Fasting-induced dissociation of cationic and secretory events in pancreatic islets

Control of Insulin Secretion by Production of Reactive Oxygen Species: Study Performed in Pancreatic Islets from Fed and 48-Hour Fasted Wistar Rats

Mitochondria and NADPH oxidase are important sources of reactive oxygen species in particular the superoxide radical (ROS) in pancreatic islets. These molecules derived from molecular oxygen are involved in pancreatic β-cells signaling and control of insulin secretion. We examined the involvement of ROS produced through NADPH oxidase in the leucine- and/or glucose-induced insulin secretion by pancreatic islets from fed or 48-hour fasted rats. Glucose-stimulated insulin secretion (GSIS) in isolated islets was evaluated at low (2.8 mM) or high (16.7 mM) glucose concentrations in the presence or absence of leucine (20 mM) and/or NADPH oxidase inhibitors (VAS2870–20 μM or diphenylene iodonium—DPI—5 μM). ROS production was determined in islets treated with dihydroethidium (DHE) or MitoSOX Red reagent for 20 min and dispersed for fluorescence measurement by flow cytometry. NADPH content variation was examined in INS-1E cells (an insulin secreting cell line) after incubation in the presence of glucose (2.8 or 16.7 mM) and leucine (20 mM). At 2.8 mM glucose, VAS2870 and DPI reduced net ROS production (by 30%) and increased GSIS (by 70%) in a negative correlation manner (r = -0.93). At 16.7 mM glucose or 20 mM leucine, both NADPH oxidase inhibitors did not alter insulin secretion neither net ROS production. Pentose phosphate pathway inhibition by treatment with DHEA (75 μM) at low glucose led to an increase in net ROS production in pancreatic islets from fed rats (by 40%) and induced a marked increase (by 144%) in islets from 48-hour fasted rats. The NADPH/NADP⁺ ratio was increased when INS-1E cells were exposed to high glucose (by 4.3-fold) or leucine (by 3-fold). In conclusion, increased ROS production through NADPH oxidase prevents the occurrence of hypoglycemia in fasting conditions, however, in the presence of high glucose or high leucine levels, the increased production of NADPH and the consequent enhancement of the activity of the antioxidant defenses mitigate the excess of ROS production and allow the secretory process of insulin to take place.

Low-protein diet in adult male rats has long-term effects on metabolism

Nutritional insults during developmental plasticity have been linked with metabolic diseases such as diabetes in adulthood. We aimed to investigate whether a low-protein (LP) diet at the beginning of adulthood is able to program metabolic disruptions in rats. While control rats ate a normal-protein (23%; NP group) diet, treated rats were fed a LP (4%; LP group) diet from 60 to 90 days of age, after which an NP diet was supplied until they were 150 days old. Plasma levels of glucose and insulin, autonomous nervous system (ANS), and pancreatic islet function were then evaluated. Compared with the NP group, LP rats exhibited unchanged body weight and reduced food intake throughout the period of protein restriction; however, after the switch to the NP diet, hyperphagia of 10% (P<0.05), and catch-up growth of 113% (P<0.0001) were found. The LP rats showed hyperglycemia, insulin resistance, and higher fat accretion than the NP rats. While the sympathetic tonus from LP rats reduced by 28%, the vagus tonus increased by 21% (P<0.05). Compared with the islets from NP rats, the glucose insulinotropic effect as well as cholinergic and adrenergic actions was unaltered in the islets from LP rats. Protein restriction at the beginning of adulthood induced unbalanced ANS activity and fat tissue accretion later in life, even without functional disturbances in the pancreatic islets.

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.

Modulation of the insulinotropic action of glibenclamide and glimepiride by nutrient secretagogues in pancreatic islets from normoglycemic and hyperglycemic rats

In perifused pancreatic islets from euglycemic rats, the secretory response to either glibenclamide or glimepiride (1.0 microM each) increases as a function of the concentration of D-glucose (2.8-16.7 mM) present in the perifusion medium. On the contrary, the sulfonylurea-induced increment in 45Ca efflux from prelabeled islets decreases at increasing concentrations of the hexose. Neither glibenclamide nor glimepiride affect D-glucose metabolism in isolated islets, as judged from the production of 3HOH from D-[5-3H]glucose or the generation of 14CO2, as well as 14C-labeled amino acids and acidic metabolites, from D-[3,4-14C]glucose, D-[2-14C]glucose and D-[6-14C]glucose. The insulinotropic action of the hypoglycemic sulfonylureas is not impaired in islets prepared from rats infused for 48 hr with a hypertonic solution of D-glucose. The dimethyl ester of succinic acid is more efficient than D-glucose in supporting the insulin-releasing effect of glibenclamide or glimepiride. Thus, although the insulinotropic action of hypoglycemic sulfonylureas appears unaffected in a model of B-cell glucotoxicity, a potentiation of their secretory effects might be expected, in non-insulin-dependent diabetes, from the combined administration of succinic acid methyl ester.

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.

Effects of single dose irradiation on pancreatic beta-cell function

We have irradiated abdominal cavity of 23 rats with 10 Gy irradiation-induced hypoglycemia on the fourth day after intervention. Islets collected at this time showed an impaired insulin secretion without affecting insulin content. This impairment persisted after one month follow-up with reduced number of beta-cells in morphological examination.

Insulin secretion in the isolated islets of single-, regular-fasted and fed rats

To investigate the alterations in insulin secretion induced by habituation to single daily meal, adult rats were trained on a regular-fasting scheme (2-hr feeding/22-hr fasting) for 4 weeks. Insulin secretion induced by nutrient secretagogues (D-glucose and L-leucine) and the rates of 45Ca2+ outflow and 14C-glucose oxidation were studied in isolated islets obtained from these animals and in fed and 22-hr single-fasted rats. As expected, in the 22-hr fasted group, insulin secretion was drastically decreased although not abolished while 45Ca2+ outflow and 14C-glucose oxidation rates were only partially reduced. However, the regular-fasted rats did not secrete insulin in response to the nutrient secretagogues; calcium entry was not detected although a partial reduction in 14C-glucose oxidation rate was observed. These results suggest that regular fasting induces alterations in pancreatic B-cell glycolytic pathways leading to impairment of calcium efflux and insulin secretion. Such impairment is more pronounced than that induced by a single 22-hr fast.

Vitamin D and pancreatic islet function. II. Dynamics of insulin release and cationic fluxes

Pancreatic islets were prepared from control and vitamin D-deprived rats 2 or 5 weeks after weaning and, in the latter case, after 3 or 6 days treatment with exogenous vitamin D3 (60 nmol per day). The islets were prelabelled with both 86Rb and 45Ca and placed in a perfusion chamber. Vitamin D deprivation or administration failed to affect 86Rb outflow whether prior or after stimulation of the islets by a rise in either extracellular D-glucose or Ca2+ concentration. However, vitamin D deprivation decreased and vitamin D administration enhanced the basal 45Ca fractional outflow rate, as well as the magnitude of changes in both 45Ca and insulin release evoked by the rise in either D-glucose or extracellular Ca2+. It is proposed that the alteration in 45Ca fluxes and insulin release attributable to changes in the supply of vitamin D are, to a large extent, independent of the changes in nutrient catabolism conceivably associated with vitamin D deprivation and administration.

Can desensitization of the B-cell to D-glucose be simulated in cultured pancreatic islets?

In order to investigate the phenomenon of B-cell desensitization to D-glucose, rat pancreatic islets were cultured for 20-44h in the presence of increasing concentrations of D-glucose in the 5.6 to 27.8 mM range, and then incubated for 30 to 120 min for measurement of secretory, metabolic and ionic variables. After culture in the presence of 5.6 mM D-glucose, the release of insulin evoked by D-glucose (16.7 mM) was less marked than that seen in islets cultured in the presence of 11.1 mM D-glucose. In the latter islets, the secretory response to D-glucose (8.3 mM or more) was still modest, especially over short periods of incubation, but was markedly enhanced by either theophylline or forskolin. The release of insulin evoked by D-glucose in the presence of theophylline was little affected by either Ca2+ concentration of the culture medium or length of culture period (20h vs 44h). The culture-induced alteration in the responsiveness to D-glucose coincided with a smaller relative increase of D-[5(3)H]glucose utilization, D-[U-14C]glucose oxidation or net 45Ca uptake at increasing concentrations of the hexose. It contrasted with a well-preserved secretory response to nonnutrient secretagogues. Although these findings could be interpreted as evidence of B-cell desensitization to D-glucose, the fact that the secretory behavior of the islets was not vastly different whether they were first cultured at physiological (8.3 mM) or higher (11.1 to 27.8 mM) concentrations of D-glucose suggests that this experimental design may not be an optimal model for the functional alteration of the B-cell in hyperglycemic non-insulin-dependent diabetic subjects.