Changes in food intake, metabolic parameters and insulin resistance are induced by an isoenergetic, medium-chain fatty acid diet and are associated with modifications in insulin signalling in isolated rat pancreatic islets.

Long-chain fatty acids are capable of inducing alterations in the homoeostasis of glucose-stimulated insulin secretion (GSIS), but the effect of medium-chain fatty acids (MCFA) is poorly elucidated. In the present study, we fed a normoenergetic MCFA diet to male rats from the age of 1 month to the age of 4 months in order to analyse the effect of MCFA on body growth, insulin sensitivity and GSIS. The 45% MCFA substitution of whole fatty acids in the normoenergetic diet impaired whole body growth and resulted in increased body adiposity and hyperinsulinaemia, and reduced insulin-mediated glucose uptake in skeletal muscle. In addition, the isolated pancreatic islets from the MCFA-fed rats showed impaired GSIS and reduced protein kinase Ba (AKT1) protein expression and extracellular signal-related kinase isoforms 1 and 2 (ERK(1/2)) phosphorylation, which were accompanied by increased cellular death. Furthermore, there was a mildly increased cholinergic sensitivity to GSIS. We discuss these findings in further detail, and advocate that they might have a role in the mechanistic pathway leading to the compensatory hyperinsulinaemic status found in this animal model.

Angiotensin II induces superoxide generation via NAD(P)H oxidase activation in isolated rat pancreatic islets.

Angiotensin II (Ang II) controls blood pressure, electrolyte balance, cell growth and vascular remodeling. Ang II activates NAD(P)H oxidase in several tissues with important function in the control of insulin secretion. Considering the concomitant occurrence of hypertension, insulin resistance and pancreatic B cell secretion impairment in the development of type II diabetes the aim of the present study was to evaluate the effect of ANG II on NAD(P)H oxidase activation in isolated pancreatic islets. We found that ANGII-induced superoxide generation via NAD(P)H oxidase activation and increased protein and mRNA levels of NAD(P)H oxidase subunits (p47(PHOX) and gp91(PHOX)).

Insulinotropic effect of the tumor promoter 12-O-tetradecanoylphorbol-13-acetate in rat pancreatic islets.

In isolated rat pancreatic islets, the tumor-promoting agent 12-O-tetradecanoylphorbol-13-acetate (TPA), when used in the 2.10(-9) to 2.10(-7) M range, was found to stimulate insulin release both in the absence and presence of glucose. The non-tumor-promoting agent 4-methylphorbol-12,13-didecanoate failed to stimulate insulin release. The insulinotropic capacity of TPA was enhanced by glucose in a dose-related fashion. In the absence of glucose, the TPA-stimulated release of insulin was a slowly induced and not rapidly reversible phenomenon. It was inhibited by antimycin A, by epinephrine, at low temperatures, and in the absence of extracellular Ca2+ or the presence of cytochalasin B, was unaffected by the organic calcium antagonist D600 or indomethacin, and was potentiated by theophylline. No obvious effect of TPA upon 86Rb or 32P efflux and 45Ca net uptake could be detected in the isolated islets. However, TPA caused a progressive increase in both 45Ca fractional outflow rate and cyclic adenosine 3':5'-monophosphate content in the islets. It is proposed that the insulinotropic action of TPA may be due, in part at least, to interference with the transport of calcium by native ionophores.

The coupling of metabolic to secretory events in pancreatic islets. The possible role of glutathione reductase.

The participation of glutathione reductase in the process of nutrient-stimulated insulin release was investigated in rat pancreatic islets exposed to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU). BCNU caused a time-and dose-related, irreversible inhibition of glutathione reductase activity. This coincided with a fall in both GSH/GSSG ratio and the thiol content of the islets. Pretreatment of the islets with BCNU inhibited the oxidation of glucose and its stimulant action upon both 45Ca net uptake and insulin release. Although BCNU (up to 0.5 mM) failed to affect the oxidation of L-leucine and L-glutamine, it also caused a dose-related inhibition of insulin release evoked by the combination of these two amino acids. The latter inhibition was apparently not fully accounted for by the modest to negligible effects of BCNU upon 45Ca uptake, 45Ca efflux, 86Rb efflux and cyclic AMP production. Since BCNU failed to inhibit insulin release evoked by the association of Ba2+ and theophylline, these results support the view that glutathione reductase participates in the coupling of metabolic to secretory events in the process of nutrient-stimulated insulin release. However, the precise modality of such a participation, for example the control of intracellular Ca2+ distribution, remains to be elucidated.

The stimulus-secretion coupling of amino acid-induced insulin release: metabolism and cationic effects of leucine.

When isolated rat pancreatic islets are exposed to L-leucine (20 mM), the rate of NH4 production is close to the summed rates of L-[1-14C] leucine decarboxylation and alpha-ketoisocarproate production, whereas the rates of acetoacetate production and L-[U-14C]-leucine oxidation are compatible with conversion of each mole of the amino acid to one mole of acetoacetate and three moles of CO2. ATP content, ATP/ADP ratio, and adenylate charge are maintained at normal values by L-leucine, whereas the NADH/NAD+ ratio (but not the NADPH/NADP+ ratio) is significantly increased. The release of insulin evoked by L-leucine is potentiated by 2-ketoisovalerate, unaffected by L-valine, and inhibited by menadione. L-leucine mimicks the effect of D-glucose on 86Rb+ and 45Ca2+ handling by the islets. However, relative to its rate of oxidation, the insulinotropic effect of L-leucine is less marked than that of D-glucose. This may be due, in part at least, to a decrease in the oxidation of endogenous nutrients. It is concluded that the metabolic, cationic, and secretory effects of L-leucine in isolated islets are not incompatible with the fuel hypothesis for insulin release.

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.

Hexose metabolism in pancreatic islets: pyruvate carboxylase activity.

The anaplerotic hypothesis for insulin release postulates that an increased generation of malonyl-CoA, acyl residues and diacylglycerol in nutrient-stimulated pancreatic islets may couple the catabolism of nutrient secretagogues to more distal events in the secretory sequence. In the light of this hypothesis, pyruvate carboxylase activity was measured in rat pancreatic islets using two distinct radioisotopic procedures. The first procedure is based on the conversion of oxalacetate generated from pyruvate to 14C-labelled citrate in the presence of [1-14C]acetyl-CoA and citrate synthase. The second technique involves the conversion of 14C-labelled oxalacetate generated from [1-14C]pyruvate to radioactive aspartate in the presence of L-glutamate and glutamate-oxalacetate transaminase. Pyruvate carboxylase activity amounted to 10 pmol/min per islet, was restricted to mitochondria, displayed a Km for pyruvate close to 0.4 mM, and demonstrated dependency towards ATP (apparent Ka close to 0.1 mM), Mg2+ and acetyl-CoA. It is proposed that pyruvate carboxylase activity accounts for the generation of 14C-labelled amino acids other than alanine in islets exposed to D-[3,4-14C]glucose and participates to the pyruvate/citrate shuttle for the transport of acetyl-CoA out of the mitochondria in nutrient-stimulated islets.

Cholinergic stimulation of ion fluxes in pancreatic islets.

Cholinergic agents are known to stimulate the hydrolysis of polyphosphoinositides in pancreatic islets. The effect of carbamylcholine upon ion fluxes in the islet cells was investigated. Carbamylcholine provoked a rapid but poorly sustained increase in 45Ca and 86Rb outflow from perifused islets. Such a cationic response was observed at different glucose concentrations (zero to 16.7 mM), at three concentrations of carbamylcholine (10 microM, 100 microM and 1.0 mM), and in the absence or presence of extracellular Ca2+. It coincided with a biphasic stimulation of insulin release, both the cationic and secretory responses being abolished in the presence of atropine (10 microM). At variance with nutrient secretagogues, carbamylcholine failed to affect the net production of cyclic AMP and caused a transient decrease in 32P outflow from islets prelabelled with [32P]phosphate. It is proposed that cholinergic agents mobilize Ca2+ from intracellular sites, possibly through generation of inositol, 1,4,5-triphosphate from phosphatidylinositol 4,5-bisphosphate. The intracellular redistribution of Ca2+ does not appear sufficient, however, to account fully for the secretory response, which may also involve activation of protein kinase C by diacylglycerol.

The stimulus-secretion coupling of glucose-induced insulin release. XLVI. Physiological role of L-glutamine as a fuel for pancreatic islets.

Exogenous L-glutamine is actively metabolized in rat pancreatic islets. The rate of L-glutamine deamidation largely exceeds the rate of glutamate conversion to gamma-aminobutyrate and alpha-ketoglutarate. The latter conversion occurs in part by oxidative deamination, and in part by transamination reactions coupled with the conversion of 2-keto acids (pyruvate, oxaloacetate), themselves derived from the metabolism of glutamine, to their corresponding amino acids (alanine, aspartate). An important fraction of malate formed from alpha-ketoglutarate leaves the Krebs cycle and is converted to pyruvate, the process being apparently associated with the induction of a more reduced state in cytosolic redox couples. L-Glutamine abolishes the oxidation of endogenous nutrients is documented by the fact that the glutamine-induced increase in O2 consumption is much lower than expected from the rate of 14CO2 output from islets exposed to L-[U-14C]glutamine, L-Glutamine, although decreasing K+ conductance, fails to stimulate insulin release both in the absence and presence of D-glucose. It is proposed that L-glutamine represents a major fuel for pancreatic islets under physiological conditions.

Stimulus-secretion coupling of glucose-induced insulin release. Timing of early metabolic, ionic, and secretory events.

The timing of the early metabolic, ionic, and secretory responses to glucose in rat pancreatic islets was monitored by measuring, at 12 sec intervals, the concentrations of glucose, lactic, and pyruvic acids, 32P, 86Rb, 45ca, and insulin in the effluent of perifused prelabeled islets. The increase in glucose concentration from zero to 16.7 mM was complete within 133 sec. The output of organic acids increased after 24 sec of exposure to glucose and, in the case of lactic acid, fell slightly after the initial elevation. The phosphate flush was initiated only after 96 sec of exposure to glucose, whereas the decreases in 86Rb and 45Ca outflow were both detectable within 72 sec of stimulation. The secondary rise in 45Ca efflux was first seen after 157 sec of stimulation and its time course was not vastly different from that of insulin release. These data indicate that, in the secretory sequence, metabolic changes precede both the remodelling of ionic fluxes and the stimulation of insulin release. The results are compatible with the view that the secondary rise in 45Ca outflow is attributable, in part at least, to the glucose-induced decrease in K conductance (but not to the increase in phosphate outflow), with resulting membrane depolarization and gating of voltage-dependent Ca channels.

Stimulus-secretion coupling of glucose-induced insulin release. Effect of intracellular acidification upon calcium efflux from islet cells.

When pancreatic islets were exposed to a medium of normal pH (7.4) equilibrated with a gaz mixture containing 30% (instead of 5%) CO2, the intracellular pH of the islet cells, as judged by the apparent space of distribution of 14C-DMO, was decreased. The intracellular acidification was associated with a delayed decrease of [U-14C] glucose oxidation, but no major change in glucose-stimulated proinsulin biosynthesis, 45 calcium net uptake, or insulin release. The increase in PCO2 provoked an immediate and sustained decrease in the fractional outflow rate of 45Ca from prelabeled and perfused islets. The latter decrease was most marked under conditions associated with stimulated 40Ca-45Ca exchange (i.e., at glucose 16.7 mM and normal Ca2+ concentration), but was also present when a process of Na+-Ca2+ countertransport accounted for the major part of 45Ca efflux (e.g., in the absence of both glucose and extracellular Ca2+). These findings are compatible with the view that the generation of H+ derived from the metabolism of glucose in islet cells plays a role in the sugar-induced decrease in 45Ca fractional outflow rate.

Stimulus-secretion coupling of amino acid-induced insulin release VII. The B-cell memory for L-glutamine.

In the absence of another exogenous nutrient, L-glutamine does not stimulate insulin release from rat pancreatic islets or isolated perfused pancreases. L-glutamine, however, augments insulin release evoked by L-leucine. These two amino acids could interact by providing both the substrate (L-glutamate) and an activator (L-leucine) for the reaction catalyzed by glutamate dehydrogenase. Under suitable experimental conditions, as little as 0.5 mM L-glutamine is sufficient to enhance leucine-stimulated insulin release. When the pancreases or islets are first exposed to L-glutamine and then stimulated with L-leucine, the rate of secretion is much higher than that evoked by L-leucine in tissue not first exposed to L-glutamine. The memory of a prior exposure to L-glutamine persists for at least 25 min after removal of the latter amino acid from the extracellular fluid. This memory phenomenon is not dependent on the presence of Ca2+ in the extracellular fluid during the first exposure to L-glutamine, but is suppressed when such a prior exposure is performed in the absence of extracellular K+. The memory phenomenon could be due, in part at least, to inhibition by L-glutamine of K+ conductance in the B-cell plasma membrane. Moreover, the amount of L-glutamate which accumulates in islets exposed to L-glutamine is sufficient to maintain, for a much longer period than with other nutrient secretagogues, a sustained increase in catabolic fluxes after removal of the amino acid from the extracellular fluid.

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.

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.

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.

The effect of controlled feeding conditions on the metabolic characteristics of rats.

To evaluate the effect of feeding conditions on blood glucose, insulin and free fatty acid concentrations, rats were maintained on a 2-hr feeding/22-hr fasting (regular-fasted) schedule for 4 weeks. The animals were then subjected to glucose or insulin loads immediately prior to the usual meal time. Animals fasted for only 22 hr (single-fasted) just before the experiments, and rats having access to food ad lib were similarly loaded and tested. The results demonstrate that the regular fasting regime induces certain metabolic alterations well described in the literature for the single-fasted-period to become more pronounced, specifically, a reduction in insulin secretion and a probably increase in peripheral responsiveness to this hormone. In addition, glucose loading was more effective in lowering plasma free fatty acid concentration in rats restricted to a regular fasting scheme.

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.

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.

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.

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.