Effects of prostacyclin and its stable analog, iloprost, upon insulin secretion in isolated pancreatic islets

We investigated the effects of prostacylin in three concentrations of 2.7 nM, 53.8 nM and 267 nM on insulin release by isolated rat islets. The lowest concentration produced no significant effects. The middle concentration led to stimulation followed by inhibition of the reaction studied, while the highest concentration strongly depressed insulin release. These effects of prostacyclin appeared to be specified, because they were mimicked by its stable analog, iloprost, but not by its metabolite, 6-keto-PGF_1α. The results suggest that prostacyclin generated in islets might exert locally an influencing upon insulin release.     

Secretin uncouples glucose inhibition of glucagon-producing cells resulting in a simultaneous stimulation of both glucagon and insulin release

The effect of secretin on glucagon and insulin release and its interaction with glucose has been studied in cultured mouse pancreatic islets by column perifusion. Glucose alone showed the well-known stimulation of insulin release and inhibition of glucagon release. Addition of 10 mM secretin increased glucagon secretion at 3 mM D-glucose by 300% while no change in insulin release could be seen at this low glucose concentration. At maximal stimulation of insulin release by 20 mM D-glucose addition of 10 nM secretin increased insulin release by 30%. Despite this insulin concentration and the high glucose concentration an increase in glucagon secretion of 1800% was found. These effects of secretin were dose-dependent at 10 mM D-glucose with 1 nM secretin being the lowest effective dose.     

Vasoactive intestinal polypeptide enhances hormone content and insulin release in cultured fetal rat islets

The effect of porcine vasoactive intestinal polypeptide (VIP) on development of the biphasic insulin release response in cultured fetal rat islets was investigated. Fetal islets, 21.5 days gestational age, were cultured for 7 days in RPMI 1640 culture medium containing either 2.8 or 11.1 mM glucose adn subsequently challenged with 16.7 mM glucose in a perfusion system. Islets were exposed to VIP at a final concentration of 13.2 nM by adding the peptide to the perifusion buffer (acute exposure) or by adding it to the culture medium throughout the culture period (chronic exposure). Islet hormone and DNA contents were also quantitated at the end of the culture period. Acute exposure to VIP resulted in no alterations of the insulin release pattern after culture in the presence of either glucose concentration. However, chronic treatment of islets with 13.2 nM VIP in the presence of 2.8 mM glucose resulted in significant increases in the maximum rate of insulin release during the first phase and the total amount of insulin release during both phases. Similarly, islets cultured in the presence of 11.1 mM glucose and 13.2 nM VIP demonstrated enhanced biphasic insulin release patterns with increased maximum rate and total amount of release during both phases. The presence of VIP and 2.8 mM glucose increased islet glucagon and somatostatin contents, but islet DNA and insulin contents remained unchanged. These findings indicate that VIP plays a significant role in the in vitro development of the biphasic insulin release pattern and may be a factor controlling the maturation of the fetal islet in vivo.     

Reduced sensitivity to dexamethasone of pancreatic islets from obese (fa/fa) rats.

The direct effects of dexamethasone exposure on insulin secretion from islets of fa/fa rats and their lean littermates (Fa/?) were compared. After 72 h culture in 1 nM dexamethasone, glucose (27.5 mM)-stimulated insulin secretion over 90 min from islets of lean rats was significantly decreased compared with islets cultured without dexamethasone (12.9 +/- 1.4 vs. 5.7 +/- 1.0% of total islet content, p < 0.05). Higher doses of dexamethasone for 24-48 h culture produced similar effects. For islets of fa/fa rats, the minimum inhibitory concentration of dexamethasone was 10-fold higher, and islets required at least 48 h exposure for inhibitory effects to be observed. Dexamethasone also decreased the insulin response by islets to glybenclamide, indicating that dexamethasone effects were not specific to glucose transport or metabolism. The results suggest that islets of fa/fa rats may be less sensitive to direct inhibitory effects of glucocorticoids on glucose-stimulated insulin release than islets of lean animals.     

A novel action of activin A: stimulation of insulin secretion in rat pancreatic islets

The present study was conducted to examine an action of activin A on insulin secretion from rat pancreatic islets. In a batch incubation system, activin A stimulated insulin secretion in a dose-dependent manner at concentrations higher than 1 nM. Furthermore, activin A greatly potentiated glucose-induced insulin release. When islets were perifused with 1 nM activin A, insulin secretion was barely affected in this system. However, the insulin response to 16.7 mM glucose was greatly enhanced. Both the first and second phases of insulin response were enhanced by 1 nM activin A. These results indicate that, in addition to its known actions on pituitary-gonadal and hematopoietic systems, activin A modulates the function of pancreatic islets and stimulates insulin secretion.     

Endothelin-1 induces prostacyclin release from bovine aortic endothelial cells

The effects of endothelin-1 (ET-1) on the release of prostacyclin from cultured bovine aortic endothelial cells were studied. ET-1 induced a time- and dose-dependent release of 6-keto PGF1 alpha, the stable metabolite of prostacyclin, with an apparent EC50 value of 3.0 +/- 0.9 nM (n = 6). ET-1 up to a concentration of 500 nM did not affect cellular integrity. Preincubation of the cells for 30 min with 10 microM indomethacin inhibited ET-1 (100 nM) - induced prostacyclin release by 90%. These findings indicate that ET-1 can directly stimulate prostacyclin release from endothelial cells probably through a receptor mediated mechanism.     

Neuropeptide W in the rat pancreas: potentiation of glucose-induced insulin release and Ca2+ influx through L-type Ca2+ channels in beta-cells and localization in islets

Neuropeptide W (NPW) is a regulatory peptide that acts via two subtypes of G protein-coupled receptors, GPR7 and GPR8. Evidence has been provided that NPW is involved in the central regulation of energy homeostasis and feeding behavior. In this study, we examined the effects of NPW on insulin release and localization of NPW in the rat pancreas. NPW (10-100 nM) significantly increased insulin release in the presence of 8.3 mM, but not 2.8 mM, glucose in the isolated rat islets. By fura-2 microfluorometry, NPW (1-100 nM) concentration-dependently increased cytosolic Ca(2+) concentration ([Ca(2+)](i)) at 8.3 mM glucose in rat single beta-cells. The NPW-induced [Ca(2+)](i) increase was abolished under external Ca(2+)-free conditions and by an L-type Ca(2+) channel blocker nifedipine (10 microM). RT-PCR analysis revealed that mRNA for NPW was expressed in the rat pancreas and hypothalamus. Double immunohistochemical analysis showed that NPW-immunoreactivity was found in islets and co-localized with insulin-containing beta-cells, but not glucagon-containing alpha-cells and somatostatin-containing delta-cells. These results suggest that NPW could serve as a local modulator of glucose-induced insulin release in rat islets. NPW directly activates beta-cells to enhance Ca(2+) influx through voltage-dependent L-type Ca(2+) channels and potentiates glucose-induced insulin release.     

Islet-cell metabolism during insulin release. Effects of glucose, citrate, octanoate, tolbutamide, glucagon and theophylline

1. Concentrations of glucose 6-phosphate and 6-phosphogluconate were studied in islets of Langerhans isolated from rat pancreas and incubated in the presence of various agents that induce insulin release. 2. In response to rising concentrations of extracellular glucose (2-10mm) there is a linear increase in the intracellular concentration of glucose 6-phosphate, though this is not the case for 6-phosphogluconate, the intracellular concentration of which only increases when the external glucose concentration exceeds 5mm. 3. Tolbutamide, octanoate and citrate, all of which promote insulin secretion from isolated islets, increase the intracellular concentrations of glucose 6-phosphate and 6-phosphogluconate. The results obtained in the presence of octanoate and citrate are compatible with an inhibitory effect of citrate on islet-cell phosphofructokinase. 4. Theophylline and glucagon when incubated with islets in vitro promote insulin release and cause a rise in 6-phosphogluconate concentration and not in that of glucose 6-phosphate. 5. It is suggested that the further metabolism of glucose 6-phosphate through a pathway other than glycolysis is essential for insulin release. One such pathway involves its oxidation to 6-phosphogluconate, which seems to be a necessary accompaniment of insulin secretion due to glucose. The possibility that agents other than glucose promote insulin release by enhancing the oxidation of glucose 6-phosphate through this pathway is discussed.     

Calbindin-D(28k) controls [Ca(2+)](i) and insulin release. Evidence obtained from calbindin-d(28k) knockout mice and beta cell lines

The role of the calcium-binding protein, calbindin-D(28k) in potassium/depolarization-stimulated increases in the cytosolic free Ca(2+) concentration ([Ca(2+)](i)) and insulin release was investigated in pancreatic islets from calbindin-D(28k) nullmutant mice (knockouts; KO) or wild type mice and beta cell lines stably transfected and overexpressing calbindin. Using single islets from KO mice and stimulation with 45 mM KCl, the peak of [Ca(2+)](i) was 3.5-fold greater in islets from KO mice compared with wild type islets (p < 0.01) and [Ca(2+)](i) remained higher during the plateau phase. In addition to the increase in [Ca(2+)](i) in response to KCl there was also a significant increase in insulin release in islets isolated from KO mice. Evidence for modulation by calbindin of [Ca(2+)](i) and insulin release was also noted using beta cell lines. Rat calbindin was stably expressed in betaTC-3 and betaHC-13 cells. In response to depolarizing concentrations of K(+), insulin release was decreased by 45-47% in calbindin expressing betaTC cells and was decreased by 70-80% in calbindin expressing betaHC cells compared with insulin release from vector transfected betaTC or betaHC cells (p < 0.01). In addition, the K(+)-stimulated intracellular calcium peak was markedly inhibited in calbindin expressing betaHC cells compared with vector transfected cells (225 nM versus 1,100 nM, respectively). Buffering of the depolarization-induced rise in [Ca(2+)](i) was also observed in calbindin expressing betaTC cells. In summary, our findings, using both isolated islets from calbindin-D(28k) KO mice and beta cell lines, establish a role for calbindin in the modulation of depolarization-stimulated insulin release and suggest that calbindin can control the rate of insulin release via regulation of [Ca(2+)](i).     

Autocrine regulation of glucose metabolism in pancreatic islets

We evaluated the possible autocrine modulatory effect of insulin on glucose metabolism and glucose-induced insulin secretion in islets isolated from normal hamsters. We measured 14CO2 and 3H2O production from d-[U-14C]glucose and d-[5-3H]glucose, respectively, in islets incubated with 0.6, 3.3, 8.3, and 16.7 mM glucose alone or with 5 or 15 mU/ml insulin, anti-insulin guinea pig serum (1:500), 25 microM nifedipine, or 150 nM wortmannin. Insulin release was measured (radioimmunoassay) in islets incubated with 3.3 or 16.7 mM glucose with or without 75, 150, and 300 nM wortmannin. Insulin significantly enhanced 14CO2 and 3H2O production with 3.3 mM glucose but not with 0.6, 8.3, or 16.7 mM glucose. Addition of anti-insulin serum to the medium with 8.3 and 16.7 mM glucose decreased 14CO2 and 3H2O production significantly. A similar decrease was obtained in islets incubated with 8.3 and 16.7 mM glucose and wortmannin or nifedipine. This latter effect was reversed by adding 15 mU/ml insulin to the medium. Glucose metabolism was almost abolished when islets were incubated in a Ca2+-deprived medium, but this effect was not reversed by insulin. No changes were found in 14CO2 and 3H2O production by islets incubated with 3.3 mM glucose and anti-insulin serum, wortmannin, or nifedipine in the media. Addition of wortmannin significantly decreased insulin release induced by 16.7 mM glucose in a dose-dependent manner. Our results suggest that insulin exerts a physiological autocrine stimulatory effect on glucose metabolism in intact islets as well as on glucose-induced insulin release. Such an effect, however, depends on the glucose concentration in the incubation medium.     

Mechanism of action of growth-hormone-releasing hormone in stimulating insulin secretion in vitro from isolated rat islets and dispersed islet cells

Human growth-hormone-releasing hormone [(1-44)NH2] (hGHRH) was a potent stimulus for insulin release from rat islets of Langerhans in vitro; the optimum concentration used was 10(-11) M. The dose response curves for hGHRH effects on insulin secretion were notably different in intact islets of Langerhans compared to cultured dispersed islet cells. Pancreatic islets responded to a very low hGHRH concentration (10(-12) M), but at a higher hGHRH concentration (10(-9) M) no stimulation of insulin release was observed. When somatostatin antiserum was included in the incubation medium, hGHRH (10(-9) M) stimulated insulin release from intact islets. In cultured dispersed islet cells, which are principally insulin-secreting B cells, hGHRH directly and potently stimulated insulin release even at a concentration of 10(-9) M. Addition of somatostatin (10(-7), 10(-8) M) significantly reduced the hGHRH-induced insulin-secretory responses of dispersed islet cells. hGHRH (10(-11)-10(-9) M) raised islet cAMP levels; individually, hGHRH and theophylline exerted positive effects on insulin release, their combined effect was greater than that caused by either one. We conclude that hGHRH directly affects insulin secretion in vitro by a cAMP-dependent mechanism, and that the difference in responses of intact islets versus islet cells to increasing concentrations of hGHRH may be related to hGHRH-induced release of somatostatin in intact rat islets.     

Rapid insulinotropic action of low doses of bisphenol-A on mouse and human islets of Langerhans: role of estrogen receptor β

Bisphenol-A (BPA) is a widespread endocrine-disrupting chemical (EDC) used as the base compound in the manufacture of polycarbonate plastics. It alters pancreatic β-cell function and can be considered a risk factor for type 2 diabetes in rodents. Here we used ERβ-/- mice to study whether ERβ is involved in the rapid regulation of K(ATP) channel activity, calcium signals and insulin release elicited by environmentally relevant doses of BPA (1 nM). We also investigated these effects of BPA in β-cells and whole islets of Langerhans from humans. 1 nM BPA rapidly decreased K(ATP) channel activity, increased glucose-induced [Ca(2+)](i) signals and insulin release in β-cells from WT mice but not in cells from ERβ-/- mice. The rapid reduction in the K(ATP) channel activity and the insulinotropic effect was seen in human cells and islets. BPA actions were stronger in human islets compared to mouse islets when the same BPA concentration was used. Our findings suggest that BPA behaves as a strong estrogen via nuclear ERβ and indicate that results obtained with BPA in mouse β-cells may be extrapolated to humans. This supports that BPA should be considered as a risk factor for metabolic disorders in humans.     

Tacrolimus suppresses glucose-induced insulin release from pancreatic islets by reducing glucokinase activity

Tacrolimus is widely used for immunosuppressant therapy, including various organ transplantations. One of its main side effects is hyperglycemia due to reduced insulin secretion, but the mechanism remains unknown. We have investigated the metabolic effects of tacrolimus on insulin secretion at a concentration that does not influence insulin content. Twenty-four-hour exposure to 3 nM tacrolimus reduced high glucose (16.7 mM)-induced insulin secretion (control 2.14 +/- 0.08 vs. tacrolimus 1.75 +/- 0.02 ng.islet(-1).30 min(-1), P < 0.01) without affecting insulin content. In dynamic experiments, insulin secretion and NAD(P)H fluorescence during a 20-min period after 10 min of high-glucose exposure were reduced in tacrolimus-treated islets. ATP content and glucose utilization of tacrolimus-treated islets in the presence of 16.7 mM glucose were less than in control (ATP content: control 9.69 +/- 0.99 vs. tacrolimus 6.52 +/- 0.40 pmol/islet, P < 0.01; glucose utilization: control 103.8 +/- 6.9 vs. tacrolimus 74.4 +/- 5.1 pmol.islet(-1).90 min(-1), P < 0.01). However, insulin release from tacrolimus-treated islets was similar to that from control islets in the presence of 16.7 mM alpha-ketoisocaproate, a mitochondrial fuel. Glucokinase activity, which determines glycolytic velocity, was reduced by tacrolimus treatment (control 65.3 +/- 3.4 vs. tacrolimus 49.9 +/- 2.8 pmol.islet(-1).60 min(-1), P < 0.01), whereas hexokinase activity was not affected. These results indicate that glucose-stimulated insulin release is decreased by chronic exposure to tacrolimus due to reduced ATP production and glycolysis derived from reduced glucokinase activity.     

Somatostatin inhibits insulin release via SSTR2 in hamster clonal beta-cells and pancreatic islets

Somatostatin (SST) inhibits pancreatic endocrine secretion. It is generally accepted that SSTR2 and SSTR5 mediate the inhibition of glucagon and insulin release, respectively. The present study was performed to test the hypothesis that SSTR2, but not SSTR5, mediates SST-induced inhibition of insulin release in hamster beta-cells. Both hamster clonal beta-cells HIT-T15 and pancreatic islets were used to test this hypothesis. Both SST and a nonpeptide SSTR2 agonist L-779,976 (1-100 nM) inhibited insulin release from HIT-T15 and islets in a concentration-dependent manner. In contrast, nonpeptide agonists for SSTR1, 3, 4 and 5 at the highest concentration studied (1 microM) failed to inhibit insulin release. PRL-2903, a peptide SSTR2 antagonist (0.1-1 muicroM), antagonized SST-induced inhibition of insulin release in a concentration-dependent manner. Taken together, we conclude that, in hamster beta-cells, SST inhibits insulin release via SSTR2 but not SSTR5.     

Effects of mannoheptulose and DL-glyceraldehyde on glucose induced insulin release and adenosine 3',5'-monophosphate levels in isoalted islets of rat pancreas.

The effects of mannoheptulose and DL-glyceraldehyde on glucose-induced insulin release and cycli AMP levels in islets isolated from rat pancreas were investigated. Mannoheptulose inhibition on glucose-induced insulin release was observed after only 5-min incubation period, indicating an inhibitory effect on the early phase of insulin release. This inhibition on insulin release was accompanied with the simultaneous depression of cyclic AMP levels in islets. By the addition of DL-glyceraldehyde to the medium in which glucose and mannoheptulose were present, the depressed cyclic AMP levels in islets were recovered to the control level completely but the restoration of insulin release in the early phase was not complete. In the absence of glucose, DL-glyceraldehyde did not demonstrate a significant increase of insulin release during 5 min incubation, though a marked stimulation was observed after 30-min incubation. Cyclic AMP levels in islets were not affected by DL-glyceraldehyde. When DL-glyceraldehyde was added to the medium with glucose, significant inhibition of glucos-induced insulin release in its early phase was observed without the reduction of cyclic AMP levels in islets. From these findings, the following possibilities are suggested and discussed. 1. Maintenance of the cyclic AMP levels in islets is a necessary but insufficient condition for glucose-induced insulin release particularly for its early phase. 2. Glucose-induced insulin release seems to depend on both the binding of glucose with glucoreceptor and the supply of some metabolites. Mannoheptulose inhibits both mechanisms. DL-glyceraldehyde may supply metabolites but competitively inhibit the binding of glucose to the glucoreceptor.     

Effects of galnon, a non-peptide galanin-receptor agonist, on insulin release from rat pancreatic islets

Galanin is a neurotransmitter peptide that suppresses insulin secretion. The present study aimed at investigating how a non-peptide galanin receptor agonist, galnon, affects insulin secretion from isolated pancreatic islets of healthy Wistar and diabetic Goto-Kakizaki (GK) rats. Galnon stimulated insulin release potently in isolated Wistar rat islets; 100 microM of the compound increased the release 8.5 times (p<0.001) at 3.3 mM and 3.7 times (p<0.001) at 16.7 mM glucose. Also in islet perifusions, galnon augmented several-fold both acute and late phases of insulin response to glucose. Furthermore, galnon stimulated insulin release in GK rat islets. These effects were not inhibited by the presence of galanin or the galanin receptor antagonist M35. The stimulatory effects of galnon were partly inhibited by the PKA and PKC inhibitors, H-89 and calphostin C, respectively, at 16.7 but not 3.3 mM glucose. In both Wistar and GK rat islets, insulin release was stimulated by depolarization of 30 mM KCl, and 100 microM galnon further enhanced insulin release 1.5-2 times (p<0.05). Cytosolic calcium levels, determined by fura-2, were increased in parallel with insulin release, and the L-type Ca2+-channel blocker nimodipine suppressed insulin response to glucose and galnon. In conclusion, galnon stimulates insulin release in islets of healthy rats and diabetic GK rats. The mechanism of this stimulatory effect does not involve galanin receptors. Galnon-induced insulin release is not glucose-dependent and appears to involve opening of L-type Ca2+-channels, but the main effect of galnon seems to be exerted at a step distal to these channels, i.e., at B-cell exocytosis.     

Regulatory effects of 5'-deoxypyridoxal on glutamate dehydrogenase activity and insulin secretion in pancreatic islets

It has been known that glutamate, generated by glutamate dehydrogenase (GDH), acts as an intracellular messenger in insulin exocytosis in pancreatic beta cells. Here we demonstrate the correlation of GDH activity and insulin release in rat pancreatic islets perfused with 5'-deoxypyridoxal. Perfusion of islets with 5'-deoxypyridoxal, an effective inhibitor of GDH, reduced the islet GDH activity at concentration-dependent manner. Treatment of 5'-deoxypyridoxal up to 2 mM did not affect the cell viability. There was reduction in V(max) values on average about 60%, whereas no changes in K(m) values for substrates and coenzymes were observed. The concentration of GDH on the Western blot analysis and the level of GDH mRNA remained unchanged. The concentration of glutamate decreased by 52%, whereas the concentration of 2-oxoglutarate increased up to 2.3-fold in the presence of 5'-deoxypyridoxal. 5'-Deoxypyridoxal had no effects on inhibition by GTP and activation by ADP or L-leucine of islet GDH. In parallel with the inhibition of GDH activity, perfusion of islets with 5'-deoxypyridoxal reduced insulin release up to 2.5-fold. Although precise mechanism for correlation between GDH activity and insulin release remains to be studied further, our results suggest a possibility that the inhibitory effect of 5'-deoxypyridoxal on islet GDH activity may correlate with its effect on insulin release.     

Inhibitory effect of rat amylin on the insulin responses to glucose and arginine in the perfused rat pancreas

Amylin, a 37-amino acid polypeptide, is the main component of amyloid deposits in the islets of Langerhans, and has been identified in the B-cell secretory granules. We have investigated the effect of rat amylin on the insulin and glucagon release by the isolated, perfused rat pancreas. Amylin infusion at 750 nM, markedly reduced unstimulated insulin release (ca. 50%, P less than 0.025), whereas it did not modify glucagon output. At the same concentration, amylin also blocked the insulin response to 9 mM glucose (ca. 80%, P less than 0.025) without affecting the suppressor effect of glucose on glucagon release. The inhibitory effect of amylin on glucose-induced insulin secretion was confirmed by lowering the amylin concentration (500 nM) and increasing the glucose stimulus (11 mM); again, no effect of amylin on glucagon release was observed. Finally, amylin, at 500 nM, reduced the insulin response to 3.5 mM arginine (ca. 40%, P less than 0.025) without modifying the secretion of glucagon elicited by this amino acid. It can be concluded that, in the rat pancreas, the inhibitory effect of homologous amylin on unstimulated insulin secretion, as well as on the insulin responses to metabolic substrates (glucose and arginine), favours the concept of this novel peptide as a potential diabetogenic agent.     

Expression of glucagon-like peptide-1 (GLP-1) receptor and the effect of GLP-1-(7-36) amide on insulin release by pancreatic islets during rat ontogenic development.

The expression of glucagon-like peptide-1 (GLP-1) receptor and the effects of GLP-1-(7-36) amide (t-GLP-1) on glucose metabolism and insulin release by pancreatic islets during rat development were studied. GLP-1 receptor mRNA was found in significant amounts in pancreatic islets from all age groups studied, GLP-1 receptor expression being maximal when pancreatic islets were incubated at physiological glucose concentration (5.5 mM), but decreasing significantly when incubated with either 1.67 or 16.7 mM glucose. Glucose utilization and oxidation by pancreatic islets from fetal and adult rats rose as a function of glucose concentration, always being higher in fetal than in adult islets. The addition of t-GLP-1 to the incubation medium did not modify glucose metabolism but gastric inhibitory polypeptide and glucagon significantly increased glucose utilization by fetal and adult pancreatic islets at 16.7 mM glucose. At this concentration, glucose produced a significant increase in insulin release by the pancreatic islets from 10-day-old and 20-day-old suckling rats and adult rats, whereas those from fetuses showed only a significant increase when glucose was raised from 1.67 to 5.5 mM. t-GLP-1 elicited an increase in insulin release by pancreatic islets from all the experimental groups when the higher glucose concentrations were used. Our findings indicate that GLP-1 receptors and the effect of t-GLP-1 on insulin release are already present in the fetus, and they therefore exclude the possibility that alterations in the action of t-GLP-1 are responsible for the unresponsiveness of pancreatic beta cells to glucose in the fetus, but stimulation of t-GLP-1 release by food ingestion in newborns may partially confer glucose competence on beta cells.     

Dimethylurea: A radical scavenger that protects isolated pancreatic islets from the effects of alloxan and dihydroxyfumarate exposure

The hydroxyl radical scavenger dimethylurea was tested $\text{in vitro}$ for possible effectiveness in protecting insulin secreting cells from the deleterious effects of alloxan and dihydroxyfumarate. A five min exposure of isolated rat pancreatic islets to alloxan (0.15 and 0.20 mg/ml) caused a concentration dependent decrease in subsequent glucose-stimulated insulin release. The presence of 40 mM dimethylurea during alloxan exposure attenuated or eliminated the inhibition of insulin release caused by alloxan. Exposure of islets to autooxidizing dihydroxyfumarate, a known generator of hydroxyl free radicals, also caused an inhibition of glucose-stimulated insulin release. This effect was also eliminated when dimethylurea was present during the exposure period. These results support the concept that alloxan produces its insulin inhibitory effects $\text{in vitro}$ via the generation of hydroxyl free radicals.