Effect of alloxan on islet tissue permeability: protection and reversal by NADPH

Protection of B cells against the effect of alloxan

Alloxan induces diabetes in laboratory animals through the destruction of the endocrine pancreatic B cells. The mechanism of alloxan toxicity is still obscure. This study was conducted to investigate the effects of superoxide dismutase (SOD) or reduced nicotinamide adenine dinucleotide (NADPH) treatment on the B cells in isolated rat islets prior to alloxan treatment. Islets were treated with SOD (1000 U) or 0.1 mM NADPH for 10 min followed by alloxan treatment (0.18 mg) for 5 min. Insulin secretion was studied in samples incubated for 60 min in media supplemented with glucose (1.8 mg/ml). Morphological examinations were conducted on fixed samples after the alloxan treatment. SOD significantly protected the islets from the cytotoxic effect of alloxan. Although alloxan decreased insulin secretion to 35% of the control, SOD increased this level to 73% of the control values. NADPH did not provide any protection to the islets. Insulin secretion from islets treated with NADPH and alloxan was not different from that after alloxan treatment alone. Morphological changes were observed in the islets treated with alloxan alone or alloxan in the presence of NADPH. Islets exhibited multiple cellular necrosis, marked degranulation and extensive vesiculation of the endoplasmic reticulum and Golgi complex. Mitochondrial enlargement with disrupted cristae and mitochondrial ruptures were prominent. However, islets treated with SOD and alloxan were similar to the control except for the enlarged mitochondria. The increased insulin secretion from islets treated with SOD and alloxan reinforces the free radical hypothesis of alloxan toxicity. The markedly enlarged mitochondria was one of the targets through which alloxan destroyed the B cells.

Metabolism and transport of dehydroascorbic acid in erythrocytes of "spontaneous diabetic BB/W" Wistar rats

Rates of uptake and reduction of dehydroascorbic acid in erythrocytes of "Spontaneous diabetic BB/W" and control Wistar rats were determined. Lysed cells reduced 14C-dehydroascorbic acid more rapidly than intact cells did, suggesting that membrane transport is a rate-limiting step. Diabetic rats had lower plasma levels of ascorbic acid but more rapid reduction of dehydroascorbic acid than control animals. The results indicate more rapid transport of dehydroascorbic acid into erythrocytes of prediabetic "BB/W" rats than Wistar rats.

Alloxan-induced glucoprivic feeding deficits are blocked by D-glucose and amygdalin

Intracerebroventricular injection of alloxan, a pancreatic beta (B) cell cytotoxin, impairs glucoprivic feeding in rats. The goal of this experiment was to determine whether alloxan-induced impairment of glucoprivic feeding can be attenuated by agents which antagonize alloxan's toxicity in the B cell. Therefore, alloxan was co-administered into the fourth ventricle alone or in combination with D-glucose, L-glutamine, or amygdalin, all known antagonists of alloxan's B cell toxicity, or with L-glucose, which does not antagonize B cell toxicity. We found that alloxan produced deficits in glucoprivic feeding which were not attenuated by co-administration with L-glucose or L-glutamine. Alloxan/L-glucose treated rats ate 11% and 14% of control intake, respectively, after systemic administration of 2-deoxy-D-glucose (2DG, 250 mg/kg) and fourth ventricular 5-thioglucose (5TG, 120 micrograms/5 microliter). Alloxan/L-glutamine rats ate 20% and 22% of control intake after 2DG and 5TG, respectively. In contrast, D-glucose and amygdalin (15 mM) completely blocked alloxan-induced impairment of glucoprivic feeding and amygdalin (10 mM) exerted a partial protective effect. These behavioral results may indicate that in the brain, susceptibility to alloxan toxicity depends upon cellular characteristics shared with the B cell.

Cytotoxic effects of alloxan treatment in vitro on monolayer cultures of neonatal rat pancreas

Monolayer cultures of neonatal rat pancreas were exposed briefly (5 min) to alloxan and were then maintained for several days (up to 14 days) to examine the long-term effects of alloxan exposure on B cells. Alloxan (0-10 mM) caused a dose-dependent reduction in rates of insulin release during the first 24 h after treatment. This reduction persisted throughout the period of observation. Rates of glucagon release were unaltered by all concentrations of alloxan tested. Spontaneously decomposed alloxan had little or no effect on rates of insulin release. The simultaneous presence of high concentrations of glucose during exposure of cells to alloxan exerted a rapid protective effect against alloxan toxicity that was both glucose and alloxan dose dependent. Alloxan treatment of pancreatic monolayer cultures may allow for a comparison of those actions of alloxan that are directly on and specific for islet B cells. Furthermore, alloxan-treated cultures may represent a unique in vitro system for studying the physiology and biochemistry of other islet cell types in cultures in which B cells and insulin are reduced, as well as for studying glucose interactions with the B cell.

Protection against alloxan-induced diabetes in mice by stimulation of alpha 2-adrenergic receptors

A single intravenous injection of alloxan in mice induced hyperglycemia in a dose dependent fashion. This diabetogenic action of alloxan was prevented by a single intraperitoneal injection of the alpha 2-adrenergic agonists, i.e. oxymetazoline, clonidine or epinephrine 40 min prior to the injection of alloxan. The alpha 1-adrenergic agonists, i.e. methoxamine and phenylephrine, and a beta-adrenergic agonist, isoproterenol, failed to prevent the diabetogenic action of alloxan. The inhibitory effect of clonidine on alloxan-induced diabetes was antagonized by yohimbine or phentolamine, but not by prazosin. Although alpha 2-adrenergic agonists caused a transient hyperglycemia at the time of alloxan administration (40 min after the administration of alpha 2-adrenergic agonists), the plasma glucose level at the time of alloxan injection did not correlate with the anti-diabetogenic effect of alpha 2-adrenergic agents. These results clearly demonstrate that the alpha 2-adrenergic mechanism which inhibits insulin release from pancreatic B cells prevented the diabetogenic action of alloxan in mice.

Non-enzymatic reduction of alloxan by reduced nicotinamide nucleotide

Much evidence has been reported that the diabetogenic action of alloxan is caused by the formation of cytotoxic free radicals during the autoxidation of dialuric acid, a reduction product of alloan, to alloxan. The mechanism by which alloxan is reduced in vivo to dialuric acid, however, is unknown. The non-enzymatic reaction of alloxan with NAD(P)H was studied as a possible candidate for the reduction of alloxan. The reaction was carried out at 37 degrees in 50 mM phosphate buffer (mostly at pH 7.0) and was followed by measuring the decrease in absorbance at 340 nm. NADH and NADPH were found to be stoichiometrically oxidized by alloxan to NAD and NADP respectively. When the alloxan concentration (1.0 mM) was kept constant and the concentration of NAD(P)H (0.05 to 0.2 mM) was varied, the rate of decrease in the relative concentration of NAD(P)H was almost constant, suggesting that the autoxidation of dialuric acid by O2 was rapid enough to neglect its presence in the medium. The reaction between alloxan and NAD(P)H was accelerated by decreasing the pH. Both the rate of decrease in NAD(P)H concentration and the rate of O2 consumption resulting from autoxidation of the dialuric acid formed by reduction of alloxan were not affected by the presence of 20 mM D-glucose. Ethylene formation by the reaction of methional with . OH, one of the autoxidation products of dialuric acid, was clearly reduced by the presence of alpha- or beta-D-glucose (20 mM), but there was no significant difference between the effects of the two anomers. These results with D-glucose ruled out the possibility that the protection of beta-cells by D-glucose against the diabetogenicity of alloxan can be explained either by its inhibitory action on dialuric acid formation or by its scavenging effect on . OH.

Alloxan-induced luminol luminescence as a tool for investigating mechanisms of radical-mediated diabetogenicity

Chemiluminescence of luminol in a cell-free system was used to investigate the mechanism of alloxan-dependent free-radical generation. In the presence of alloxan and reduced glutathione (GSH), luminescence was greatly stimulated by FeSO4. Replacing GSH by oxidized glutathione or NAD(P)(H), or replacing FeSO4 by CuSO4, ZNSO4 or FeCl3, did not yield chemiluminescence. The chemiluminescence of a mixture of alloxan. GSH, FeSO4 and luminol was inhibited by catalase, superoxide dismutase, scavengers of hydroxyl radicals (sodium benzoate, n-butanol, D-mannitol, dimethyl sulphoxide) or metal-ion chelators (EDTA, diethylenetriaminepenta-acetic acid, diethyldithiocarbamate. desferroxamine), D-glucose, L-glucose, D-mannose, D-fructose, 3-O-methyl-D-glucose, NAD+, NADH, NADP+ or NADPH, but not by urea or enzymically inactive superoxide dismutase. The results support the hypothesis that the diabetogenic action of alloxan is mediated by hydroxyl radicals generated in an iron-catalysed reaction. Protection against alloxan in vivo depends both on the chemical reactivity of protector with radicals or radical-generating systems and on the stereospecific requirement of some strategic site in the B-cell.

Trypan Blue as a marker of plasma membrane permeability in alloxan-treated mouse islet cells

Suspensions of pancreatic islet cells from noninbred ob/ob-mice were incubated with Trypan Blue. Microscope photometry showed that apparently viable cells excluded the dye completely, whereas the nuclei of nonviable cells accumulated Trypan Blue by a saturable process. The nucleus-to-medium dye gradient was more then 30:1 in media containing 0.1% or less Trypan Blue. The apparent affinity constant for nuclear binding of the dye was 3.1 X 10(4)l/mol. Albumin partially inhibited the nuclear staining. More than 0.5% Trypan Blue in the medium was toxic per se. In the absence of albumin, 0.5 or 20 mmol/l alloxan, 1 mmol/l N-ethylmaleimide, or 0.1 mmol/l chloromercuribenzene-p-sulphonic acid, but not 20 mmol/l streptozotocin, increased the frequency of islet cells stained with 0.1% Trypan Blue. The absorbance of nuceli was also increased in cells treated with alloxan or N-ethylmaleimide, but not in those treated with chloromercuribenze-p-sulphonic acid. It is concluded that alloxan rapidly increases the permeability of the plasma membrane in mouse beta-cells. This action of alloxan appears to be more acute than any such effect of streptozotocin.