Hypercholesterolaemia, signs of islet microangiopathy and altered angiogenesis precede onset of type 2 diabetes in the Goto-Kakizaki (GK) rat

AIMS/HYPOTHESIS

The adult non-obese Goto-Kakizaki (GK) rat model of type 2 diabetes, particularly females, carries in addition to hyperglycaemia a genetic predisposition towards dyslipidaemia, including hypercholesterolaemia. As cholesterol-induced atherosclerosis may be programmed in utero, we looked for signs of perinatal lipid alterations and islet microangiopathy. We hypothesise that such alterations contribute towards defective pancreas/islet vascularisation that might, in turn, lead to decreased beta cell mass. Accordingly, we also evaluated islet inflammation and endothelial activation in both prediabetic and diabetic animals.

METHODS

Blood, liver and pancreas were collected from embryonic day (E)21 fetuses, 7-day-old prediabetic neonates and 2.5-month-old diabetic GK rats and Wistar controls for analysis/quantification of: (1) systemic variables, particularly lipids; (2) cholesterol-linked hepatic enzyme mRNA expression and/or activity; (3) pancreas (fetuses) or collagenase-isolated islet (neonates/adults) gene expression using Oligo GEArray microarrays targeted at rat endothelium, cardiovascular disease biomarkers and angiogenesis, and/or RT-PCR; and (4) pancreas endothelial immunochemistry: nestin (fetuses) or von Willebrand factor (neonates).

RESULTS

Systemic and hepatic cholesterol anomalies already exist in GK fetuses and neonates. Hyperglycaemic GK fetuses exhibit a similar percentage decrease in total pancreas and islet vascularisation and beta cell mass. Normoglycaemic GK neonates show systemic inflammation, signs of islet pre-microangiopathy, disturbed angiogenesis, collapsed vascularisation and altered pancreas development. Concomitantly, GK neonates exhibit elevated defence mechanisms.

CONCLUSIONS/INTERPRETATION

These data suggest an autoinflammatory disease, triggered by in utero programming of cholesterol-induced islet microangiopathy interacting with chronic hyperglycaemia in GK rats. During the perinatal period, GK rats show also a marked deficient islet vascularisation in conjunction with decreased beta cell mass.

The GK rat beta-cell: a prototype for the diseased human beta-cell in type 2 diabetes?

Increasing evidence indicates that decreased functional beta-cell mass is the hallmark of type 2 diabetes (T2D) mellitus. Nowadays, the debate focuses on the possible mechanisms responsible for abnormal islet microenvironment, decreased beta-cell number, impaired beta-cell function, and their multifactorial aetiologies. This review is aimed to illustrate to what extend the Goto-Kakizaki rat, one of the best characterized animal models of spontaneous T2D, has proved be a valuable tool offering sufficient commonalities to study these aspects. We propose that the defective beta-cell mass and function in the GK model reflect the complex interactions of multiple pathogenic players: (i) several independent loci containing genes responsible for some diabetic traits (but not decreased beta-cell mass); (ii) gestational metabolic impairment inducing an epigenetic programming of the pancreas (decreased beta-cell neogenesis and/or proliferation) which is transmitted to the next generation; and (iii) loss of beta-cell differentiation due to chronic exposure to hyperglycemia/hyperlipidemia, inflammatory mediators, oxidative stress and to perturbed islet microarchitecture.

Defective functional β-cell mass and Type 2 diabetes in the Goto-Kakizaki rat model

Increasing evidence indicates that decreased functional β-cell mass is the hallmark of Type 2 diabetes mellitus. Therefore, the debate focuses on the possible mechanisms responsible for abnormal islet microenvironment, decreased β-cell number, impaired β-cell function and their multifactorial etiologies. The information available on the Goto-Kakizaki/Par rat line, one of the best characterized animal models of spontaneous Type 2 diabetes mellitus, are reviewed in such a perspective. We propose that the defective β-cell mass and function in the Goto-Kakizaki/Par model reflect the complex interactions of multiple pathogenic players, including several independent loci containing genes responsible for some diabetic traits (but not decreased β-cell mass), gestational metabolic impairment inducing an epigenetic programming of the pancreas (decreased β-cell neogenesis), which is transmitted to the next generation, and loss of β-cell differentiation due to chronic exposure to hyperglycemia, inflammatory mediators, oxidative stress and perturbed islet microarchitecture.

Underestimation of D-glucose utilisation as judged from the conversion of D-[3-(3)H]glucose to (3)HOH

AIMS/HYPOTHESIS

The conversion of D-[3-(3)H]glucose to (3) HOH is currently measured to assess D-glucose utilisation. The validity of such a procedure was re-evaluated.

METHODS

The conversion of D-[3-(3)H]glucose and D-[5-(3)H]glucose to (3) HOH was measured in rat pancreatic islets, parotid cells and erythrocytes. The tritiation of lipids were also examined in islets exposed to D-[3-(3)H]glucose or D-[5-(3)H]glucose.

RESULTS

In rat pancreatic islets and parotid cells, but not in rat erythrocytes, the generation of (3) HOH from D-[3-(3)H]glucose underestimates the rate of D-glucose utilisation, this being apparently attributable to a partial escape from detritiation of [1-(3)H]glycerone-3-phosphate. Such an escape phenomenon resulted in a higher tritiation of lipids in pancreatic islets exposed to D-[3-(3)H]glucose, rather than D-[5-(3)H]glucose. Its relative extent was affected by a number of environmental factors such as the cell type under consideration, the metabolic status of the animals, and the extracellular concentration of D-glucose.

CONCLUSION/INTERPRETATION

These findings impose a reservation on the use of D-[3-(3)H]glucose conversion to (3)HOH as a tool to assess the utilisation of the hexose in some cell types.

beta-cell function and viability in the spontaneously diabetic GK rat: information from the GK/Par colony

The GK rat model of type 2 diabetes is especially convenient to dissect the pathogenic mechanism necessary for the emergence of overt diabetes because all adult rats obtained in our department (GK/Par colony) to date have stable basal mild hyperglycemia and because overt diabetes is preceded by a period of normoglycemia, ranging from birth to weaning. The purpose of this article is to sum up the information so far available related to the biology of the beta-cell in the GK/Par rat. In terms of beta-cell function, there is no major intrinsic secretory defect in the prediabetic GK/Par beta-cell, and the lack of beta-cell reactivity to glucose (which reflects multiple intracellular abnormalities), as seen during the adult period when the GK/Par rats are overtly diabetic, represents an acquired defect (perhaps glucotoxicity). In terms of beta-cell population, the earliest alteration so far detected in the GK/Par rat targets the size of the beta-cell population. Several convergent data suggest that the permanently reduced beta-cell mass in the GK/Par rat reflects a limitation of beta-cell neogenesis during early fetal life, and it is conceivable that some genes among the set involved in GK diabetes belong to the subset of genes controlling early beta-cell development.

Metabolic and secretory interactions between D-glucose and D-fructose in islets from GK rats

The metabolism of D-glucose and/or D-fructose was investigated in both pancreatic islets and parotid cells of control and hereditarily diabetic Goto-Kakizaki (GK) rats. In the islets from GK rats, a preferential alteration of the oxidative response to D-glucose coincided with an impaired secretory response to the aldohexose. Such a metabolic alteration was not found in the parotid cells of GK rats. Whether in islet or parotid cells, D-fructose little affected the catabolism of glucose in either control or GK rats. The metabolism of D-fructose and the effect of D-glucose thereupon were essentially comparable in control and GK rats in both pancreatic islets and parotid cells. In both cell types, the comparison between the metabolism of D-glucose and D-fructose in cells simultaneously exposed to the two hexoses suggested a far from negligible contribution of fructokinase to the phosphorylation of D-fructose. Although the catabolism of the ketohexose and its modulation by D-glucose were closely comparable in islets from control and GK rats, the insulinotropic action of the ketohexose, relative to that of the aldohexose, was severely impaired in the GK rats. The present work thus emphasizes the specificity of the alteration in D-glucose metabolism in islets, as opposed to extrapancreatic cells, of GK rats. It also reveals in the islets of GK rats a further secretory anomaly apparently not attributable to the impairment of nutrient catabolism in the islet cells of these diabetic animals.

Metabolic and secretory response to D-fructose in pancreatic islets from adult rats injected with streptozotocin during the neonatal period

The metabolic and secretory responses to D-glucose and/or D-fructose were measured in pancreatic islets prepared from either control rats or animals that had been injected with streptozotocin during the neonatal period (STZ rats). The STZ rats displayed higher plasma D-glucose concentrations, but lower plasma insulin concentrations, islet insulin content, as well as basal and nutrient-stimulated insulin release. This coincided with lower rates of D-[U-(14)C]hexose oxidation and D-[5-(3)H]hexose utilization. In both control and STZ rats, D-fructose failed to affect significantly the metabolism of d-glucose, while the aldohexose increased the ratio between D-[U-(14)C]fructose oxidation and D-[5-(3)H]fructose conversion to (3)HOH. Such a ratio was higher than that found with radioactive D-glucose in islets exposed to both hexoses, whether in control or STZ rats, indicating a far-from-negligible contribution of fructokinase to the phosphorylation of D-fructose. Despite these analogies between both the respective fate of D-glucose and D-fructose and the reciprocal metabolic effects of the two hexoses in islets from control and STZ rats, the secretory response to the ketohexose in islets from STZ rats was preferentially suppressed, relative to that evoked by the aldohexose. This gives support to the idea that the insulinotropic action of D-fructose may not be entirely accounted for by its nutritional value in islet cells.

Decreased pancreatic islet response to L-leucine in the spontaneously diabetic GK rat: enzymatic, metabolic and secretory data

AIMS/HYPOTHESIS

Pancreatic islets from hereditarily non-insulin-dependent diabetic Goto-Kakizaki (GK) rats have a deficient insulin response not only to D-glucose but also to L-leucine. Our aim was to explain the cellular mechanism(s) underlying the beta-cell unresponsiveness to this amino acid.

METHODS

Freshly collagenase isolated islets from GK rats and healthy Wistar control rats matched with them for sex and age were compared. Leucine uptake, metabolic fluxes and insulin secretory capacity were investigated on batch incubated-islets. Enzymatic activities were measured on sonicated islets.

RESULTS

In GK rat islets, neither leucine transport nor leucine transaminase activity was disturbed. By contrast, 14CO2 production from either L-[U-14C]leucine or L-[1-14C]leucine was decreased. The L-[U-14C]leucine oxidation: L-[1-14C]leucine decarboxylation ratio was unaffected, indicating that the acetyl-CoA generated from leucine undergoes normal oxidation in the Krebs cycle. The leucine non-metabolizable analogue 2-amino-bicyclo[2,2,1]heptane-2-carboxylic acid induced insulin release and enhanced the secretory response to leucine as in controls, whereas leucine failed to amplify the response to the leucine analogue. Moreover, the potentiating action of L-glutamine on leucine-mediated insulin release was preserved. This coincided with normal glutamate dehydrogenase activity and L-[U-14C]glutamine oxidation. Finally, the secretory response to the leucine deamination product 2-ketoisocaproate was decreased, as was the 2-keto[1-14C]isocaproate oxidation.

CONCLUSION/INTERPRETATION

In islet beta cells from GK rats, the defective secretory response to leucine cannot be ascribed to a deteriorated leucine-stimulated glutamate metabolism but rather to an impaired leucine catabolism. A reduced generation of acetyl-CoA from 2-ketoisocaproate, due to the defective oxidative decarboxylation of this keto-acid by the mitochondrial branched-chain 2-ketoacid dehydrogenase, is incriminated.

Effect of the new imidazoline derivative S-22068 (PMS 847) on insulin secretion in vitro and glucose turnover in vivo in rats

We have investigated the possible mechanisms underlying the antihyperglycaemic effect of the imidazoline derivative S-22068. In vitro, in the presence of 5 mmol/l glucose, S-22068 (100 micromol/l) induced a significant and sustained increase in insulin secretion from isolated, perifused, rat islets and a marked sensitization to a subsequent glucose challenge (10 mmol/l). S-22068 (100 micromol/l was able to antagonize the stimulatory effect of diazoxide on 86Rb efflux from preloaded islets incubated in the presence of 20 mmol/l glucose. Experiments were also performed to investigate whether S-22068 can alter glucose turnover and peripheral insulin sensitivity in vivo in mildly diabetic rats and obese, insulin resistant, Zucker rats. Neither glucose production nor individual tissue glucose utilization was modified by S-22068 in either group of rats. Similar results were obtained whether the studies were performed under basal conditions or during euglycaemic/hyperinsulinemic clamps. The results suggest that S-22068 exerts part of its antihyperglycaemic effect by promoting insulin secretion without alteration of peripheral insulin sensitivity.

Galphaolf identification by RT-PCR in purified normal pancreatic B cells and in islets from rat models of non-insulin-dependent diabetes

Recent reports using immunohistochemistry have shown that Galphaolf which shares 88% homology with Galphas was expressed in pancreatic islets. To test the specificity of the expression of this G protein isotype in rat islet cells, B and non-B cells were separated by flow cytometry. The expression of Galphaolf and adenylyl cyclases (AC) of types II, III, V, and VI was evaluated by reverse-transcriptase polymerase chain reaction (RT-PCR). Since alterations in the expression of AC III were recently reported in the GK rat (a model of non-insulin-dependent diabetes mellitus, NIDDM), we also have analyzed the mRNA expression of Galphaolf and AC isoforms in pancreatic islets from GK rats and from adult rats neonatally treated by streptozotocin (nSTZ rats), another model of NIDDM. Southern blots of amplicons generated with specific primers of Galphaolf revealed the presence of a 540-bp band only in B cells. AC of types II, III, V, and VI were expressed both in B and non-B cells. However, AC III mRNA was clearly more abundant in non-B than in B cells. Moreover, in B cells the expression of AC VI was higher than that of AC V, whereas equal expressions of AC V and AC VI were found in non-B cells. In GK rat islets, the mRNA expressions of Galphaolf, AC II, and AC III were clearly increased and no change in AC V and AC VI was found. In nSTZ rat islets, Galphaolf expression was barely detectable, but AC II and AC III mRNA levels were higher than those observed in controls. In conclusion, Galphaolf mRNA appeared specifically expressed in islet B cells and was increased in GK islets. The steady-state mRNA levels of AC II and AC III were clearly increased in the islets of the two rat models of NIDDM. Thus, alterations in the expression of G protein isotypes and AC isoforms could contribute to the diabetic phenotype.

Impaired pancreatic beta cell function in the fetal GK rat. Impact of diabetic inheritance

The Goto-Kakisaki (GK) rat is a genetic model of non-insulin-dependent diabetes. At 21.5 d of age we found that GK fetuses had an increased plasma glucose concentration, a decreased plasma insulin level, and a reduced pancreatic beta cell mass. To investigate the beta cell function during fetal life we used a hyperglycemic clamp protocol applied to the mothers, which allowed us to obtain a steady-state hyperglycemia in the corresponding fetuses. At variance, with Wistar (W) fetuses, plasma insulin concentration in GK fetuses did not rise in response to hyperglycemia. In contrast, GK fetal pancreas released insulin in response to glucose in vitro to the same extent as W fetal pancreas. Such a discrepancy between the in vivo and in vitro results suggests that the lack of pancreatic reactivity to glucose as seen in vivo is extrinsic to the fetal GK beta cell. Finally, the importance of gestational hyperglycemia was investigated by performing crosses between GK and W rats. Fetuses issued from crosses between W mother and GK father or GK mother and W father had a beta cell mass close to normal values and were still able to increase their plasma insulin levels in response to hyperglycemia in vivo. Our data suggest that hyperglycemia in utero does not influence the severity of the decrease of the beta cell mass or the lack of the insulin secretory response to glucose in the fetal GK rat. Moreover they indicate that conjunction of GK genes originating from both parents is necessary in order for these defects to be fully expressed.

Effects of dehydroepiandrosterone in rats injected with streptozotocin during the neonatal period

Control rats and diabetic animals injected with streptozotocin during the neonatal period were either maintained on a standard diet or given access to food supplemented with dehydroepiandrosterone (DHEA, 0.2%) for 11 days before sacrifice. In both control and diabetic rats, DHEA feeding augmented the activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase and cytosolic NADP-linked malate dehydrogenase in liver, but not so in either the parotid gland or pancreatic islets. DHEA lowered, in both control and diabetic rats, the ratio between D-glucose oxidation and utilization and the rate of insulin release in pancreatic islets exposed to a high concentration of D-glucose, as well as the insulin concentration and insulin/glucose ratio in plasma. These findings support the view that, in diabetes, DHEA, by increasing sensitivity to insulin, may allow islet B-cells to avoid the otherwise unfavorable consequences of chronic hyperactivity.

Impaired phosphoinositide metabolism in glucose-incompetent islets of neonatally streptozotocin-diabetic rats

The effects of nutrient and neurotransmitter stimuli on insulin release, loss of phosphoinositides (PI), and production of inositol phosphates (InsP) were investigated in islets from neonatally streptozotocin-injected (nSTZ) rats. In islets from nSTZ rats, insulin secretory responses to 16.7 mM D-glucose and 10.0 mM D-glyceraldehyde were reduced compared with controls. Contents in phosphatidylinositol 4-monophosphate [PtdIns(4)P] and phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], but not in phosphatidylinositol, were diminished. Glucose effects on breakdown of PtdIns(4)P and PtdIns(4,5)P2 and on total InsP accumulation were both reduced. D-Glucose was unable to increase the levels of both inositol trisphosphate isomers, Ins(1,3,4)P3 and Ins(1,4,5)P3. Glyceraldehyde also failed to promote InsP formation. By contrast, the ability of 1.0 mM carbachol or 300 nM cholecystokinin to stimulate insulin secretion and InsP generation was still observed. Thus a disturbed coupling between nutrient recognition and activation of phospholipase C, possibly together with a shortage of available polyphosphoinositides, could be responsible for the altered islet PI turnover in the nSTZ rats. It is proposed that such defects may contribute to the impairment of glucose-stimulated insulin secretion in this model of non-insulin-dependent diabetes mellitus.

Decreased ATP-induced synthesis and Ca(2+)-stimulated degradation of polyphosphoinositides in pancreatic islets from neonatally streptozotocin-diabetic rats

Phosphoinositide (PI) synthesis and hydrolysis were investigated in pancreatic islet homogenates from neonatal streptozotocin diabetic (n-STZ) and control rats. In the diabetics, ATP, in absence of Ca2+, failed to increase the amount of phosphatidylinositol 4-phosphate (PtdInsP) and phosphatidyl inositol 4, 5-bisphosphate (PtdInsP2) at variance with the pattern in controls. Also, the Ca(2+)-stimulated generation of inositol phosphates (InsP) was dramatically decreased, whether in the absence or presence of ATP. Moreover, phosphatidylinositol (PtdIns) kinase activity was reduced while PtdInsP kinase activity was not impaired. These data suggest that the suppressed formation of PtdInsP and subsequent PtdInsP2 synthesis, concomitantly with a decreased Ca(2+)-stimulated phospholipase C activity, may participate to the alteration of the PI pathway, the limitation of the InsP production, and finally the impairment of the insulin release in the n-STZ model of non-insulin-dependent diabetes.

Decreased glucose-induced cAMP and insulin release in islets of diabetic rats: reversal by IBMX, glucagon, GIP

The first aim of the study was to investigate the possibility that a defect on the islet adenosine 3',5'-cyclic monophosphate (cAMP) production could be involved in the failure of the glucose-induced insulin secretion in the neonatal streptozotocin diabetic rats. Exposure to glucose concentration that induced a rise of the cAMP content in the control islets did not elicit any significant increase in cAMP in diabetic islets. Forskolin, isobutyl methylxanthine (IBMX), glucagon, or pertussis toxin amplified the cAMP accumulation and the insulin release to the same extent in both types of islets. Somatostatin, prostaglandin E2, UK-14304, or galanin inhibited cAMP accumulation and insulin release to the same extent in both types of islets. Our second purpose was to investigate whether the use of activators of adenylate cyclase could restore the beta-cell competence to glucose in diabetic rats. The addition of IBMX, glucagon, or gastric inhibitory polypeptide (GIP) to perifused islets of diabetic rats amplified their insulin response to glucose, and a clear biphasic pattern of the release was regained. In conclusion, although there is no major alteration of the functionality of the adenylate cyclase in the beta-cells of the diabetic rats, we have identified a defective glucose-induced cAMP generation that could be explained by a block in the step(s) linking glucose metabolism and activation of adenylate cyclase.

Pancreatic islet mitochondrial glycerophosphate dehydrogenase deficiency in two animal models of non-insulin-dependent diabetes mellitus

Western blotting of pancreatic islet extracts from either hereditarily diabetic Goto-Kakizaki rats (GK rats) or animals injected with streptozotocin during the neonatal period (STZ rats) demonstrated a pronounced decrease of immunoreactive mitochondrial glycerophosphate dehydrogenase (m-GDH), when compared to results obtained in islets from control rats. By contrast, the islet glucokinase protein content was either unaffected (GK rats) or much less severely decreased than that of m-GDH (STZ rats). These findings indicate that the impaired activity of m-GDH previously documented in islet homogenates from diabetic rats coincides with a decreased content of this enzyme in the endocrine pancreas.

Mitochondrial deoxyribonucleic acid content is specifically decreased in adult, but not fetal, pancreatic islets of the Goto-Kakizaki rat, a genetic model of noninsulin-dependent diabetes

Considerable interest has recently been focused on the putative role of mutations in the mitochondrial genome for the development of noninsulin-dependent diabetes. The Goto-Kakizaki (GK) rat, a genetic model of defective insulin secretion and hyperglycemia, is characterized by partial maternal inheritance. Because the mitochondrial genome is known to be maternally transmitted, the aim of this study was to investigate whether the GK syndrome can be explained in terms of alterations of the mitochondrial DNA (mtDNA). For this purpose, pancreatic islets were isolated from adult and fetal control Wistar and diabetic GK rats. Using electron microscopy, the ultrastructural morphology of beta-cell mitochondria was analyzed in control and GK islets. It was found that the beta-cells of adult GK rats had a significantly smaller mitochondrial volume and an increased number of mitochondria per unit tissue volume as compared with the beta-cells of corresponding control islets. Moreover, mtDNA and mtRNA were isolated from the islets and, as a control tissue, from liver, and subsequently analyzed using Southern and Northern blot techniques. No major deletions or restriction fragment polymorphism could be detected in mtDNA from both GK liver and GK islets. The mtDNA sequence of the transfer RNAleu(UUS) gene was identical in both strains of rats. mtDNA contents of fetal GK islets and fetal GK liver were not different from those of fetal Wistar rats. However, adult GK islets contained markedly less mtDNA than the corresponding control islets, contrary to the mtDNA contents of adult liver, which were similar in the two strains. The lower islet mtDNA contents were paralleled by a decreased content of islet mtRNA (12S ribosomal RNA and cytochrome b messenger RNA). Islet insulin messenger RNA contents were similar in GK and Wistar rats. In conclusion, our results do not support a role of a genetic defect in mtDNA as a cause of the GK syndrome. Instead, mtDNA damage may occur specifically in islet cells as a consequence of the disturbed metabolic environment of the adult GK rat. It is speculated that a long-lasting metabolic dysfunction may induce mtDNA damage and/or inhibition of mtDNA replication leading to a gradual and late decrease in the mitochondrial volume fraction and subsequently an impaired capacity for oxidative metabolism.

Restricted effect of formycin A and non-glucidic nutrients upon insulin release in islets from rats with hereditary or acquired non-insulin-dependent diabetes

Pancreatic islets isolated from control rats, Goto-Kakizaki rats and adult rats that were injected with streptozotocin during the neonatal period were incubated for two successive period of 90 min each in the presence of D-glucose (11.1 mM) with or without formycin A (1.0 mM), and in the presence of the dimethyl ester of succinic acid (SAD, 10.0 mM) with or without palmitate (1.0 mM). Although formycin A augmented glucose-stimulated insulin release in both control and diabetic rats, it failed to compensate for the impaired secretory response to D-glucose in the latter animals. Likewise, non-glucidic nutrients such as SAD and/or palmitate failed to display a more efficient insulinotropic action, relative to basal insulin output, in diabetic than control rats. These results indicate that both formycin A and non-glucidic nutrients are unable, through their immediate insulinotropic action, to restore a normal output of insulin in islets of animals with inherited or acquired non-insulin-dependent diabetes.

Enzymatic and secretory activities in pancreatic islets of non-insulin-dependent diabetic rats after short-term infusion of succinic acid monomethyl ester

The monomethyl ester of succinic acid (SME) was recently proposed as a novel tool for stimulation of proinsulin biosynthesis and insulin release in animal models of non-insulin-dependent diabetes mellitus. In the present study, either saline or SME (14 mmol/day) was infused for 3 days to control rats, animals injected with streptozotocin during the neonatal period, and Goto-Kakizaki rats with inherited diabetes. The infusion of SME failed to correct the anomalies found in the islets of diabetic rats, namely, a decreased activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase, a low insulin content, and an impaired secretory response to various nutrient secretagogues including D-glucose, 2-ketoisocaproate, and the combination of L-leucine and L-glutamine. These findings raise the question of whether a more prolonged administration of SME is required to raise the insulin store and improve the secretory potential of the endocrine pancreas in animals with type 2 diabetes.

Pancreatic islet response to dicarboxylic acid esters in rats with type 2 diabetes: enzymatic, metabolic and secretory aspects

This study aimed to compare the metabolic and secretory responses of pancreatic islets from animals with non-insulin-dependent diabetes to D-glucose with the effects of the methyl esters of succinic acid (SME) and glutamic acid (GME). The insulin secretory response to D-glucose was impaired in islets from rats with diabetes which was either inherited (Goto-Kakizaki (GK) rats) or acquired (streptozotocin-treated (STZ) rats). This coincided with a preferential alteration of oxidative relative to total glycolysis in intact islets and a selective defect of FAD-linked mitochondrial glycerophosphate dehydrogenase (m-GDH) in islet homogenates. This enzymatic defect was also found in purified B cells from STZ rats. It contrasted both with unaltered activities of glutamate dehydrogenase and succinate dehydrogenase in the islets of diabetic animals and with a normal or even increased activity of m-GDH in the livers of GK and STZ rats. The oxidation of [1,4-14C]SME and [U-14C]GME appeared decreased in islets of GK or STZ animals when compared with control rats, but no significant difference between control and diabetic rats was observed when the oxidative data were expressed relative to the rate of [U-14C]GME hydrolysis. Nevertheless, the absolute values for insulin release evoked by a non-metabolized analogue of L-leucine (BCH), by SME and by the association of BCH with either SME or GME were invariably lower in islets of GK and STZ rats than in those of control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Hexose metabolism in pancreatic islets UDP-glucose pyrophosphorylase activity

UDP-glucose pyrophosphorylase was measured in rat pancreatic islets, the generation of D-glucose 1-phosphate from UDP-glucose and PPI being eventually coupled to the generation of L-[U-14C]glutamate from 14C-labelled alpha-ketoglutarate. The activity of the enzyme was about one order of magnitude lower in islet than liver homogenates. The affinity of the enzyme for either UDP-glucose or PPi was comparable, however, in liver and islets. The activity of UDP-glucose pyrophosphorylase was somewhat lower in islets from animals with inherited or acquired diabetes mellitus than in those from control rats. These findings are considered in connection with the accumulation of glycogen in islets of hyperglycemic animals.

Glucose refractoriness of pancreatic beta-cells in rat models of non-insulin dependent diabetes

A decreased insulin response, preferentially to glucose, has been considered a hallmark of non-insulin dependent diabetes mellitus (Type 2) in humans. Syndromes resembling human diabetes occur spontaneously in many animal species and can also be induced by treating animals with drugs or viruses, excising their pancreases or manipulating their diet. Among these models, rat diabetes induced by neonatal streptozotocin administration (n-STZ models) has been first recognized as an adequate tool to study the long-term consequences of a gradually reduced beta-cell mass. More recently, the GK (Goto Kakisaki) Wistar rat has become available and is now considered as a promising spontaneous rat model of non-insulin dependent diabetes. We and others have found that defects in insulin secretion and action develop in the n-STZ and the GK models, which in many ways resemble those described in human non-insulin dependent diabetes. This review is aimed to sum up with a comparative approach, the informations so far collected in the n-STZ and GK models concerning the cellular mechanisms leading to the desensitization of their beta-cells to glucose. Taken together, the data reinforce the view that the impairment of glucose-induced insulin release in n-STZ and GK rats is clearly related to a defect in oxidative glycolysis. This leads to a severe decrease in the mitochondrial oxidative catabolism of glucose-derived pyruvate. Its coincides with a lower ATP/ADP ratio in glucose-stimulated islets and a subsequent alteration of ionic events tightly coupled to the fuel function of the hexose in islet cells, i.e. the decrease in K+ conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Metabolic, ionic, and secretory response to D-glucose in islets from rats with acquired or inherited non-insulin-dependent diabetes

The metabolic, ionic, and secretory response to D-glucose was investigated in islets of adult rats either injected with streptozotocin during the neonatal period (STZ rats) or presenting with inherited diabetes (GK rats). At a high concentration of D-glucose (16.7 mM), the ATP/ADP ratio was lower in islets from STZ and GK than control rats. This coincided with an impaired response of perifused islets to a rise in D-glucose concentration in terms of stimulation of insulin release, suppression of effluent radioactivity from islets prelabeled with [2-3H]adenosine, reduction in 86Rb efflux, and induction of a phosphate flush in islets prelabeled with 32P(i). The ratio in either D-[5-3H]glucose utilization or D-[2-14C]glucose oxidation at high/low hexose concentration, as well as the paired ratio between D-[2-14C]glucose oxidation and D-[5-3H]glucose utilization in islets incubated at a high concentration of the hexose, was also lower in STZ and GK rats than in control rats. Such was not the case, however, from the oxidation of [2-14C]pyruvate. Instead, the latter 2-keto acid, when tested at a 5.0 mM concentration, improved more efficiently the overall oxidative response of the islets to a rise in D-glucose concentration in STZ and GK rats than in control animals. It is proposed, therefore, that in both STZ and GK rats, the B-cell secretory defect is primarily attributable to an anomaly in oxidative glycolysis. In islets exposed to a high concentration of D-glucose, this metabolic deficiency results in impaired ATP generation, altered closing of ATP-responsive K+ channels, and, hence, diminished insulin output.

Preferential alteration of oxidative relative to total glycolysis in pancreatic islets of two rat models of inherited or acquired type 2 (non-insulin-dependent) diabetes mellitus

In islets from both adult rats injected with streptozotocin during the neonatal period and spontaneously diabetic rats obtained by repeated selective breedings (GK rats), the ratio between D-[3,4-14C]glucose oxidation and D-[5-3H]glucose conversion to 3HOH was 25% lower than in islets from control rats, indicating an impaired contribution of oxidative to total glycolysis. No primary defect in the Krebs cycle was found in the islets of diabetic rats, as judged from the ratio between either D-[2-14C]glucose or D-[6-14C]glucose and D-[3,4-14C]glucose oxidation. Therefore, we propose that a preferential alteration of oxidative glycolysis in the pancreatic beta cell may contribute to the impairment of glucose-induced insulin release not only in a cytotoxic but also in a spontaneous model of non-insulin-dependent diabetes mellitus.

Metabolic response to nonglucidic nutrient secretagogues and enzymatic activities in pancreatic islets of adult rats after neonatal streptozotocin administration

In islets from adult rats injected with streptozotocin during the neonatal period, both a nonmetabolized analog of L-leucine and 3-phenylpyruvate augmented 14CO2 output from islets either prelabeled with L-[U-14C]glutamine or exposed to D-[2-14C]glucose and D-[6-14C]glucose, in a manner qualitatively comparable to that found in islets from control rats. The islets of diabetic rats differed, however, from those of control rats by their unresponsiveness to both the L-leucine analog and a high concentration of D-glucose in terms of increasing 3HOH generation from [2-3H]glycerol, an impaired sparing action of the hexose upon 14CO2 output from islets prelabeled with [U-14C]palmitate, and, most importantly, by a decreased rate of D-[2-14C]glucose and D-[6-14C]glucose oxidation when either incubated at a high concentration of the hexose (16.7 mM) or stimulated by nonglucidic nutrient secretagogues at a low concentration of D-glucose (2.8 mM). In islet homogenates, the activity of glyceraldehyde phosphate dehydrogenase, glutamate decarboxylase, and NADP-malate dehydrogenase was lower in diabetic than control islets. Such was not the case for glutamate-alanine transaminase, glutamate-aspartate transaminase, or glutamate dehydrogenase. The neonatal injection of streptozotocin thus affected, in the adult rats, the activity of several islet enzymes. Nevertheless, the metabolic data suggest that an impaired circulation in the glycerol phosphate shuttle, as observed in response to stimulation of the islets by either a high concentration of D-glucose or nonglucidic nutrient secretagogues, represents an essential determinant of the preferential impairment of glucose-induced insulin release in this model of non-insulin-dependent diabetes.

Functional and metabolic perturbations in isolated pancreatic islets from the GK rat, a genetic model of noninsulin-dependent diabetes

Spontaneously diabetic nonobese GK rats exhibit high basal plasma glucose and insulin levels and a poor insulin secretory response to glucose. We studied insulin biosynthesis, insulin release, and glucose metabolism in freshly isolated islets from GK rats and control Wistar rats. In GK rats, islet insulin content was decreased when expressed per islet but normal when related to DNA content. In the presence of a low concentration (2.8 mM) of glucose both (pro)insulin and total protein biosynthesis was doubled in islets from GK rats. As judged from the (pro)insulin/total protein synthesis ratio, (pro)insulin biosynthesis was normally stimulated by 16.7 mM glucose. In islets from diabetic rats both basal and glucose-stimulated insulin release were dramatically decreased. A reduced secretory response to 10 mM leucine or 10 mM leucine plus 10 mM glutamine and a lack of response to 10 mM monomethylsuccinate were observed. By contrast the insulinotropic capacity of nonnutrient secretagogues such as 62 microM gliclazide or the combination of 2 mM Ba2+ and 1.4 mM theophylline in the absence of extracellular Ca2+ remained normal. Glucose oxidation (estimated as the production of 14CO2 from D-[6-14C]glucose) was severely impaired, whereas no major alteration of glycolytic flux (as judged from the conversion of D-[5-3H]glucose to 3H2O) could be detected. Accordingly, the D-[6-14C]glucose oxidation/D-[5-3H]glucose use ratio was less markedly increased in response to a rise in glucose concentration in islets from GK rats than in islets from control rats. Thus, in islets from diabetic GK rats, glucose-induced insulin release but not insulin biosynthesis was impaired. This defect is associated with, and probably at least in part due to, a deficient oxidative metabolism of glucose in islet mitochondria.

Enzymic and metabolic anomalies in islets of diabetic rats: relationship to B cell mass

A preferential impairment of the pancreatic B cell secretory response to D-glucose occurs in adult rats injected with streptozotocin during the neonatal period. Three possible explanations for such a preferential defect were investigated in the present study. First, the time course for 3-O-methyl-D-glucose uptake by islets suggested that the anomaly in hexose transport was mainly attributable to a decrease in the space accessible to the D-glucose analog commensurate with the decrease in B cell mass, rather than to a delayed equilibration of hexose concentration across the B cell plasma membrane. Second, the activity of glucose-6-phosphatase was found to be equally low in islets from diabetic and control rats, ruling out the futile cycling between D-glucose and D-glucose 6-phosphate as a cause for the preferential alteration of the secretory response to the hexose. Third, the activity of flavine adenine dinucleotide-linked glycerophosphate dehydrogenase was found to be decreased to a greater relative extent than the B cell mass. This coincided with an impaired generation of 3HOH from L-[2-3H] glycerol in intact islets. It is proposed, therefore, that an altered circulation in the glycerol phosphate shuttle may play a major role in the impaired process of glucose-stimulated insulin release in this model of noninsulin-dependent diabetes.

Study of hexose transport, glycerol phosphate shuttle and Krebs cycle in islets of adult rats injected with streptozotocin during the neonatal period

At 3-4 degrees C, the transport of 3-O-methyl-D-glucose (30 mM) was severely impaired in islets prepared from adult rats injected with streptozotocin during the neonatal period. However, at 37 degrees C, the first and second phase of glucose-stimulated insulin release were decreased to the same relative extent in perifused islets of diabetic, as compared to control, animals. Moreover, the time-related increase in the oxidative response of the islets to 16.7 mM D-glucose was less pronounced in diabetic than control rats. The activity of the mitochondrial FAD-linked glycerophosphate dehydrogenase in islet homogenates of diabetic rats only represented one-fifth of that found in control rats, whereas the activity of the cytosolic NAD-glycerophosphate dehydrogenase was comparable in both types of rats. This coincided with the fact that a rise in D-glucose concentration from 2.8 to 16.7 mM failed to increase significantly L-[2-3H]glycerol conversion to 3HOH in islets from diabetic rats, in contrast to the situation found in control animals. The activity of 2-ketoglutarate dehydrogenase in islet homogenates when expressed per microgram protein was not different in control and diabetic rats. Likewise, the ratio between D-[6-14C]glucose oxidation and D-[3,4-14C]glucose oxidation and the capacity of either a non-metabolized analog of L-leucine or 3-phenylpyruvate to preferentially stimulated D-[6-14C]glucose oxidation relative to D-[5-3H]glucose utilization were both unaffected in islets from diabetic rats. These findings argue against the existence of a primary defect in the Krebs cycle of diabetic rats. It is proposed that, despite an obvious alteration of the hexose transport system in the islet cells of diabetic rats, the preferential impairment of the B-cell secretory response to D-glucose, as distinct from other secretagogues, in this model of non-insulin-dependent diabetes is mainly attributable to the low activity of FAD-linked glycerophosphate dehydrogenase, resulting in a decreased metabolic flow through the glycerol phosphate shuttle and a reduced rate of aerobic glycolysis.

Neonatal streptozotocin injection: a model of glucotoxicity?

Adult rats injected with streptozotocin during the neonatal period displayed in the fed state moderate hyperglycemia. However, the percentages of glycated hemoglobin in erythrocytes and glycated lactate dehydrogenase in liver and pancreatic islets, as well as the sorbitol and glycogen content of the islets, were not significantly increased. Likewise, in intact islets, the ouabain-sensitive inflow of 86Rb+, and the ratio between 3H2O production from D-[2-3H]glucose and D-[5-3H]glucose were not different in control and streptozotocin-injected rats. These findings suggest that the alteration in both the mitochondrial catabolism of D-glucose and secretory response to the hexose previously documented in the islets of the latter animals are not attributable to factors such as the excessive nonenzymatic glycation of cytosolic proteins, sorbitol or glycogen accumulation, or impaired Na+, K(+)-adenosine triphosphatase (ATPase) activity. Although a contributive role of glucotoxicity in the impaired function of beta cell in this model of non-insulin-dependent diabetes should not be ruled out, it is speculated that streptozotocin might also cause a long-term damage of key mitochondrial dehydrogenases in the pancreatic beta cells and, possibly, their precursor cells.

Beta-cell insensitivity to glucose in the GK rat, a spontaneous nonobese model for type II diabetes

In early 1988, a colony of GK rats was started in Paris with progenitors issued from F35 of the original colony reported by Goto and Kakisaki. When studied longitudinally up to 8 mo, GK rats showed as early as 1 mo (weaning) significantly higher basal plasma glucose (9 mM) and insulin levels (doubled), altered glucose tolerance (intravenous glucose), and a very poor insulin secretory response to glucose in vivo compared with Wistar controls. Males and females were similarly affected. Studies of in vitro pancreatic function were carried out with the isolated perfused pancreas preparation. Compared with nondiabetic Wistar rats, GK rats at 2 mo showed a significantly increased basal insulin release, no insulin response to 16 mM glucose, and hyperresponse to 19 mM arginine. Pancreatic insulin stores were only 50% of that in Wistar rats. Perfusion of GK pancreases for 50 or 90 min with buffer containing no glucose partially improved the insulin response to 16 mM glucose and markedly diminished the response to 19 mM arginine, whereas the responses by Wistar pancreases were unchanged. These findings are similar to those reported in rats with non-insulin-dependent diabetes induced by neonatal streptozocin administration and support the concept that chronic elevation in plasma glucose may be responsible, at least in part, for the beta-cell desensitization to glucose in this model. The GK rat seems to be a valuable model for identifying the etiology of beta-cell desensitization to glucose.

Impairment of glycerol phosphate shuttle in islets from rats with diabetes induced by neonatal streptozocin

In islets from adult rats injected with streptozocin during the neonatal period, the oxidative and secretory responses to D-glucose are more severely affected than those evoked by L-leucine. A possible explanation for such a preferential defect was sought by comparing the rate of aerobic glycolysis, taken as the sum of D-[3,4-14C]glucose conversion to labeled CO2, pyruvate, and amino acid, with the total glycolytic flux, as judged from the conversion of D-[5-3H]glucose to 3H2O. A preferential impairment of aerobic relative to total glycolysis was found in islets from diabetic rats incubated at either low or high D-glucose concentration. This coincided in islet mitochondria of diabetic rats with a severe decrease in both the basal (no-Ca2+) generation of 3H2O from L-[2-3H]glycerol-3-phosphate and the Ca2(+)-induced increment in [3H]glycerophosphate detritiation. The mitochondria of diabetic rats were also less efficient than those of control animals in generating 14CO2 from [1-14C]-2-ketoglutarate. The diabetes-induced alteration of 2-ketoglutarate dehydrogenase in islet mitochondria was less marked, however, than that of the FAD-linked glycerophosphate dehydrogenase and was not associated with any change in responsiveness to Ca2+. Sonicated islet mitochondria of diabetic rats displayed normal to slightly elevated glutamate dehydrogenase activity. We propose, therefore, that the preferential impairment of the oxidative and secretory responses of islet cells to D-glucose in this experimental model of diabetes may be at least partly attributable to an altered transfer of reducing equivalents into the mitochondria as mediated by the glycerol phosphate shuttle.

Impairment of the mitochondrial oxidative response to D-glucose in pancreatic islets from adult rats injected with streptozotocin during the neonatal period

Pancreatic islets removed from adult rats injected with streptozotocin during the neonatal period display an impaired secretory response to D-glucose and, to a lesser extent, to L-leucine. Despite normal to elevated hexokinase and glucokinase activities in the islets of these glucose-intolerant animals and despite normal mitochondrial binding of the hexokinase isoenzymes, the metabolic response to a high concentration of D-glucose is severely affected, especially in terms of D-[6-14C]glucose oxidation. Thus, the ratio in D-[6-14C]glucose oxidation/D-[5-3H]glucose utilization is much less markedly increased in response to a rise in hexose concentration and, at a high concentration of D-glucose (16.7 mmol/l), less markedly decreased by the absence of Ca2+ and presence of cycloheximide in diabetic than control rats. This metabolic defect contrasts with (1) a close-to-normal or even increased capacity of the islets of diabetic rats to oxidize D-[6-14C]glucose, [2-14C]pyruvate, L-[U-14C]glutamine and L-[U-14C]leucine at low, non-insulinotropic, concentrations of these substrates; (2) a lesser impairment of the oxidation of L-[U-14C]leucine tested in high concentration (20 mmol/l), the effect of Ca2+ deprivation upon the latter variable being comparable in diabetic and control rats; (3) an unaltered transamination of either [2-14C]pyruvate or L-[U-14C]leucine; and (4) a modest perturbation of glycolysis. The most obvious alteration in glycolysis consists in a lesser increase of the glycolytic flux in response to a rise of D-glucose concentration in diabetic than control rats, this coinciding with an apparent decrease in affinity of glucokinase for the hexose.(ABSTRACT TRUNCATED AT 250 WORDS)

The desensitization of normal B-cells to glucose in vitro is transient and not related to high glucose levels

To examine the postulated phenomenon of glucotoxicity toward the B-cell, islets isolated from normal adult rats were cultured for 1-6 days in RPMI medium at various glucose concentrations. Insulin release and (pro)insulin biosynthesis by these islets were then measured in short term incubations. The 1-day cultured islets (at 9.7 mM glucose) displayed a deficient glucose-stimulated insulin release which was partially restored in the presence of forskolin (5 microM). By contrast they exhibited a consistent insulin release in response to ketoisocaproate (10 mM), 12-O-tetradecanoylphorbol-13-acetate (2 microM), or the combination of Ba2+ (2 mM) and theophylline (1.4 mM) in the absence of extracellular Ca2+. Desensitization of their B-cells was not specific for glucose, since glyceraldehyde (10 mM) or leucine (10 mM) also failed to stimulate insulin release. Desensitization was not related to glucose concentration (5.6, 9.7, or 16.7 mM) in the medium during the 1-day culture period and was restricted to the secretory function, with no impairment of the biosynthesis process. The desensitization to glucose was transient, and high glucose levels (9.7 and 16.7 mM) in the culture medium favored restoration of the subsequent secretory response to the hexose. Under conditions of recovery of B-cell sensitivity to glucose in vitro (5 days at 9.7 mM glucose), the secretory response to acute glucose was in fact significantly enhanced after an additional exposure (1 day) to very high glucose levels (22 or 55 mM). The present results argue against 1) the possibility that islets suffer from some unspecific decreased viability after a 1-day culture period; and 2) the hypothesis that glucose insensitivity in the 1-day cultured islets is primarily caused by a direct deleterious effect of high glucose concentrations on the B-cells. They, rather, reinforce the view that high glucose levels are actually crucial in the preservation of the insulin secretory response to glucose of islets maintained in tissue culture.

The rat models of non-insulin dependent diabetes induced by neonatal streptozotocin

This review is intended to describe the characteristics of the rat models of non-insulin-dependent diabetes induced by neonatal streptozotocin administration (n-STZ models), to sum-up the information so far collected and to highlight the potential of these models for diabetes research. The n-STZ models can now be recognized as adequate tools for the elucidation of the mechanisms leading to: 1) regeneration of the beta cells, 2) the functional "exhaustion" of the beta cells, 3) the emergence of defects in insulin action. They appear well-suited to study the effects of the modulating factors involved in the appearance and/or deterioration of non-insulin-dependent diabetes (obesity, gestation, content of the diet). They are potentially appropriate for investigations in diabetes pharmacotherapy.

Evaluation of the pancreatic B-cell function in the rat after prenatal exposure to streptozotocin or N-nitrosomethylurea

Female rats were treated with calibrated doses of N-nitrosomethylurea (NMU) or streptozotocin (STZ) at the end of their pregnancy (20.5 day post coïtum) and the pancreatic B-cell function in their progeny was studied. We also investigated whether or not these nitrosamines were able to directly damage fetal B-cell function using isolated fetal islets maintained in vitro. STZ (1 mM) was found to be cytotoxic toward B-cell function in fetal islets maintained in vitro: inhibitory effect on both glucose-stimulated insulin biosynthesis and release were observed. NMU (1 mM) exhibited a similar though less toxic effect. So the question was raised as to whether any toxicity against the B cells in the fetus can develop upon exposure of the pregnant mother to these nitrosamines. STZ administration (35 mg/Kg) to pregnant mothers exerted clear-cut effects on the endocrine pancreas of their offspring: their pancreatic insulin stores were decreased by 35% when measured 1 day after birth. This alteration was maintained: in 2 month-old offspring a mild hyperglycemia (9.9 +/- 0.3 vs 8.5 +/- 0.2 mM in control rats, p less than 0.01), a decrease of the pancreatic insulin stores (by 45%) and an impaired insulin secretion in response to glucose and arginine was observed. These defects were only detectable in the male progeny. NMU administration (30 or 50 mg/Kg) to pregnant mothers did not produce in the offspring any significant alteration of their pancreatic insulin stores and plasma glucose levels, what the ages considered (1 day or 2 months).(ABSTRACT TRUNCATED AT 250 WORDS)

Insulin production and glucose metabolism in isolated pancreatic islets of rats with NIDDM

Rats with non-insulin-dependent diabetes mellitus (NIDDM) induced by neonatal injection of streptozocin are known to have a deficient insulin response to glucose. To evaluate to what extent this glucose insensitivity can be attributed to a perturbation of the islet glucose metabolism, we estimated the rates of glucose phosphorylation, glucose utilization, oxygen consumption, and glucose oxidation in islets isolated from normal and NIDDM rats and compared these values with rates of islet insulin biosynthesis and release in vitro. The data confirm that islets from rats with NIDDM display a deficient response to glucose of both insulin biosynthesis and release that is still present after an overnight culture of the islets at 5.5 mM glucose. Furthermore, they show that islets of these rats have 1) normal low- and high-Km glucose-phosphorylating activities and no major alteration of the glucose utilization rate, 2) decreased insulin release in response to glyceraldehyde, 3) decreased rates of basal respiration and glucose oxidation and a markedly reduced stimulation by glucose of both islet oxygen consumption and glucose oxidation, and 4) decreased glucose-stimulated net 45Ca uptake. We conclude that the relative unresponsiveness to glucose of islets from NIDDM rats is associated with, and perhaps due to, a deficient islet glucose metabolism. This defect is not due to gross alterations in the glycolytic pathway but probably reflects alteration in the islet mitochondria function.

Influence of D-glucose upon the respiratory and secretory response of insulin-producing tumor cells to 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid

In insulin-producing cells of the RINm5F line, the nonmetabolized analogue of L-leucine, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid decreases O2 consumption, lowers ATP content, and inhibits insulin release despite stimulation of both NH4 production and 14CO2 output from cells prelabeled with L-[U-14C]glutamine. The metabolic and secretory effects of 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid are opposed to those of D-glucose, which increases respiration, ATP content, and insulin release, while lowering NH4+ production and 14CO2 output from the prelabeled cells. D-Glucose also antagonizes the inhibitory action of 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid upon both respiration and secretion. These findings suggest that, in tumor as in normal islet cells, the regulation of insulin release by exogenous nutrients depends on the availability of endogenous ATP.

32P, 86Rb+ and 45Ca2+ handling by tumoral insulin-secreting cells (RINm5F line)

In perifused tumoral islet cells (RINm5F line), which were prelabelled with either [32P]orthophosphate, 86Rb+ or 45Ca2+, the administration of D-glucose (1.4, 2.8 or 16.7 mM) increased the efflux of 32P, decreased the outflow of 86Rb, increased slightly the efflux of 45Ca from cells perifused in the presence of Ca2+, and decreased modestly the outflow of 45Ca from cells perifused in the absence of Ca2+. D-glucose also stimulated the net uptake of 45Ca2+. When Ba2+ (2 mM) was used, in the absence of Ca2+, instead of D-glucose as an insulin secretagogue, the efflux of 32P was little affected, but the outflow of 45Ca was dramatically increased. These changes are qualitatively similar to those occurring in normal islet cells. Nevertheless, the ionic response to D-glucose appeared, as a rule, less marked in tumoral than normal islet cells. Moreover, the concentration-response relationship was shifted to a lower range of hexose concentrations in the RINm5F cells.

Effect of cytochalasin B on glucose uptake, utilization, oxidation and insulinotropic action in tumoral insulin-producing cells

Cytochalasin B (17-3 microM) virtually abolished 3-O-methyl-D-[U-14C]glucose uptake and D-[5-3H]glucose utilization in tumoral insulin-producing cells of the RINm5F line. This coincided with a marked decrease in D-[U-14C]glucose oxidation and suppression of the stimulant action of D-glucose upon insulin release. Cytochalasin B, however, augmented basal insulin release by the tumoral cells. The RINm5F cells appeared much more sensitive than normal islet cells to cytochalasin B, as judged by the relative magnitude of inhibition in either hexose uptake or utilization. In both cell types, the inhibitory action of cytochalasin B upon glucose metabolism seemed to be competitive, being more marked at low than high glucose concentration. These results are interpreted in support of the view that a decreased efficiency of hexose transport across the plasma membrane represents an essential deficiency of the RINm5F cells.

Metabolic and secretory response of tumoral-insulin producing cells to D-fructose and D-galactose

At variance with normal islet cells, tumoral insulin-producing cells of the RINm5F line were found to display a positive secretory response not solely to D-glucose and D-mannose, but also to D-fructose and D-galactose. All hexoses increased the ATP/ADP ratio, exerted a sparing action upon the oxidation of endogenous nutrients in cells prelabelled with either L-[U-14C]glutamine or [U-14C]palmitate, increased the output of lactic acid and, as judged from data collected in the presence of D-[U-14C]hexoses, underwent oxidation in the RINm5F cells. The secretory response to these four hexoses appeared commensurate with the extent of their metabolic effects.

Opposite effects of D-glucose and a nonmetabolized analogue of L-leucine on respiration and secretion in insulin-producing tumoral cells (RINm5F)

D-Glucose increased the cytosolic NADH/NAD+ ratio (but not the cytosolic NADPH/NADP+ ratio), augmented O2 uptake, raised the ATP/ADP ratio, decreased 86Rb outflow, and stimulated insulin release in tumoral insulin-producing cells of the RINm5F line. L-Leucine and 4-methyl-2-oxopentanoate also stimulated insulin secretion. In the RINm5F cells, as in normal islet cells, the nonmetabolized analogue of L-leucine, 2-aminobicyclo[2,2,1]heptane-2-carboxylic acid (BCH), activated glutamate dehydrogenase, augmented L-[U-14C]glutamine oxidation, and induced a more reduced state of cytosolic redox couples. However, in sharp contrast to either its effect in normal islet cells or that of D-glucose in the tumoral cells, BCH severely decreased O2 uptake, lowered the ATP/ADP ratio, increased 86Rb outflow, and inhibited insulin release in the RINm5F cells. These findings are interpreted to support the concept that the rate of ATP generation represents an essential determinant of the secretory response of insulin-producing cells to nutrient secretagogues.

Hexose metabolism in pancreatic islets. Absence of glucose-6-phosphatase in rat islet cells

Homogenates of either rat or mouse pancreatic islets, pure rat B cells or insulin-producing cells of the RINm5F line catalyzed the hydrolysis of D-glucose-6-phosphate. Relative to protein content, the enzymic activity, which was mainly associated with particulate rather than soluble subcellular material, was much lower in endocrine pancreatic cells than in liver. The rat islet enzyme differed from liver glucose-6-phosphate by its lower affinity for D-glucose-6-phosphate, its lower pH optimum, its greater relative efficiency towards L-glycerol-3-phosphate as distinct from D-glucose-6-phosphate, its restricted lability during exposure to pH 5.0, its inability to act as a glucose-6-phosphate:glucose phosphotransferase, and its insensitivity to inhibition by D-glucose. It is concluded that rat islet cells are virtually devoid of true glucose-6-phosphatase activity.

3-O-methyl-D-glucose transport in tumoral insulin-producing cells

Tumoral insulin-producing cells of the RINm5F line were exposed at different temperatures, and for various lengths of time to increasing concentrations of 3-O-methyl-D-[U-14C]glucose. The uptake of the hexose represented a temperature-sensitive and saturable process, so that no rapid equilibration of hexose concentrations across the plasma membrane was reached, especially at low temperature and/or high concentrations of 3-O-methyl-D-glucose. The uptake of 3-O-methyl-D-[U-14C]glucose was not affected by a prior loading of the cells with the unlabeled hexose and its release from prelabeled cells was observed in the absence of any concentration gradient across the plasma membrane. The uptake of D-[U-14C]glucose and utilization of D-[5-3H]glucose was inhibited by 3-O-methyl-D-glucose, which failed, however, to affect D-[U-14C]glucose oxidation. At variance with the situation found in normal insulin-producing cells, the transport of D-glucose into the tumoral cells may thus play a regulatory role in its metabolism.

D-glucose transport and concentration in tumoral insulin-producing cells

The uptake of D-[U-14C]glucose and D-[5-3H]glucose by tumoral insulin-producing cells of the RINm5F line was measured over 3-30 min of incubation at 7-37 degrees C. The apparent distribution space of the hexose ranged from values as low as the L-[1-14C]glucose space to values 10 times higher than the 3H2O space. Although a major fraction of the radioactivity recovered in the cellular pellet corresponded to the metabolites of D-glucose, the results suggested that the transport of D-glucose into the tumoral cells represents a saturable and temperature-dependent process. When D-glucose was measured by an enzymic procedure in cells incubated for 30 min at 37 degrees C in the presence of 50.0 mM D-glucose, the apparent distribution space of the hexose remained lower than the intracellular water space. These results indicate that the RINm5F cells have lost an essential attribute of the glucose-sensing device in normal insulin-producing cells, namely the ability to ensure the equilibration of D-glucose concentrations across the plasma membrane.

Hexose metabolism in pancreatic islets: compartmentation of hexokinase in islet cells

Hexokinase activity was found in both soluble (cytosolic) and particulate subcellular fractions prepared from rat pancreatic islet homogenates. The bound enzyme was associated with mitochondria rather than secretory granules. Relative to the total hexokinase activity, the amount of bound enzyme was higher in islet homogenates prepared at pH 6.0 (72 +/- 7%) than in islets homogenized at pH 7.4 (38 +/- 1%). The affinity of hexokinase for equilibrated D-glucose was not different in the cytosolic and mitochondrial fractions. In both fractions, hexokinase displayed a greater affinity for alpha- than beta-D-glucose, but a higher maximal velocity with the beta- than alpha-anomer. Glucose 6-phosphate inhibited to a greater extent cytosolic than mitochondrial hexokinase. A high Km glucokinase-like enzymic activity was also present in both subcellular fractions. It is proposed that the ambiguity of hexokinase plays a propitious role in the glucose-sensing function of pancreatic islet cells.

Sensitivity to pentamidine but resistance to streptozotocin of tumoral insulin-producing cells (RINm5F line)

Pancreatic islets removed from normal rats and tumoral insulin-producing cells of the RINm5F line were examined for their respective sensitivity to two distinct beta-cytotoxic agents, namely pentamidine and streptozotocin. After 20 h of culture in the presence of pentamidine (5-500 microM), a dose-related decrease in both insulin output and D-[5-3H] glucose utilization was observed in pancreatic islets. Under identical conditions, pentamidine caused a dose-related destruction of RINm5F cells and, in the remaining cells, also impaired D-[5-3H] glucose utilization. When pancreatic islets were exposed for 60 or 120 min to streptozotocin (3.77 mM), both the utilization of D-[5-3H] glucose and oxidation of D-[U-14C] glucose were impaired. However, under identical conditions, the RINm5F cells appeared resistant to the cytotoxic action of streptozotocin. Since streptozotocin, but not pentamidine, is thought to be transported into islet cells at the intervention of a hexose carrier, these findings support the concept that a low efficiency of this carrier-mediated process represents a fundamental defect of the RINm5F cells.

Impaired uptake of D-glucose by tumoral insulin-producing cells

At variance with the situation found in normal pancreatic islets, no equilibration of extracellular and intracellular D-glucose concentrations occurs in tumoral insulin-producing cells of the RINm5F line. This unexpected behaviour may account, in part at least, for the abnormal kinetics of glucose utilization in the tumoral cells and their poor secretory response to this hexose.

Anomeric specificity of mammalian hexokinase

The anomeric specificity of hexokinase was examined in crude homogenates of rat parotid gland, erythrocytes and pancreatic islets. At 8 degrees C, the alpha/beta ratio in maximal velocity averaged 0.73, 0.66 and 0.75 in the parotid, erythrocytes and pancreatic islets, respectively. Hexokinase displayed a greater affinity for alpha- than beta-D-glucose as judged from three criteria: the Km value, the reaction velocity measured with mixtures of the two anomers and their effect upon the phosphorylation of D-[U-14C] glucose in anomeric equilibrium. The latter procedure yielded an alpha/beta ratio in Km close to 0.51, 0.49 and 0.39 in parotid, erythrocytes and pancreatic islets, respectively. Within the limits of this study, the anomeric specificity of mammalian hexokinase would appear to be a mirror image of that of yeast hexokinase.

Temperature dependency of the anomeric specificity of yeast and bovine hexokinases

The phosphorylation of alpha- and beta-D-glucose by either yeast hexokinase or beef heart hexokinase was measured at both 10 and 30 degrees C. At 30 degrees C, the anomeric specificity of yeast hexokinase represented a mirror image of that of bovine hexokinase, in terms of both maximal velocity and affinity. A decrease in temperature apparently accentuated the anomeric difference in both maximal velocity and affinity of bovine hexokinase. Such a difference consisted in a higher maximal velocity with beta- than alpha-D-glucose, but a greater affinity for the alpha- than beta-anomer. In yeast hexokinase, however, the decrease in temperature suppressed the anomeric difference in maximal velocity and inversed the anomeric difference in affinity. In the case of both enzymes, the fall in temperature decreased more the maximal velocity recorded with alpha-D-glucose than that measured with beta-D-glucose, and severely lowered the Km for alpha-D-glucose, whilst failing to affect significantly the Km for beta-D-glucose. These findings, which allow to reconcile prior apparently conflicting data, reveal that the anomeric behaviour of hexokinase is affected by the ambient temperature. Our data also support the view that hexokinase underwent a phylogenic evolution in terms of its anomeric specificity.

Evidence for normal in vitro Ca2+-stimulated insulin release in rats with non-insulin-dependent diabetes

Non-insulin-dependent diabetes (NIDD) was induced in adult rats following a neonatal injection of streptozotocin. Rats with NIDD exhibited moderate basal hyperglycemia (less than 2g/l) with defective glucose-induced insulin secretion and preserved or enhanced responsiveness to various non-glucose secretagogues. The possible sites of defects in the B cells of these diabetic rats are at present unknown. Extracellular Ca2+ at concentrations exceeding 10 mmol/l causes a dose-related stimulation of insulin release by the perfused normal rat pancreas. This technique provokes an increase of cytosolic Ca2+ accumulation in the B cell sufficient to trigger activation of the microtubular-microfilamentous effector system responsible for the exocytosis of insulin granules. It is therefore possible to compare functional efficiency of this process in the B cells of diabetic rats to that in the normal B cells. Results obtained in response to Ca2+ stimulation over the range 10-13.5 mmol/l provide evidence that the insulin release is stimulated in the pancreas of diabetic rats to the same extent as that in the controls, in both the kinetic and the quantitative aspect of the response. These data are compatible with the idea that in the B cells of rats with NIDD : 1) the effector system of insulin secretion is not grossly altered and 2) the lesion responsible for the loss of glucose-induced insulin secretion occurs before the effector system, involving either the mechanism by which glucose is metabolized as previously proposed by us, or another step yet to be identified.