Metformin disrupts insulin secretion, causes proapoptotic and oxidative effects in rat pancreatic beta-cells in vitro

Metformin is the first-line drug to treat type 2 diabetes mellitus. Its mechanism of action is still debatable, and recent studies report that metformin attenuates oxidative stress. This study evaluated the in vitro antioxidant effects of a broad range of metformin concentrations on insulin-producing cells. The cell cycle, metabolism, glucose-stimulated insulin secretion, and cell death were evaluated to determine the biguanide effects on beta-cell function and survival. Antioxidant potential was based on reactive oxygen species (ROS), reduced glutathione (GSH), oxidative stress biomarker levels, and antioxidant enzyme and transcriptional factor Nrf2 activities. The results demonstrate that metformin disrupted GSIS in a concentration-dependent manner, lowered insulin content, and attenuated beta-cell metabolism. At high concentrations, metformin induced cell death and cell cycle arrest as well as increased ROS generation, consequently reducing GSH content. Although carbonylated protein content was elevated, indicating oxidative stress, the antioxidant enzyme and Nrf2 activities were not altered. In conclusion, our results show that metformin disrupts pancreatic beta-cell functionality but does not exert a putative antioxidant effect. It is important to note that the drug could potentially affect beta-cells, especially at high circulating levels.

Early Cytokine-Induced Transient NOX2 Activity Is ER Stress-Dependent and Impacts β-Cell Function and Survival

In type 1 diabetes (T1D) development, proinflammatory cytokines (PIC) released by immune cells lead to increased reactive oxygen species (ROS) production in β-cells. Nonetheless, the temporality of the events triggered and the role of different ROS sources remain unclear. Isolated islets from C57BL/6J wild-type (WT), NOX1 KO and NOX2 KO mice were exposed to a PIC combination. We show that cytokines increase O₂^•− production after 2 h in WT and NOX1 KO but not in NOX2 KO islets. Using transgenic mice constitutively expressing a genetically encoded compartment specific H₂O₂ sensor, we show, for the first time, a transient increase of cytosolic/nuclear H₂O₂ in islet cells between 4 and 5 h during cytokine exposure. The H₂O₂ increase coincides with the intracellular NAD(P)H decrease and is absent in NOX2 KO islets. NOX2 KO confers better glucose tolerance and protects against cytokine-induced islet secretory dysfunction and death. However, NOX2 absence does not counteract the cytokine effects in ER Ca²⁺ depletion, Store-Operated Calcium Entry (SOCE) increase and ER stress. Instead, the activation of ER stress precedes H₂O₂ production. As early NOX2-driven ROS production impacts β-cells’ function and survival during insulitis, NOX2 might be a potential target for designing therapies against early β-cell dysfunction in the context of T1D onset.

Intermittent Fasting for Twelve Weeks Leads to Increases in Fat Mass and Hyperinsulinemia in Young Female Wistar Rats

Fasting is known to cause physiological changes in the endocrine pancreas, including decreased insulin secretion and increased reactive oxygen species (ROS) production. However, there is no consensus about the long-term effects of intermittent fasting (IF), which can involve up to 24 hours of fasting interspersed with normal feeding days. In the present study, we analyzed the effects of alternate-day IF for 12 weeks in a developing and healthy organism. Female 30-day-old Wistar rats were randomly divided into two groups: control, with free access to standard rodent chow; and IF, subjected to 24-hour fasts intercalated with 24-hours of free access to the same chow. Alternate-day IF decreased weight gain and food intake. Surprisingly, IF also elevated plasma insulin concentrations, both at baseline and after glucose administration collected during oGTT. After 12 weeks of dietary intervention, pancreatic islets displayed increased ROS production and apoptosis. Despite their lower body weight, IF animals had increased fat reserves and decreased muscle mass. Taken together, these findings suggest that alternate-day IF promote β -cell dysfunction, especially in developing animals. More long-term research is necessary to define the best IF protocol to reduce side effects.

NADPH oxidase-2 does not contribute to β-cell glucotoxicity in cultured pancreatic islets from C57BL/6J mice

High glucose-induced oxidative stress and increased NADPH oxidase-2 (NOX2) activity may contribute to the progressive decline of the functional β-cell mass in type 2 diabetes. To test that hypothesis, we characterized, in islets from male NOX2 knockout (NOX2-KO) and wild-type (WT) C57BL/6J mice cultured for up to 3 weeks at 10 or 30 mmol/l glucose (G10 or G30), the in vitro effects of glucose on cytosolic oxidative stress using probes sensing glutathione oxidation (GRX1-roGFP2), thiol oxidation (roGFP1) or H₂O₂ (roGFP2-Orp1), on β-cell stimulus-secretion coupling events and on β-cell apoptosis. After 1-2 days of culture in G10, the glucose stimulation of insulin secretion (GSIS) was ∼1.7-fold higher in NOX2-KO vs. WT islets at 20-30 mmol/l glucose despite similar rises in NAD(P)H and intracellular calcium concentration ([Ca²⁺]_i) and no differences in cytosolic GRX1-roGFP2 oxidation. After long-term culture at G10, roGFP1 and roGFP2-Orp1 oxidation and β-cell apoptosis remained low, and the glucose-induced rises in NAD(P)H, [Ca²⁺]_i and GSIS were similarly preserved in both islet types. After prolonged culture at G30, roGFP1 and roGFP2-Orp1 oxidation increased in parallel with β-cell apoptosis, the glucose sensitivity of the NADPH, [Ca²⁺]_i and insulin secretion responses increased, the maximal [Ca²⁺]_i response decreased, but maximal GSIS was preserved. These responses were almost identical in both islet types. In conclusion, NOX2 is a negative regulator of maximal GSIS in C57BL/6J mouse islets, but it does not detectably contribute to the in vitro glucotoxic induction of cytosolic oxidative stress and alterations of β-cell survival and function.

Long-term disruption of maternal glucose homeostasis induced by prenatal glucocorticoid treatment correlates with miR-29 upregulation

Excess of glucocorticoids (GCs) during pregnancy is strongly associated with the programming of glucose intolerance in the offspring. However, the impact of high GC levels on maternal metabolism is not clearly documented. This study aimed to test the hypothesis that mothers exposed to elevated levels of GCs might also display long-term disturbances in glucose homeostasis. Dexamethasone (DEX) was administered noninvasively to the mothers via drinking water between the 14th and the 19th days of pregnancy. Mothers were subjected to glucose and insulin tolerance tests at 1, 2, 3, 6, and 12 mo postweaning. Pregnant rats not treated with DEX and age-matched virgin rats were used as controls. Pancreatic islets were isolated at the 20th day of pregnancy and 12 mo postweaning in order to evaluate glucose-stimulated insulin secretion. The expression of the miR-29 family was also studied due to its responsiveness to GCs and its well-documented role in the regulation of pancreatic β-cell function. Rats treated with DEX during pregnancy presented long-term glucose intolerance and impaired insulin secretion. These changes correlated with 1) increased expression of miR-29 and its regulator p53, 2) reduced expression of syntaxin-1a, a direct target of miR-29, and 3) altered expression of genes related to cellular senescence. Our data demonstrate that the use of DEX during pregnancy results in deleterious outcomes to the maternal metabolism, hallmarked by reduced insulin secretion and glucose intolerance. This maternal metabolic programming might be a consequence of time-sustained upregulation of miR-29s in maternal pancreatic islets.

Fish oil supplementation for two generations increases insulin sensitivity in rats

We investigated the effect of fish oil supplementation for two consecutive generations on insulin sensitivity in rats. After the nursing period (21 days), female rats from the same prole were divided into two groups: (a) control group and (b) fish oil group. Female rats were supplemented with water (control) or fish oil at 1 g/kg body weight as a single bolus for 3 months. After this period, female rats were mated with male Wistar rats fed on a balanced chow diet (not supplemented). Female rats continued to receive supplementation throughout gestation and lactation periods. The same treatment was performed for the next two generations (G1 and G2). At 75 days of age, male offspring from G1 and G2 generations from both groups were used in the experiments. G1 rats did not present any difference with control rats. However, G2 rats presented reduction in glycemia and lipidemia and improvement in in vivo insulin sensitivity (model assessment of insulin resistance, insulin tolerance test) as well as in vitro insulin sensitivity in soleus muscle (glucose uptake and metabolism). This effect was associated with increased insulin-stimulated p38 MAP kinase phosphorylation and lower n-6/n-3 fatty acid ratio, but not with activation of proteins from insulin signaling (IR, IRS-1 and Akt). Global DNA methylation was decreased in liver but not in soleus muscle. These results suggest that long-term fish oil supplementation improves insulin sensitivity in association with increased insulin-stimulated p38 activation and decreased n-6:n-3 ratio in skeletal muscle and decreased global DNA methylation in liver.

Diet-induced obesity impairs AKT signalling in the retina and causes retinal degeneration

Retinopathy, a common complication of diabetes, is characterized by an unbalanced production of nitric oxide (NO), a process regulated by nitric oxide synthase (NOS). We hypothesized that retinopathy might stem from changes in the insulin receptor substrate (IRS)/PI3K/AKT pathway and/or expression of NOS isoforms. Thus, we analysed the morphology and apoptosis index in retinas of obese rats in whom insulin resistance had been induced by a high-fat diet (HFD). Immunoblotting analysis revealed that the retinal tissue of HFD rats had lower levels of AKT(1) , eNOS and nNOS protein than those of samples taken from control animals. Furthermore, immunohistochemical analyses indicated higher levels of iNOS and 4-hydroxynonenal and a larger number of apoptotic nuclei in HFD rats. Finally, both the inner and outer retinal layers of HFD rats were thinner than those in their control counterparts. When considered alongside previous results, these patterns suggest two major ways in which HFD might impact animals: direct activity of ingested fatty acids and/or via insulin-resistance-induced changes in intracellular pathways. We discuss these possibilities in further detail and advocate the use of this animal model for further understanding relationships between retinopathy, metabolic syndrome and type 2 diabetes.

Expression of NADPH oxidase in human pancreatic islets

AIMS

NADPH oxidase (NOX) is a known source of superoxide anions in phagocytic and non-phagocytic cells. In this study, the presence of this enzyme in human pancreatic islets and the importance of NADPH oxidase in human β-cell function were investigated.

MAIN METHODS AND KEY FINDINGS

In isolated human pancreatic islets, the expression of NADPH oxidase components was evidenced by real-time PCR (p22(PHOX), p47(PHOX) and p67(PHOX)), Western blotting (p47(PHOX) and p67(PHOX)) and immunohistochemistry (p47(PHOX), p67(PHOX) and gp91(PHOX)). Immunohistochemistry experiments showed co-localization of p47(PHOX), p67(PHOX) and gp91(PHOX) (isoform 2 of NADPH oxidase-NOX2) with insulin secreting cells. Inhibition of NADPH oxidase activity impaired glucose metabolism and glucose-stimulated insulin secretion.

SIGNIFICANCE

These findings demonstrate the presence of the main intrinsic components of NADPH oxidase comprising the NOX2 isoform in human pancreatic islets, whose activity also contributes to human β-cell function.

Metabolic disorders and adipose tissue insulin responsiveness in neonatally STZ-induced diabetic rats are improved by long-term melatonin treatment

Diabetes mellitus is a product of low insulin sensibility and pancreatic β-cell insufficiency. Rats with streptozotocin-induced diabetes during the neonatal period by the fifth day of age develop the classic diabetic picture of hyperglycemia, hypoinsulinemia, polyuria, and polydipsia aggravated by insulin resistance in adulthood. In this study, we investigated whether the effect of long-term treatment with melatonin can improve insulin resistance and other metabolic disorders in these animals. At the fourth week of age, diabetic animals started an 8-wk treatment with melatonin (1 mg/kg body weight) in the drinking water at night. Animals were then killing, and the sc, epididymal (EP), and retroperitoneal (RP) fat pads were excised, weighed, and processed for adipocyte isolation for morphometric analysis as well as for measuring glucose uptake, oxidation, and incorporation of glucose into lipids. Blood samples were collected for biochemical assays. Melatonin treatment reduced hyperglycemia, polydipsia, and polyphagia as well as improved insulin resistance as demonstrated by constant glucose disappearance rate and homeostasis model of assessment-insulin resistance. However, melatonin treatment was unable to recover body weight deficiency, fat mass, and adipocyte size of diabetic animals. Adiponectin and fructosamine levels were completely recovered by melatonin, whereas neither plasma insulin level nor insulin secretion capacity was improved in diabetic animals. Furthermore, melatonin caused a marked delay in the sexual development, leaving genital structures smaller than those of nontreated diabetic animals. Melatonin treatment improved the responsiveness of adipocytes to insulin in diabetic animals measured by tests of glucose uptake (sc, EP, and RP), glucose oxidation, and incorporation of glucose into lipids (EP and RP), an effect that seems partially related to an increased expression of insulin receptor substrate 1, acetyl-coenzyme A carboxylase and fatty acid synthase. In conclusion, melatonin treatment was capable of ameliorating the metabolic abnormalities in this particular diabetes model, including insulin resistance and promoting a better long-term glycemic control.

Cytotoxicity and cytoprotective effects of citrus flavonoids on insulin-secreting cells BRIN-BD11: beneficial synergic effects

Flavonoids, in general, have potent antioxidant activity and they can be used in treating chronic diseases involving oxidative stress, such as diabetes mellitus. The purpose of this study was to evaluate the cytotoxicity and cytoprotective effects of citrus flavonoids on the functionality of BRIN-BD11 cells. The assessment of cytotoxic and cytoprotective flavonoid tested was performed using the MTT reduction assay. The flavonoids did not show cytotoxic effects in any of the tested concentrations (5-20 µM) and also negative insulinotropic effects were not observed. To cytoprotective assay, the IC50 of H2O2 in treatment of 2 h (acute oxidative stress) was measured (350 µM). Moreover, under acute oxidative stress, the isolated flavonoids (10 µM) had no cytoprotective effects. Besides an antioxidant role of the flavonoids was only observed when using in association. Thus future experiments are needed, varying the experimental condition, to better evaluate the possible mechanisms of action of these flavonoids.

Angiotensin II-induced JNK activation is mediated by NAD(P)H oxidase in isolated rat pancreatic islets

Angiotensin II (AII), the active component of the renin angiotensin system (RAS), plays a vital role in the regulation of physiological processes of the cardiovascular system, but also has autocrine and paracrine actions in various tissues and organs. Many studies have shown the existence of RAS in the pancreas of humans and rodents. The aim of this study was to evaluate potential signaling pathways mediated by AII in isolated pancreatic islets of rats. Phosphorylation of MAPKs (ERK1/2, JNK and p38MAPK), and the interaction between proteins JAK/STAT were evaluated. AII increased JAK2/STAT1 (42%) and JAK2/STAT3 (100%) interaction without altering the total content of JAK2. Analyzing the activation of MAPKs (ERK1/2, JNK and p38MAPK) in isolated pancreatic islets from rats we observed that AII rapidly (3 min) promoted a significant increase in the phosphorylation degree of these proteins after incubation with the hormone. Curiously JNK protein phosphorylation was inhibited by DPI, suggesting the involvement of NAD(P)H oxidase in the activation of protein.

Oleic acid modulates metabolic substrate channeling during glucose-stimulated insulin secretion via NAD(P)H oxidase

Positive acute effects of fatty acids (FA) on glucose-stimulated insulin secretion (GSIS) and reactive oxygen species (ROS) formation have been reported. However, those studies mainly focused on palmitic acid actions, and reports on oleic acid (OA) are scarce. In this study, the effect of physiological OA levels on β-cell function and the mechanisms involved were investigated. Analyses of insulin secretion, FA and glucose oxidation, and ROS formation showed that, at high glucose concentration, OA treatment increases GSIS in parallel with increased ROS content. At high glucose, OA oxidation was increased, accompanied by a suppression of glucose oxidation. Using approaches for protein knockdown of FA receptor G protein-coupled receptor 40 (GPR40) and of p47(PHOX), a reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase component, we observed that GPR40 does not mediate OA effects on ROS formation and GSIS. However, in p47(PHOX) knockdown islets, OA-induced ROS formation and the inhibitory effect of OA on glucose metabolism was abolished. Similar results were obtained by pharmacological inhibition of protein kinase C, a known activator of NAD(P)H oxidase. Thus, ROS derived from OA metabolism via NAD(P)H oxidase are an inhibitor of glucose oxidation. Put together, these results indicate that OA acts as a modulator of glucose oxidation via ROS derived from its own metabolism in β-cells.

Regulation of insulin secretion and reactive oxygen species production by free fatty acids in pancreatic islets

Free fatty acids regulate insulin secretion through metabolic and intracellular signaling mechanisms such as induction of malonyl-CoA/long-chain CoA pathway, production of lipids, GPRs (G protein-coupled receptors) activation and the modulation of calcium currents. Fatty acids (FA) are also important inducers of ROS (reactive oxygen species) production in β-cells. Production of ROS for short periods is associated with an increase in GSIS (glucose-stimulated insulin secretion), but excessive or sustained production of ROS is negatively correlated with the insulin secretory process. Several mechanisms for FA modulation of ROS production by pancreatic β-cells have been proposed, such as the control of mitochondrial complexes and electron transport, induction of uncoupling proteins, NADPH oxidase activation, interaction with the renin-angiotensin system, and modulation of the antioxidant defense system. The major sites of superoxide production within mitochondria derive from complexes I and III. The amphiphilic nature of FA favors their incorporation into mitochondrial membranes, altering the membrane fluidity and facilitating the electron leak. The extra-mitochondrial ROS production induced by FA through the NADPH oxidase complex is also an important source of these species in β-cells.

NAD(P)H oxidase participates in the palmitate-induced superoxide production and insulin secretion by rat pancreatic islets

Nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase complex has been shown to be involved in the process of glucose-stimulated insulin secretion (GSIS). In this study, we examined the effect of palmitic acid on superoxide production and insulin secretion by rat pancreatic islets and the mechanism involved. Rat pancreatic islets were incubated during 1 h with 1 mM palmitate, 1% fatty acid free-albumin, 5.6 or 10 mM glucose and in the presence of inhibitors of NAD(P)H oxidase (DPI--diphenyleneiodonium), PKC (calphostin C) and carnitine palmitoyl transferase-I (CPT-I) (etomoxir). Superoxide content was determined by hydroethidine assays. Palmitate increased superoxide production in the presence of 5.6 and 10 mM glucose. This effect was dependent on activation of PKC and NAD(P)H oxidase. Palmitic acid oxidation was demonstrated to contribute for the fatty acid induction of superoxide production in the presence of 5.6 mM glucose. In fact, palmitate caused p47(PHOX) translocation to plasma membrane, as shown by immunohistochemistry. Exposure to palmitate for 1 h up-regulated the protein content of p47(PHOX) and the mRNA levels of p22(PHOX), gp91(PHOX), p47(PHOX), proinsulin and the G protein-coupled receptor 40 (GPR40). Fatty acid stimulation of insulin secretion in the presence of high glucose concentration was reduced by inhibition of NAD(P)H oxidase activity. In conclusion, NAD(P)H oxidase is an important source of superoxide in pancreatic islets and the activity of NAD(P)H oxidase is involved in the control of insulin secretion by palmitate.

Low doses of hydrogen peroxide impair glucose-stimulated insulin secretion via inhibition of glucose metabolism and intracellular calcium oscillations

The inhibitory effect of hydrogen peroxide (H(2)O(2)) on glucose-stimulated insulin secretion was previously reported. However, the precise mechanism involved was not systematically investigated. In this study, the effects of low concentrations of H(2)O(2) (5-10 micromol/L) on glucose metabolism, intracellular calcium ([Ca(2+)](i)) oscillations, and dynamic insulin secretion in rat pancreatic islets were investigated. Low concentrations of H(2)O(2) impaired insulin secretion in the presence of high glucose levels (16.7 mmol/L). This phenomenon was observed already after 2 minutes of exposure to H(2)O(2). Glucose oxidation and the amplitude of [Ca(2+)](i) oscillations were dose-dependently suppressed by H(2)O(2). These findings indicate that low concentrations of H(2)O(2) reduce insulin secretion in the presence of high glucose levels via inhibition of glucose metabolism and consequent impairment in [Ca(2+)](i) handling.

Association of NAD(P)H oxidase with glucose-induced insulin secretion by pancreatic beta-cells

We previously described the presence of nicotinamide adenine dinucleotide phosphate reduced form [NAD(P)H]oxidase components in pancreatic beta-cells and its activation by glucose, palmitic acid, and proinflammatory cytokines. In the present study, the importance of the NAD(P)H oxidase complex for pancreatic beta-cell function was examined. Rat pancreatic islets were incubated in the presence of glucose plus diphenyleneiodonium, a NAD(P)H oxidase inhibitor, for 1 h or with the antisense oligonucleotide for p47(PHOX) during 24 h. Reactive oxygen species (ROS) production was determined by a fluorescence assay using 2,7-dichlorodihydrofluorescein diacetate. Insulin secretion, intracellular calcium responses, [U-(14)C]glucose oxidation, and expression of glucose transporter-2, glucokinase and insulin genes were examined. Antisense oligonucleotide reduced p47(PHOX) expression [an important NAD(P)H oxidase cytosolic subunit] and similarly to diphenyleneiodonium also blunted the enzyme activity as indicated by reduction of ROS production. Suppression of NAD(P)H oxidase activity had an inhibitory effect on intracellular calcium responses to glucose and glucose-stimulated insulin secretion by isolated islets. NAD(P)H oxidase inhibition also reduced glucose oxidation and gene expression of glucose transporter-2 and glucokinase. These findings indicate that NAD(P)H oxidase activation plays an important role for ROS production by pancreatic beta-cells during glucose-stimulated insulin secretion. The importance of this enzyme complex for the beta-cell metabolism and the machinery involved in insulin secretion were also shown.

Activation of insulin and IGF-1 signaling pathways by melatonin through MT1 receptor in isolated rat pancreatic islets

Melatonin diminishes insulin release through the activation of MT1 receptors and a reduction in cAMP production in isolated pancreatic islets of neonate and adult rats and in INS-1 cells (an insulin-secreting cell line). The pancreas of pinealectomized rats exhibits degenerative pathological changes with low islet density, indicating that melatonin plays a role to ensure the functioning of pancreatic beta cells. By using immunoprecipitation and immunoblotting analysis we demonstrated, in isolated rat pancreatic islets, that melatonin induces insulin growth factor receptor (IGF-R) and insulin receptor (IR) tyrosine phosphorylation and mediates the activities of the PI3K/AKT and MEK/ERKs pathways, which are involved in cell survival and growth, respectively. Thus, the effects of melatonin on pancreatic islets do not involve a reduction in cAMP levels only. This indoleamine may regulate growth and differentiation of pancreatic islets by activating IGF-I and insulin receptor signaling pathways.

Diabetes associated cell stress and dysfunction: role of mitochondrial and non-mitochondrial ROS production and activity

It is now widely accepted, given the current weight of experimental evidence, that reactive oxygen species (ROS) contribute to cell and tissue dysfunction and damage caused by glucolipotoxicity in diabetes. The source of ROS in the insulin secreting pancreatic beta-cells and in the cells which are targets for insulin action has been considered to be the mitochondrial electron transport chain. While this source is undoubtably important, we provide additional information and evidence for NADPH oxidase-dependent generation of ROS both in pancreatic beta-cells and in insulin sensitive cells. While mitochondrial ROS generation may be important for regulation of mitochondrial uncoupling protein (UCP) activity and thus disruption of cellular energy metabolism, the NADPH oxidase associated ROS may alter parameters of signal transduction, insulin secretion, insulin action and cell proliferation or cell death. Thus NADPH oxidase may be a useful target for intervention strategies based on reversing the negative impact of glucolipotoxicity in diabetes.

Glucose, palmitate and pro-inflammatory cytokines modulate production and activity of a phagocyte-like NADPH oxidase in rat pancreatic islets and a clonal beta cell line

AIMS/HYPOTHESIS

Acute or chronic exposure of beta cells to glucose, palmitic acid or pro-inflammatory cytokines will result in increased production of the p47(phox) component of the NADPH oxidase and subsequent production of reactive oxygen species (ROS).

METHODS

Rat pancreatic islets or clonal rat BRIN BD11 beta cells were incubated in the presence of glucose, palmitic acid or pro-inflammatory cytokines for periods between 1 and 24 h. p47(phox) production was determined by western blotting. ROS production was determined by spectrophotometric nitroblue tetrazolium or fluorescence-based hydroethidine assays.

RESULTS

Incubation for 24 h in 0.1 mmol/l palmitic acid or a pro-inflammatory cytokine cocktail increased p47(phox) protein production by 1.5-fold or by 1.75-fold, respectively, in the BRIN BD11 beta cell line. In the presence of 16.7 mmol/l glucose protein production of p47(phox) was increased by 1.7-fold in isolated rat islets after 1 h, while in the presence of 0.1 mmol/l palmitic acid or 5 ng/ml IL-1beta it was increased by 1.4-fold or 1.8-fold, respectively. However, palmitic acid or IL-1beta-dependent production was reduced after 24 h. Islet ROS production was significantly increased after incubation in elevated glucose for 1 h and was completely abolished by addition of diphenylene iodonium, an inhibitor of NADPH oxidase or by the oligonucleotide anti-p47(phox). Addition of 0.1 mmol/l palmitic acid or 5 ng/ml IL-1beta plus 5.6 mmol/l glucose also resulted in a significant increase in islet ROS production after 1 h, which was partially attenuated by diphenylene iodonium or the protein kinase C inhibitor GF109203X. However, ROS production was reduced after 24 h incubation.

CONCLUSIONS/INTERPRETATION

NADPH oxidase may play a key role in normal beta cell physiology, but under specific conditions may also contribute to beta cell demise.

Propionate inhibits glucose-induced insulin secretion in isolated rat pancreatic islets

Dietary fibers, probably by generating short chain fatty acids (SCFA) through enterobacterial fermentation, have a beneficial effect on the control of glycemia in patients with peripheral insulin resistance. We studied the effect of propionate on glucose-induced insulin secretion in isolated rat pancreatic islets. Evidence is presented that propionate, one of the major SCFA produced in the gut, inhibits insulin secretion induced by high glucose concentrations (11.1 and 16.7 mM) in incubated and perfused pancreatic islets. This short chain fatty acid reduces [U-(14)C]-glucose decarboxylation and raises the conversion of glucose to lactate. Propionate causes a significant decrease of both [1-(14)C]- (84%) and [2-(14)C]-pyruvate (49%) decarboxylation. These findings indicate pyruvate dehydrogenase as the major site for the propionate effect. These observations led us to postulate that the reduction in glucose oxidation and the consequent decrease in the ATP/ADP ratio may be the major mechanism for the lower insulin secretion to glucose stimulus induced by propionate.

ERK3 associates with MAP2 and is involved in glucose-induced insulin secretion

The adaptation of pancreatic islets to pregnancy includes increased beta cell proliferation, expansion of islet mass, and increased insulin synthesis and secretion. Most of these adaptations are induced by prolactin (PRL). We have previously described that in vitro PRL treatment increases ERK3 expression in isolated rat pancreatic islets. This study shows that ERK3 is also upregulated during pregnancy. Islets from pregnant rats treated with antisense oligonucleotide targeted to the PRL receptor displayed a significant reduction in ERK3 expression. Immunohistochemical double-staining showed that ERK3 expression is restricted to pancreatic beta cells. Transfection with antisense oligonucleotide targeted to ERK3 abolished the insulin secretion stimulated by glucose in rat islets and by PMA in RINm5F cells. Therefore, we examined the participation of ERK3 in the activation of a cellular target involved in secretory events, the microtubule associated protein MAP2. PMA induced ERK3 phosphorylation that was companied by an increase in ERK3/MAP2 association and MAP2 phosphorylation. These observations provide evidence that ERK3 is involved in the regulation of stimulus-secretion coupling in pancreatic beta cells.

New Insights into Fatty Acid Modulation of Pancreatic β‐Cell Function

Insulin resistance states as found in type 2 diabetes and obesity are frequently associated with hyperlipidemia. Both stimulatory and detrimental effects of free fatty acids (FFA) on pancreatic beta cells have long been recognized. Acute exposure of the pancreatic beta cell to both high glucose concentrations and saturated FFA results in a substantial increase of insulin release, whereas a chronic exposure results in desensitization and suppression of secretion. Reduction of plasma FFA levels in fasted rats or humans severely impairs glucose-induced insulin release but palmitate can augment insulin release in the presence of nonstimulatory concentrations of glucose. These results imply that changes in physiological plasma levels of FFA are important for regulation of beta-cell function. Although it is widely accepted that fatty acid (FA) metabolism (notably FA synthesis and/or formation of LC-acyl-CoA) is necessary for stimulation of insulin secretion, the key regulatory molecular mechanisms controlling the interplay between glucose and fatty acid metabolism and thus insulin secretion are not well understood but are now described in detail in this review. Indeed the correct control of switching between FA synthesis or oxidation may have critical implications for beta-cell function and integrity both in vivo and in vitro. LC-acyl-CoA (formed from either endogenously synthesized or exogenous FA) controls several aspects of beta-cell function including activation of certain types of PKC, modulation of ion channels, protein acylation, ceramide- and/or NO-mediated apoptosis, and binding to and activating nuclear transcriptional factors. The present review also describes the possible effects of FAs on insulin signaling. We have previously reported that acute exposure of islets to palmitate up-regulates some key components of the intracellular insulin signaling pathway in pancreatic islets. Another aspect considered in this review is the potential source of fatty acids for pancreatic islets in addition to supply in the blood. Lipids can be transferred from leukocytes (macrophages) to pancreatic islets in coculture. This latter process may provide an additional source of FAs that may play a significant role in the regulation of insulin secretion.

Dehydroepiandrosterone increases β-cell mass and improves the glucose-induced insulin secretion by pancreatic islets from aged rats

The effect of dehydroepiandrosterone (DHEA) on pancreatic islet function of aged rats, an animal model with impaired glucose-induced insulin secretion, was investigated. The following parameters were examined: morphological analysis of endocrine pancreata by immunohistochemistry; protein levels of insulin receptor, IRS-1, IRS-2, PI 3-kinase, Akt-1, and Akt-2; and static insulin secretion in isolated pancreatic islets. Pancreatic islets from DHEA-treated rats showed an increased beta-cell mass accompanied by increased Akt-1 protein level but reduced IR, IRS-1, and IRS-2 protein levels and enhanced glucose-stimulated insulin secretion. The present results suggest that DHEA may be a promising drug to prevent diabetes during aging.

Perinatal salt restriction: a new pathway to programming insulin resistance and dyslipidemia in adult wistar rats

Several studies support the hypothesis that chronic diseases in adulthood might be triggered by events that occur during fetal development. This study examined the consequences of perinatal salt intake on blood pressure (BP) and carbohydrate and lipid metabolism in adult offspring of dams on high-salt [HSD; 8% (HSD2) or 4% (HSD1)], normal-salt (NSD; 1.3%), or low-salt (LSD; 0.15% NaCl) diet during pregnancy and lactation. At 12 wk of age, female Wistar rats were matched with adult male rats that were fed NSD. Weekly tail-cuff BP measurements were performed before, during, and after pregnancy. After weaning, the offspring received only NSD and were housed in metabolic cages for 24-h urine collection for sodium and potassium and nitrate and nitrite excretion measurements. At 12 wk of age, intra-arterial mean BP was measured, a euglycemic-hyperinsulinemic clamp was performed, and plasma lipids and nitrate and nitrite concentrations were determined. Tail-cuff BP was higher during pregnancy in HSD2 and HSD1 than in NSD and LSD dams. Mean BP (mm Hg) was also higher in the offspring of HSD2 (110 +/- 5) and HSD1 (107 +/- 5) compared with NSD (100 +/- 2) and LSD (92 +/- 2). Lower glucose uptake and higher plasma cholesterol and triacylglycerols were observed in male offspring from LSD dams (glucose uptake: HSD2 17 +/- 4, HSD1 15 +/- 3, NSD 11 +/- 3, LSD 4 +/- 1 mg . kg(-1) . min(-1); cholesterol: HSD2 62 +/- 6, HSD1 82 +/- 11, NSD 68 +/- 10, LSD 98 +/- 17 mg/dL; triacylglycerols: HSD2 47 +/- 15, HSD1 49 +/- 12, NSD 56 +/- 19, LSD 83 +/- 11 mg/dL). In conclusion, maternal salt intake during pregnancy and lactation has long-term influences on arterial pressure, insulin sensitivity, and plasma lipids of the adult offspring.

Changes of fatty acid composition in incubated rat pancreatic islets

OBJECTIVE

The hypothesis that changes in fatty acId composition of pancreatic islets occur during incubation was investigated.

METHODS

The content and composition of fatty acIds (FA) from rat pancreatic islets and culture medium after incubation for 1 and 3 hours in the absence or in the presence of 5.6, 8.3, or 16.7 mM glucose were determined by HPLC analysis.

RESULTS

The FA content of pancreatic islets was reduced after 1 hour incubation in the absence of glucose. However, the total FA content was restored by incubating in the presence of 5.6 mM glucose and exceeded by incubating in the presence of 8.3 mM or 16.7 mM glucose. Saturated FA contributed a substantially greater proportion of the total FA increase in comparison to unsaturated FA, being palmitic and stearic acIds the most important. The total lipId content of pancreatic islets was not increased if the period of incubation in the presence of glucose was extended to 3 hours. A substantial amount of FA was found in the medium after 1 hour incubation in the absence of glucose: 141 ng per 80 islets for saturated and 75 ng per 80 islets for unsaturated. The release of FA from islets is increased in the presence of glucose.

CONCLUSION

The release of FA from islets is a novel finding and may be related to modulation of B-cell function.

Palmitate modulates the early steps of insulin signalling pathway in pancreatic islets

Insulin stimulates its own secretion and synthesis by pancreatic beta-cells. Although the exact molecular mechanism involved is unknown, changes in beta-cell insulin signalling have been recognized as a potential link between insulin resistance and its impaired release, as observed in non-insulin-dependent diabetes. However, insulin resistance is also associated with elevated plasma levels of free fatty acids (FFA) that are well known modulators of insulin secretion by pancreatic islets. This information led us to investigate the effect of FFA on insulin receptor signalling in pancreatic islets. Exposure of pancreatic islets to palmitate caused up-regulation of several insulin-induced activities including tyrosine phosphorylation of insulin receptor and pp185. This is the first evidence that short exposure of these cells to 100 microM palmitate activates the early steps of insulin receptor signalling. 2-Bromopalmitate, a carnitine palmitoyl-CoA transferase-1 inhibitor, did not affect the effect of the fatty acid. Cerulenin, an acylation inhibitor, abolished the palmitate effect on protein levels and phosphorylation of insulin receptor. This result supports the proposition that protein acylation may be an important mechanism by which palmitate exerts its modulating effect on the intracellular insulin signalling pathway in rat pancreatic islets.

Pancreatic beta-cells express phagocyte-like NAD(P)H oxidase

The presence of a phagocyte-like NAD(P)H oxidase in pancreatic beta-cells was investigated. Three NAD(P)H oxidase components were found in pancreatic islets by RT-PCR: gp91(PHOX), p22(PHOX), and p47(PHOX). The components p67(PHOX) and p47(PHOX) were also demonstrated by Western blotting. Through immunohistochemistry, p47(PHOX) was mainly found in the central area of the islet, confirming the expression of this component by insulin-producing cells. Activation of NAD(P)H oxidase complex in the beta-cells was also examined by immunohistochemistry. The pancreatic islets presented slower kinetics of superoxide production than HIT-T15 cells, neutrophils, and macrophages, but they reached 66% that of the neutrophil nitroblue tetrazolium (NBT) reduction after 2 h of incubation. Glucose (5.6 mmol/l) increased NBT reduction by 75% when compared with control. The involvement of protein kinase C (PKC) in the stimulatory effect of glucose was confirmed by incubation of islets with phorbol myristate acetate (a PKC activator) and bysindoylmaleimide (GF109203X) (a PKC-specific inhibitor). Diphenylene iodonium [an NAD(P)H oxidase inhibitor] abolished the increase of NBT reduction induced by glucose, confirming the NAD(P)H oxidase activity in pancreatic islets. Because reactive oxygen species are involved in intracellular signaling, the phagocyte-like NAD(P)H oxidase activation by glucose may play an important role for beta-cell functioning.

Pleiotropic effects of fatty acids on pancreatic beta-cells

Hyperlipidemia is frequently associated with insulin resistance states as found in type 2 diabetes and obesity. Effects of free fatty acids (FFA) on pancreatic beta-cells have long been recognized. Acute exposure of the pancreatic beta-cell to FFA results in an increase of insulin release, whereas a chronic exposure results in desensitization and suppression of secretion. We recently showed that palmitate augments insulin release in the presence of non-stimulatory concentrations of glucose. Reduction of plasma FFA levels in fasted rats or humans severely impairs glucose-induced insulin release. These results imply that physiological plasma levels of FFA are important for beta-cell function. Although, it has been accepted that fatty acid oxidation is necessary for its stimulation of insulin secretion, the possible mechanisms by which fatty acids (FA) affect insulin secretion are discussed in this review. Long-chain acyl-CoA (LC-CoA) controls several aspects of the beta-cell function including activation of certain types of protein kinase C (PKC), modulation of ion channels, protein acylation, ceramide- and/or nitric oxide (NO)-mediated apoptosis, and binding to nuclear transcriptional factors. The present review also describes the possible effects of FA on insulin signaling. We showed for the first time that acute exposure of islets to palmitate upregulates the intracellular insulin-signaling pathway in pancreatic islets. Another aspect considered in this review is the source of FA for pancreatic islets. In addition to be exported to the medium, lipids can be transferred from leukocytes (macrophages) to pancreatic islets in co-culture. This process consists an additional source of FA that may plays a significant role to regulate insulin secretion.

Insulin secretion induced by palmitate--a process fully dependent on glucose concentration

The effect of 0.1 mM palmitate on insulin secretion by 1 hr incubated pancreatic islets was examined in the presence of different glucose concentrations (5.6 and 16.7 mM). The oxidation of both glucose and palmitate and the incorporation of [U-14C]-palmitate into lipid fractions and phospholipid species were determined. In the presence of 5.6 mM glucose, palmitate reduced insulin release by 80%. In contrast, in the presence of 16.7 mM glucose, palmitate raised the amount of insulin released by 49%. Palmitate (0.1 mM) caused a significant reduction (52%) of [U-14C]-glucose decarboxylation at 5.6 mM but it did not have any effect at 16.7 mM glucose. The decarboxylation of [U-14C]-palmitate was markedly lower (94%) in the presence of 16.7 mM, as compared to 5.6 mM glucose. [U-14C]-Palmitate was significantly incorporated into total lipid fractions in the presence of both glucose concentrations. The increase in glucose concentration from 5.6 to 16.7 mM raised by 138% the incorporation of [U-14C]-palmitate into phospholipids: phosphatidylcholine (PC), phosphatidylserine (PS), phosphatidylethanolamine (PE), phosphatidic acid (PA) and phosphatidylinositol (PI). PC and PA at 0.1 mM raised by three and four-fold, respectively, insulin release by incubated pancreatic islets. We postulated that palmitate (at 0.1 mM) promotes a deviation of glycerol-phosphate to lipid synthesis, decreasing glucose oxidation (at 5.6 mM) and possibly ATP/ADP ratio in the cytosol, leading to a reduction in insulin secretion. At 16.7 mM glucose concentration, the high glycolytic flux is now enough to provide glycerol-phosphate for lipid synthesis and carbons for the Krebs cycle. So, under this condition, ATP production might be not reduced. The increase in the production of PA and PC may explain the increase in insulin secretion observed at 16.7 mM glucose.

Daily rhythm of glucose-induced insulin secretion by isolated islets from intact and pinealectomized rat

It is well known that pinealectomy induces in rats a diminished glucose tolerance, insulin resistance, a reduction in GLUT4 content in adipose and muscular tissues, a decrease in hepatic and muscular glycogenesis, impairment of glucagon action and an increase in blood pyruvate concentration. In addition, it has been shown that melatonin suppresses insulin secretion in several experimental conditions. The objective of the present study was to investigate the daily rhythm of glucose-induced insulin secretion and glucose oxidation by isolated pancreatic islets and to investigate the effect of chronic absence of melatonin (30 days of pinealectomy) on this rhythmic process. The data obtained confirmed the presence of a strong 24-hr rhythm of insulin secretion by isolated pancreatic islets. In addition, it was demonstrated that the glucose-metabolizing ability of the B-cell follows a daily rhythm phase locked to insulin secretion rhythm. Most interesting, however, was the demonstration that the daily rhythmic processes of insulin secretion and B-cell -[U-14C]-glucose oxidation by isolated pancreatic islets is completely modified by the chronic absence of the pineal gland. Thus, pinealectomy induced in all groups an increase in 24-hr mean glucose-stimulated insulin secretion and [U-14C]-glucose oxidation, in addition to some alterations in the rhythmic amplitude and a remarkable phase-advancing of the daily curves for 8.3 mm glucose (a condition similar to that observed in fed animals and where the B-cells are supposedly more active). These observations strongly suggest that the presence of the pineal gland may be necessary for the proper synchronization of these metabolic rhythms with other circadian rhythms like activity-rest and feeding.

Secreção da insulina: efeito autócrino da insulina e modulação por ácidos graxos

A insulina exerce um papel central na regulação da homeostase da glicose e atua de maneira coordenada em eventos celulares que regulam os efeitos metabólicos e de crescimento. A sub-unidade beta do receptor de insulina possui atividade tirosina quinase intrínseca. A autofosforilação do receptor, induzida pela insulina, resulta na fosforilação de substratos protéicos intracelulares, como o substrato-1 do receptor de insulina (IRS-1). O IRS-1 fosforilado associa-se a domínios SH2 e SH3 da enzima PI 3-quinase, transmitindo, desta maneira, o sinal insulínico. A insulina parece exercer feedback positivo na sua secreção, pela interação com seu receptor em células B pancreáticas. Alterações nos mecanismos moleculares da via de sinalização insulínica sugerem uma associação entre resistência à insulina e diminuição da secreção deste hormônio, semelhante ao observado em diabetes mellitus tipo 2. Uma das anormalidades associadas à resistência à insulina é a hiperlipidemia. O aumento do pool de ácidos graxos livres circulantes pode modular a atividade de enzimas e de proteínas que participam na exocitose da insulina. Essa revisão descreve também os possíveis mecanismos de modulação da secreção de insulina pelos ácidos graxos em ilhotas pancreáticas.

Macrophages transfer [14C]-labelled fatty acids to pancreatic islets in culture

Macrophages are able to produce, export, and transfer fatty acids to lymphocytes in culture. The purpose of this study was to examine if labelled fatty acids could be transferred from macrophages to pancreatic islets in co-culture. We found that after 3 h of co-culture the transfer of fatty acids to pancreatic islets was: arachidonic >> oleic > linoleic = palmitic. Substantial amounts of the transferred fatty acids were found in the phospholipid fraction; 87.6% for arachidonic, 59.9% for oleic, 53.1% for palmitic, and 36.9% for linoleic acids. The remaining radioactivity was distributed among the other lipid fractions analysed (namely polar lipids, cholesterol, fatty acids, triacylglycerol and cholesterol ester), varying with the fatty acid used. For linoleic acid, a significant proportion (63.1%) was almost equally distributed in these lipid fractions. Also, it was observed that transfer of fatty acids from macrophages to pancreatic islets is time-dependent up to 24 h, being constant and linear with time for palmitic acid and remaining constant after 12 h for oleic acid. These results lead us to postulate that in addition to the serum, circulating monocytes may also be a source of fatty acids to pancreatic islets, mainly arachidonic acid.

Tolerable dietary lipid content that does not alter insulin secretion

Lipids, either as membrane components or as fuel, are important nutrients that can affect insulin secretion. The aim of this study was to establish the maximum tolerable amount of fat present in the diet, which does not induce significant alteration in the process of insulin secretion. For that, just-weaned male albino rats (70-90 g body weight) were fed during 6 weeks with diets for growing rodents containing 7% fat (A Group) as recommended by the American Institute of Nutrition-AIN. Two other groups in which the fat content of the diet was increased to reach 10% (B Group) or 13% (C Group) were also included. Insulin release, 86Rb+ and 45Ca2+ Fractional Outflow Rate (FOR) during the process of glucose-induced insulin secretion was determined in perfused islets isolated from these animals. No statistical differences in these parameters were detected between A and B rats. However, in the C group, a lower 86Rb+ FOR was found during the whole experiment and a poor insulin secretory response to glucose stimulus was observed. These results led us to postulate that the maximal limiting amount of total lipids present in the diet that does not impair the process of glucose-induced insulin secretion is 10%. These findings authorize future studies on the interference of different dietary lipid sources, in a content 43% more elevated than that recommended (10% against 7%), on the mechanisms of insulin secretion.

Glucose induces an acute increase of superoxide dismutase activity in incubated rat pancreatic islets

The activity of superoxide dismutase (SOD), catalase, and glutathione peroxidase (GSP) in isolated rat pancreatic islets exposed to high glucose concentration for a short period of time (60 min) was determined. High glucose concentration (16.7 mM) did not significantly alter catalase activity. GSP activity was increased by glucose at 5.6 mM, remaining elevated at higher concentrations up to 16.7 mM. However, the activity of SOD increased with glucose concentration, and this increment was closely correlated with the rate of insulin secretion (r = 0.96). High potassium (30 mM) did not increase SOD activity, suggesting that the increase in intracellular ionic calcium concentration does not stimulate this enzyme activity. alpha-Ketoisocaproic acid and pyruvate, which are metabolized through the TCA cycle, did not increase SOD activity, indicating that the stimulation of SOD activity might be triggered by a factor produced through glycolysis or the pentose phosphate pathway.

Modulation of insulin secretion by leptin

The present study examines the acute effect of leptin (50 nM) on insulin secretion and on the fractional outflow rates of 45Ca2+ and 86Rb+ from pancreatic islets isolated from male lean albino rats. Under a constant physiological glucose concentration (5.6 mM), the addition of leptin to the perifusion medium led to an increment in 45Ca2+ fractional outflow rate followed by a significant (p < 0.05) increase (26%) in the insulin release. At low glucose concentration (2.8 mM), leptin also elicited a significant (p < 0.05; 50-60%) increase in insulin secretion. However, under supraphysiological (16.7 mM) glucose concentration, the rapid first-phase insulin secretion response was abolished. At low glucose levels, islets perifused in the presence of leptin presented a lower 86Rb+ fractional outflow rate compared with perifused controls. In contrast, when glucose was switched to 16.7 mM, compared with controls, a slight increase in the 86Rb+ fractional outflow rate was observed instead. These in vitro data provide evidence that, by changing K+ fluxes, leptin might modulate insulin secretion from pancreatic islets.

Soybean- and olive-oils-enriched diets increase insulin secretion to glucose stimulus in isolated pancreatic rat islets

Islets isolated from rats fed a lipid-enriched diet have shown an impairment of insulin secretion, but there is no available data comparing the effect of diet containing different dietary fat. This may be important in preventing or facilitating the establishment of diabetes. In this study, the effect of diets enriched (10%) with different fatty acids on insulin secretion by isolated pancreatic islets was investigated. The sources of the fatty acids tested were: saturated long chain from animal fat (AF), polyunsaturated from soybean oil (SO), and monounsaturated from olive oil (OL). The results were compared with those from rats receiving a diet enriched (10%) with a balanced mixture of fatty acids (the same proportion of AF, SO, and OL). The effect of fat-rich diets on insulin release was tested in vivo by giving a glucose load (glucose tolerance test-GTT) and in vitro in perfused islets. The mechanism involved was also examined by measuring 45Ca2+ and 86Rb+ fluxes, GLUT-2 content, and glucose oxidation in isolated islets. A significant increase of insulin secretion and glucose oxidation without any alteration of the ionic movements were detected in islets from SO and OL rats. GLUT-2 content was increased in islets of the OL group but diminished in AF rats. The results led us to postulate that soybean and olive oils may increase the response of insulin secretion to glucose stimulus in pancreatic islets.

Pivotal role of leptin in insulin effects

The OB protein, also known as leptin, is secreted by adipose tissue, circulates in the blood, probably bound to a family of binding proteins, and acts on central neural networks regulating ingestive behavior and energy balance. The two forms of leptin receptors (long and short forms) have been identified in various peripheral tissues, a fact that makes them possible target sites for a direct action of leptin. It has been shown that the OB protein interferes with insulin secretion from pancreatic islets, reduces insulin-stimulated glucose transport in adipocytes, and increases glucose transport, glycogen synthesis and fatty acid oxidation in skeletal muscle. Under normoglycemic and normoinsulinemic conditions, leptin seems to shift the flux of metabolites from adipose tissue to skeletal muscle. This may function as a peripheral mechanism that helps control body weight and prevents obesity. Data that substantiate this hypothesis are presented in this review.

Possible modulatory role of non-activated lymphocytes on insulin secretion

In order to study the probable physiological role of non-activated lymphocytes on islet B-cells, we incubated and perfused rat pancreatic islets in the presence of low (2.8 mM) and high (16.7 mM) glucose concentrations after pre-exposure for 60 min to rat lymphocytes or to substances secreted by lymphocytes. Insulin secretion and 86Rb+, 45Ca2+ and [3H]-phosphoinositide metabolite fluxes were lower compared to controls when islets were pre-exposed to lymphocytes but were not different when islets were pre-exposed to substances secreted by lymphocytes. These alterations in isotope flux suggest that, when lymphocytes and islets are in contact, closure of potassium channels and a paradoxical effect of glucose load on insulin release occur in the presence of low glucose concentrations. The alterations observed are probably due to a swift and direct action of lymphocyte secretion perhaps induced by a direct of islet cells.

Impairment of insulin secretion in pancreatic islets isolated from Walker 256 tumor-bearing rats

Previous study has shown that insulin secretion in response to a glucose stimulus (16.7 mM) is reduced in islets isolated from Walker 256 tumor-bearing rats compared with controls. The ultrastructure, 45Ca2+ and 86Rb+ fractional outflow rate, phosphoinositide hydrolysis, and [U-14C]glucose decarboxylation were examined in islets isolated from tumor-bearing and control rats. The general morphological features of the islets from the control and experimental groups were very similar. The 86Rb+ fractional outflow rate was not changed, whereas the 45Ca2+ fractional outflow rate, [U-14C]glucose decarboxylation, and phosphoinositide metabolism were markedly reduced in islets from tumor-bearing rats. The changes in 45Ca2+ fractional outflow rate in islets from tumor-bearing rats were not due to impaired functioning of voltage-dependent calcium channels. By perifusing the islets in the presence of high potassium concentration, evidence was obtained that phospholipase C from islets from tumor-bearing rats reduced response to calcium. To further examine the mechanism involved in the impairment of insulin secretion by islets from tumor-bearing rats, islets isolated from normal rats were perifused after preincubation in the presence of serum from tumor-bearing rats. The results suggest that a thermolabile circulating factor is partially responsible for the changes described in islets isolated from Walker 256 tumor-bearing rats.

Undernutrition during early lactation as an alternative model to study the onset of diabetes mellitus type II

In order to characterize an alternative animal model for the study of diabetes mellitus type II onset, we compared the effects of a diet containing 8% of protein (LPD) and a normal diet containing 25% of protein supplied to the dams during the first 12 days of lactation. We studied in the pups the growth evolution and, when they develop into adults (60 days), the glucose tolerance test (GTT) and the insulin secretion, in response to stimulatory concentrations of glucose. The weight of the two groups were significantly different at 60 days of age (LPD = 179 +/- 19 g; NPD = 186 +/- 18 g). The GTT ten minutes after iv glucose administration showed a significant increase of blood glucose concentration of the LPD group (LPD = 550 +/- 17 mg/dl; NPD = 425 +/- 13 mg/dl, p < 0.001). The insulin secretion, four minutes after stimulation was found reduced in the LPD group (LPD = 1.1 +/- 0.08 muU/islet/min; NPD = 1.85 +/- 0.2 muU/islet/min.). The present study indicates insulin secretory and/or resistance impairment due to early undernutrition. Also, the data taken together suggest that undernutrition during early lactation can be used as an alternative model to study particular characteristics of the onset of diabetes mellitus type II.

Modulation of insulin secretion by feeding behavior and physical activity: possible beneficial effects on obese and aged rats

The high occurrence of diabetes in aged subjects is well known. In fact, the aging process is accompanied by obesity and presenting increased insulin release and lower peripheral responsiveness to this hormone. A recent study has shown that the changes in glucose utilization and insulin secretion during aging are abolished when obesity is somehow avoided. This fact justifies the effort to define new strategies to avoid the development of obesity during aging. In this review, beneficial effects of balanced diets, high frequency food intake and moderate exercise training on insulin secretion and its effect in rats are presented.

Insulin and prostaglandins as signals between tumor and the host immune system

The development of malignant tissue in vivo is partially favored by the immunosuppression that occurs in cancer patients. However, the signals between tumor and immune tissues remain to be identified. We present evidence that prostaglandins may act as one of these signals by a direct action on cells of the immune system, or by inhibition of insulin secretion which in turn suppresses immune function, or both.

Modulation of insulin secretion and 45Ca2+ efflux by dopamine in glucose-stimulated pancreatic islets

1. The effect of dopamine on calcium efflux and insulin secretion is examined in the present study. For this purpose, islets isolated from adult Wistar rats were perfused or incubated at 37 degrees C for 60 min. 2. The results obtained from perfused islets indicate that 100 microM dopamine, in the presence of 5.6 mM glucose, increases insulin secretion and causes a modest elevation of 45Ca2+ efflux. However, glucose stimuli (from 5.6 to 16.7 mM) provoked an unexpected reduction of insulin release, with no alteration in calcium efflux, when 100 microM dopamine was present in the perfusion medium. 3. Similar findings were obtained in incubated islets when the prolonged effect of dopamine was investigated. 4. The observations described above led us to conclude that bioactive amines might play an important role in the modulation of the glucose-induced insulin secretion.

Effect of epinephrine on 86Rb efflux, 45Ca outflow and insulin release from pancreatic islets perifused in the presence of propranolol

Pancreatic islets prelabelled with either 86Rb or 45Ca were perifused in the presence of propranolol (0.1 microM) and, when required, exposed to epinephrine (0.1 microM). In the absence of D-glucose, epinephrine failed to cause any obvious change in either 86Rb or 45Ca outflow. In the presence of 16.7 mM D-glucose, however, epinephrine lowered both 86Rb and 45Ca outflow, this coinciding with suppression of insulin release. Epinephrine also suppressed the increment in 86Rb outflow evoked by a rise in glucose-concentration from 8.3 to 16.7 mM. Epinephrine did not abolish the early fall in 45Ca efflux evoked by the administration of D-glucose (16.7 mM) to islets previously deprived of the hexose but, within the same experiments, impaired the secondary rise in effluent radioactivity. Likewise, epinephrine prevented the increase in 45Ca outflow provoked by a rise in hexose concentration from 8.3 to 16.7 mM. These findings are compatible with the recent proposal that epinephrine interferes with the entry of Ca2+ into the B-cell, as mediated by voltage-sensitive Ca2+ channels, but do not rule out a multifactorial coupling between the occupancy of alpha 2-adrenergic receptors and the eventual inhibition of insulin release.

Utilization of rat and human sera to carry out incubation and perifusion of pancreatic islets

Incubation of isolated pancreatic rat islets in a medium consisting of fresh rat or human sera and the measurement of insulin secretion after 60 min in the presence of 6.0-7.0 and 16.7 mM of glucose, were carried out. Perifusion experiments with isolated rat islets preincubated in the presence of rat or human sera and 0.2 mCi/mL of 45Ca2+ were also performed, and 45Ca2+ outflow rate and insulin secretion were analyzed. In both experiments, the usual islet responsiveness to glucose stimulation was preserved when compared with experiments in which Krebs-Henseleit (K-H) buffer was used as the medium. The results obtained demonstrate that both rat and human sera can be used as media for islets perifusion and incubation experiments, thus allowing for the effect(s) of circulating substances on insulin release to be examined.

Long-term regulation of pancreatic B-cell responsiveness to D-glucose by food availability, feeding schedule, and diet composition

The immediate metabolic, cationic, and secretory response of the insulin-producing B-cell to D-glucose is regulated, in a delayed or long-term manner, by nutritional factors such as food availability, feeding schedule, or diet composition. The B-cell keeps the memory of these nutritional manipulations so that the corresponding changes in its responsiveness to D-glucose can be documented in vitro in isolated pancreatic islets. The results of experiments conducted in starved rats, in animals exposed to an altered feeding schedule, and in rats given free access to a high-carbohydrate, high-protein, or high-lipid, as distinct from balanced, diet all suggest that a sufficient prandial hyperglycemia is essential for maintenance of an optimal metabolic and secretory behavior of the islet B-cell in response to a rise in D-glucose concentration.

Metabolic mechanisms involved in the impaired insulin secretion in pancreatic islets isolated from exercised and fasted rats

This study examined the metabolic mechanisms involved in the impaired insulin secretion by pancreatic islets isolated from fasted and exercised rats. Insulin secretion to glucose (8.3 to 16.7 mM) stimulus was lower in fasted (F), exercised (E), and fasted plus exercised (EF) rats as compared with the control group. The rate of glucose oxidation by isolated islets was reduced in F and EF rats, but it was not modified in the E group. In response to alpha-KIC (5, 10, 15, and 20 mM), insulin secretion was not different in EF and F rats, in comparison to controls, except in the E group, which showed values slightly higher. These findings suggest that changes in insulin secretion in fasted rats, associated or not to exercise training, might be a consequence of a reduced activity of the right-hand side of the Krebs cycle (from pyruvate to oxoglutarate), leading to decreased glucose oxidation. However, the exercise itself caused a significant decrease of insulin secretion without altering glycolysis and Krebs cycle activities.

Obesity is the major cause of alterations in insulin secretion and calcium fluxes by isolated islets from aged rats

To investigate the alterations in insulin secretion induced by aging, 2-month-old, 12-month-old, and 12-month old lean rats (submitted to a caloric restriction during the last month that causes a weight loss of approximately 20%) were studied. As expected, glucose intolerance and increased insulin response were observed during IV-GTT in 12-month-old rats. These effects were, however, reversed by weight loss. Insulin secretion was investigated in isolated islets both during static incubation and perifusion. In 12-month-old rats insulin secretion and 45Ca2+ efflux were lower only in the second phase of the hormonal secretion, suggesting an involvement of voltage-sensitive calcium channels in these phenomena. Considering that in vivo and in vitro alterations were reversed after weight loss, it is possible to conclude that obesity is probably a major cause of impaired insulin secretion in 12-month-old albino rats. Since 14C-glucose metabolism was not changed in islets from aged rats, the effect of obesity on insulin secretion is not due to altered glucose metabolism in pancreatic B-cells.

Insulin secretion to glucose stimulus in pancreatic islets isolated from rats fed unbalanced diets

To verify the effect of different energetic sources on insulin secretion, just-weaned male Wistar rats were divided into four groups fed as follows: high carbohydrate (HC), high protein (HP), high lipid (HL) and balanced (C) diets during five weeks. Body weight gain and daily food intake were similar among the four groups. Insulin content of the isolated islets was not different; however, insulin release to a high glucose concentration (16.7 mM) stimulus was clearly higher in islets isolated from rats fed a balanced diet as compared to the other groups (HC, HP and HL). The rates of insulin secretion in islet perfusates from rats fed unbalanced diets were also decreased, although 45Ca2+ outflow rate (FOR) was similar in all groups. Since the rate of U-14C-glucose oxidation was decreased in islets isolated from rats fed unbalanced diets, this could be one of the mechanisms for the reduced rates of insulin release observed. Therefore, the increased supply of specific fuels provoke metabolic alterations in B-cell leading to changes in insulin secretion.