W rats subjected to 30 min of insulin-induced hypoglycemic coma demonstrated a normal mitochondrial respiration following a 60 min period of glucose restitution, a finding consistent with earlier results in non-diabetic rats. However, in rats exposed to 1 week of moderate hypoglycemia (plasma glucose = 3.0 mumol.ml-1), both state 3 respiration and the respiratory control ratio (RCR) were reduced from control. In fact, when the chronic hypoglycemia was imposed following a 3-4 week period of diabetic hyperglycemia, the state 3 rate and RCR were found to be reduced to a greater degree than in chronically hypoglycemic, non-diabetic, previously normoglycemic rats. Finally, when 1 week of moderate hypoglycemia preceded a 30 min period of insulin-induced hypoglycemic coma, a disturbed pattern of mitochondrial respiration (i.e. increased state 4, decreased RCR) was found at 60 min of recovery following coma. These results indicate that chronic increases in glucose (and insulin deprivation) have no effect on cerebral mitochondrial respiratory function, whereas prolonged, albeit moderate, reductions in cerebral glucose supply result in perturbations in mitochondrial respiration. These results demonstrate the importance of an adequate glucose supply for normal mitochondrial activity.

Mitochondrial changes and associated alterations induced in mice by streptozotocin administered in vivo and in vitro

Isolated mouse liver mitochondria incubated with streptozotocin showed decreased rate and extent of Ca2+ uptake, and, dependent on the concentration of streptozotocin and the addition of alpha-ketoglutarate, glutamate, fluorocitrate or guanosine 5'-triphosphate, the retention of Ca2+ was either increased or decreased. Similar observations were made in liver mitochondria incubated with succinyl-CoA. In mitochondria isolated from the kidneys and islets of mice injected with streptozotocin, with and without additional injections of glucose and/or glucagon, the rate and extent of Ca2+ uptake were reduced and the release of accumulated Ca2+ was stimulated. Electron microscopy and X-ray microanalysis showed dislocation of Ca2+-containing precipitates from the mitochondria to the cytosol, and stereology disclosed increased mitochondrial volume in the B cells of streptozotocin-treated mice. State 3 and state 4 respiration with NAD-linked substrates was inhibited, but succinate oxidation was unaffected, in mitochondria isolated from the kidneys of mice treated with streptozotocin. In the kidneys of streptozotocin-injected mice, the concentration of succinyl-CoA was increased, that of citrate and guanosine 5'-triphosphate was decreased, that of glucose 6-phosphate, fructose 6-phosphate and fructose 1,6-diphosphate was unaffected, and the metabolite concentration ratios suggested increased mitochondrial [NAD+]/[NADH] ratio and decreased cytoplasmic [NAD+]/[NADH] ratio. It is suggested as a new hypothesis that the cytotoxicity and the diabetogenicity of streptozotocin are dependent on inhibited citric acid cycle enzyme activity (primarily that of succinyl-CoA synthetase and citrate synthetase) with altered metabolite concentrations, leading to impairment of the mitochondrial uptake of Ca2+ and the activation of the pyruvate, isocitrate and alpha-ketoglutarate dehydrogenases.

Ca2+ transport in isolated mouse liver mitochondria; role of reductive carboxylation and citrate?

The uptake of Ca2+ in isolated mouse liver mitochondria respiring on succinate in the presence of rotenone and added Pi, was inhibited by dibucaine, fluorocitrate, p-hydroxymercuribenzoate (PMB), malonate, palmitoyl-CoA, succinyl-CoA and trifluoroperazine. The release of accumulated Ca2+ was stimulated by arsenite, malonate, PMB, palmitoyl-CoA and succinyl-CoA, whereas the release was inhibited by dibucaine, fluorocitrate, trifluoroperazine, and by oligomycin, especially in the presence of ADP. The pyridine nucleotides were oxidized in mitochondria incubated with PMB. The observations suggest a possible contributory role of reductive carboxylation for the uptake of Ca2+, and a possible role of citrate for the retention of Ca2+ in isolated mouse liver mitochondria.

Inhibition by streptozotocin of the activity of succinyl-CoA synthetase in vitro and in vivo

The activity of succinyl-CoA synthetase from mouse liver and kidney was inhibited by streptozotocin in vitro. Streptozotocin behaved essentially as a non-competitive inhibitor, and the following kinetic values were obtained (in the presence of 10 nM streptozotocin): apparent Km 1.7 mM, apparent Ki 10 nM, and kcat 440 nkat X kg-1. Compared with non-diabetic control mice, the succinyl-CoA synthetase activity was significantly decreased in the islets and kidneys of mice with early (1 h) and manifest (greater than or equal to 2 days) streptozotocin diabetes, whereas the activity in the liver was not significantly altered. Inhibited succinyl-CoA synthetase activity is believed to play a prominent role in the cellular effects of streptozotocin.

Alterations in mitochondrial aconitase activity and respiration, and in concentration of citrate in some organs of mice with experimental or genetic diabetes

Mouse islets (not used for respiration), kidneys and liver were studied in early and manifest alloxan diabetes, and in genetic diabetes. In these organs the mitochondrial aconitase activity was lower, state 3 respiration with citrate or pyruvate plus malate (but not with succinate) was decreased, and the concentration of citrate was increased, compared with non-diabetic control mice. The alterations suggest a role of lowered activity of mitochondrial aconitase in alloxan diabetes, and probably also in genetic diabetes.

Inhibition by alloxan of mitochondrial aconitase and other enzymes associated with the citric acid cycle

Considerable variations were found in the in vitro effect of alloxan on mouse liver enzymes associated with the citric acid cycle. The following approximative alloxan concentrations induced 50% inhibition of enzyme activity: 10(-6)M for aconitase, 10(-4)M for NAD-linked isocitrate dehydrogenase, glutamate dehydrogenase, alpha-ketoglutarate dehydrogenase, succinyl-CoA synthetase and fumarase, and 10(-3)M for citrate synthase and NADP-linked isocitrate dehydrogenase. Pyruvate dehydrogenase, succinate dehydrogenase and malate dehydrogenase were not inhibited by 10(-3)M alloxan. The inhibition of aconitase was competitive both when using mouse liver and purified porcine heart enzyme. The Ki values for the purified enzyme in the presence of 5 microM alloxan were 0.22 microM with citrate, 4.0 microM with cis-aconitate and 0.62 microM with isocitrate as substrate. The high sensitivity of aconitase for inhibition by alloxan probably plays a prominent role for the toxic effects of alloxan.