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

Pregnancy restores insulin secretion from pancreatic islets in cafeteria diet-induced obese rats

Insulin resistance during pregnancy is counteracted by enhanced insulin secretion. This condition is aggravated by obesity, which increases the risk of gestational diabetes. Therefore, pancreatic islet functionality was investigated in control nonpregnant (C) and pregnant (CP), and cafeteria diet-fed nonpregnant (Caf), and pregnant (CafP) obese rats. Isolated islets were used for measurements of insulin secretion (RIA), NAD(P)H production (MTS), glucose oxidation (14CO2 production), intracellular Ca2+ levels (fura-2 AM), and gene expression (real-time PCR). Impaired glucose tolerance was clearly established in Caf and CafP rats at the 14th wk on a diet. Insulin secretion induced by direct depolarizing agents such as KCl and tolbutamide and increasing concentrations of glucose was significantly reduced in Caf, compared with C islets. This reduction was not observed in islets from CP and CafP rats. Accordingly, the glucose oxidation and production of reduced equivalents were increased in CafP islets. The glucose-induced Ca2+ increase was significantly lower in Caf and higher in CafP, compared with all other groups. CP and CafP islets demonstrated an increased Ca2+ oscillation frequency, compared with both C and Caf islets, and the amplitude of oscillations was augmented in CafP, compared with Caf islets. In addition, Cavα1.2 and SERCA2a mRNA levels were reduced in Caf islets. Cavα1.2, but not SERCA2a, mRNA was normalized in CafP islets. In conclusion, cafeteria diet-induced obesity impairs insulin secretion. This alteration is related to the impairment of Ca2+ handling in pancreatic islets, in especial Ca2+ influx, a defect that is reversed during pregnancy allowing normalization of insulin secretion.

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.

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.

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.

Daily delivery of dietary nitrogen to the periphery is stable in rats adapted to increased protein intake

Dietary nitrogen was traced in rats adapted to a 50% protein diet and given a meal containing 1.50 g (15)N-labeled protein (HP-50 group). This group was compared with rats usually consuming a 14% protein diet and fed a meal containing either 0.42 g (AP-14 group) or 1.50 g (AP-50 group) of (15)N-labeled protein. In the HP group, the muscle nonprotein nitrogen pool was doubled when compared with the AP group. The main adaptation was the enhancement of dietary nitrogen transferred to urea (2.2 +/- 0.5 vs. 1.3 +/- 0.1 mmol N/100 g body wt in the HP-50 and AP-50 groups, respectively). All amino acids reaching the periphery except arginine and the branched-chain amino acids were depressed. Consequently, dietary nitrogen incorporation into muscle protein was paradoxically reduced in the HP-50 group, whereas more dietary nitrogen was accumulated in the free nitrogen pool. These results underline the important role played by splanchnic catabolism in adaptation to a high-protein diet, in contrast to muscle tissue. Digestive kinetics and splanchnic anabolism participate to a lesser extent in the regulation processes.

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.

Insulin secretion, obesity, and potential behavioral influences: results from the Insulin Resistance Atherosclerosis Study (IRAS)

BACKGROUND

This work was conducted to evaluate associations of insulin secretion with overall and central obesity, dietary fats, physical activity, and alcohol.

METHODS

A frequently sampled intravenous glucose tolerance test (FSIGT) was used to assess acute insulin response to glucose (AIR) and insulin sensitivity (S(I)) among adult participants (n=675 with normal, NGT; n=332 with impaired glucose tolerance, IGT) in the Insulin Resistance Atherosclerosis Study (IRAS). Disposition index (DI) was calculated as the sum of the log-transformed AIR and S(I) to reflect pancreatic compensation for insulin resistance. Obesity was measured as body mass index (kg/m(2), BMI) and central fat distribution by waist circumference (cm). Dietary fat intake (total, saturated, polyunsaturated, oleic acid), physical activity, and alcohol intake were assessed by standardized interview.

RESULTS

In unadjusted analyses, BMI and waist were each positively correlated with AIR among NGTs (r=0.26 and 0.23, respectively; p<0.0001) but correlations were weaker among the IGTs (r=0.10, NS; r=0.13, p<0.05 for BMI and waist, respectively). BMI and waist were inversely correlated with DI among NGTs (r=-0.13 and -0.20, respectively; p<0.0001) and among IGTs (r=-0.20 and -0.19, respectively, p<0.0001). Dietary fat variables were positively related, and alcohol was inversely related, to AIR among NGTs (p<0.01) but not among IGTs. With all factors considered simultaneously in a pooled analysis of IGTs and NGTs, waist, but not BMI, was positively associated with AIR (p<0.001) and inversely associated with DI (p<0.01). None of the behavioral variables were independently related to either outcome.

CONCLUSION

Among non-diabetic patients, central obesity appears to be related to higher insulin secretion, but to lower capacity of the pancreas to respond to the ambient insulin resistance.

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.

Dietary chromium decreases insulin resistance in rats fed a high-fat, mineral-imbalanced diet

The effects of chromium (Cr) supplementation on diet-induced insulin resistance produced by feeding a high-fat, low-Cr diet were studied in rats to ascertain the role of Cr in insulin resistance. Wistar male rats were maintained for 16 weeks after weaning on a basal diet containing 40% lard, 30% sucrose, and 25% casein by weight and adequate vitamins and minerals without added Cr (-Cr). Fasting levels of insulin, glucose, and triglycerides and the responses during an intravenous glucose tolerance test (IVGTT) were compared as indices of insulin resistance and the effectiveness of dietary Cr. IVGTTs and blood sampling for data analyses were performed over a 40-minute period after IV glucose injection (1.25 g/kg body weight) in overnight-fasted animals under pentobarbital anesthesia (40 mg/kg body weight). All animals were normoglycemic (-Cr, 109 +/- 3 mg/dL; +Cr, 119 +/- 5), with fasting insulin levels elevated in the -Cr group (65 +/- 10 microU/mL) versus the +Cr group (31 +/- 4 microU/mL). Increases in plasma triglycerides in the -Cr group were not significant. Following glucose injection, the rate of glucose clearance was lower in the -Cr group (1.74 +/- 0.22 v2.39 +/- 0.11%/min), and 40-minute glucose areas in the -Cr group tended to be higher than in the +Cr group. The insulin response to glucose injection was 20% higher in the -Cr group. Forty-minute plasma triglyceride areas were lower in +Cr rats (875 +/- 62 v 1,143 +/- 97 mg/dL.min in -Cr rats). These data demonstrate that the insulin resistance induced by feeding a high-fat, nutrient-stressed diet is improved by Cr.

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.

Effect of high fat feeding on glucose tolerance in neonatally streptozotocin-treated rats at 3 and 6 months of age

In the rat, a high fat intake is believed to be associated with an increased risk for the development of glucose intolerance by inducing insulin resistance. The aim of this study was to investigate whether reduced insulin production may also play a role. Rats were treated with 0, 30, 60, and 90 mg of streptozotocin (STZ)/kg of body weight on the day of birth (0, 30, 60, and 90 nSTZ rats). At 3 or 6 months of age, glucose tolerance was assessed by the intravenous glucose tolerance test (IVGTT). STZ dose-dependently decreased first- and second-phase insulin responses and correspondingly impaired glucose tolerance. Following a 3-week high fat diet (HFD: 60% of calories as corn oil), insulin responses were higher in control as well as in STZ-treated rats both at 3 and 6 months of age. In 3-month-old rats this was accompanied by unchanged or increased glucose levels following the glucose load, whereas in 6-month-old 0 and 30 nSTZ rats glucose tolerance was slightly improved. After 6 weeks of HFD in 6-month-old rats, glucose tolerance was impaired compared to that after 3 weeks of HFD despite higher insulin responses. Continuing the HFD for up to 12 weeks further impaired glucose tolerance, but insulin responses were decreased compared to those after 6 weeks of HFD. These results indicate that very low dose neonatal STZ administration impairs glucose tolerance through decreased overall insulin responses. This may possibly be due to a reduction of B-cell number rather than an alteration of B-cell function. No clear evidence exists that a high fat intake per se negatively influences glucose-induced insulin responses, but this may become apparent after longer periods of high fat feeding.

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.