Pathogenesis of hyperglycemia in genetically obese-hyperglycemic rats, Wistar fatty: presence of hepatic insulin resistance

The present studies were designed to clarify the contribution of the liver to the development of hyperglycemia in Wistar fatty rats. The hepatic activities of insulin-inducible enzymes involved in glycolysis (glucokinase; GK and pyruvate kinase) and lipogenesis (glucose-6-phosphate dehydrogenase), were higher in fatty rats than in lean rats at 4 and 8 weeks of age because of the higher insulin levels in the former. Thereafter, the GK activities of fatty rats decreased slightly in spite of severe hyperinsulinemia, and did not differ from those of lean rats. In addition, fatty rats had higher levels of insulin-suppressible gluconeogenic enzymes, glucose-6-phosphatase (G6Pase) and fructose-1, 6-diphosphatase. These findings indicate that the hepatic enzymes of fatty rats are resistant to insulin. This postulation was supported by the fact that the hepatic enzyme activities of fatty rats showed a lower response to changes in plasma insulin levels produced by fasting and refeeding. The G6Pase/GK ratio, which indicates net glucose handling in the liver, increased in fatty rats and decreased in lean rats with advancing age, suggesting that hepatic glucose production in fatty rats becomes dominant with advancing age. The changes in hepatic glycolytic intermediates supported this suggestion; the glycolytic steps both from glucose to glucose-6-phosphate and from phospho-enolpyruvate to pyruvate in fatty rats were accelerated at 5 weeks of age, but suppressed at 12 weeks of age. These results indicate that insulin resistance in the hepatic enzyme regulation may contribute to the development of hyperglycemia in Wistar fatty rats.     

Hormonal and lipogenic and gluconeogenic enzymatic responses in LA/N-corpulent rats

Genetically obese normotensive rats, LA/N-corpulent (cp), were fed ad libitum diets containing either 54% sucrose or cooked corn starch for 12 weeks. Twenty-four rats were used for the study; half were corpulent (cp/cp) and half were lean (cp/+ or +/+). Fasting levels of plasma insulin, glucose, corticosterone, glucagon and growth hormone, and activities of liver and epididymal fat pad glucose-6-phosphate dehydrogenase (G6PD), malic enzyme (ME), and liver and kidney glucose-6-phosphatase (G6Pase), fructose 1,6-diphosphatase (FDPase), and phosphoenolpyruvate carboxykinase (PEPCK) were measured. A significant phenotype effect was observed in insulin, corticosterone, growth hormone, and liver G6PD, ME, FDPase, and kidney PEPCK, G6Pase, FDPase, and epididymal fat pad G6PD and ME (corpulent greater than lean), and glucagon (lean greater than corpulent). Diet effect (sucrose greater than starch) was significant for plasma glucose, liver ME, and kidney G6Pase. Although not significant at the P less than 0.05 level, insulin, corticosterone, liver G6PD and FDPase and kidney FDPase tended to be higher in sucrose-fed rats. This study suggests that the corpulent rat is more lipogenic and gluconeogenic than the lean, and that the hormones responsible are effective in keeping both the lipogenic and gluconeogenic enzyme activity elevated.     

The glucose-6-phosphatase/glucokinase ratio in the liver of obese-diabetic subjects

The study of G6Pase and GK activities in human liver (needle biopsies) in overnight fasted obese NIDDM patients has shown that, while G6Pase was unchanged, GK was higher (+ 55%, P less than 0.05) than in control subjects. Consequently, the G6Pase/GK ratio (which roughly reflects hepatic glucose production) was significantly reduced (-36%) in the obese diabetic group, due to more GK activity (glucose uptake). This contrasts with the activity in IDDM and nonobese NIDDM patients (where the G6Pase/GK ratio is elevated and normal, respectively) and would suggest that in the obese diabetic subjects, hepatic glucose production is not a major factor contributing to the maintenance of hyperglycemia in the overnight fasting state (leaving peripheral insulin resistance as the major cause of hyperglycemia).     

Defect in glucokinase translocation in Zucker diabetic fatty rats

Hepatic glucose fluxes and intracellular movement of glucokinase (GK) in response to increased plasma glucose and insulin were examined in 10-wk-old, 6-h-fasted, conscious Zucker diabetic fatty (ZDF) rats and lean littermates. Under basal conditions, plasma glucose (mmol/l) and glucose turnover rate (GTR; micromol.kg(-1).min(-1)) were slightly higher in ZDF (8.4 +/- 0.3 and 53 +/- 7, respectively) than in lean rats (6.2 +/- 0.2 and 45 +/- 4, respectively), whereas plasma insulin (pmol/l) was higher in ZDF (1,800 +/- 350) than in lean rats (150 +/- 14). The ratio of hepatic uridine 5'-diphosphate-glucose 3H specific activity to plasma glucose 3H specific activity ([3H]UDP-G/[3H]G; %), total hepatic glucose output (micromol.kg(-1).min(-1)), and hepatic glucose cycling (micromol.kg(-1).min(-1)) were higher in ZDF (35 +/- 5, 87 +/- 16, and 33 +/- 10, respectively) compared with lean rats (18 +/- 3, 56 +/- 6, and 11 +/- 2, respectively). [3H]glucose incorporation into glycogen (micromol glucose/g liver) was similar in lean (1.0 +/- 0.7) and ZDF (1.6 +/- 0.8) rats. GK was predominantly located in the nucleus in both rats. With elevated plasma glucose and insulin, GTR (micromol.kg(-1).min(-1)), [3H]UDP-G/[3H]G (%), and [3H]glucose incorporation into glycogen (micromol glucose/g liver) were markedly higher in lean (191 +/- 22, 62 +/- 3, and 5.0 +/- 1.4, respectively) but similar in ZDF rats (100 +/- 6, 37 +/- 3, and 1.4 +/- 0.4, respectively) compared with basal conditions. GK translocation from the nucleus to the cytoplasm occurred in lean but not in ZDF rats. The unresponsiveness of hepatic glucose flux to the rise in plasma glucose and insulin seen in prediabetic ZDF rats was associated with impaired GK translocation.     

Glucokinase is highly induced and glucose-6-phosphatase poorly repressed in liver of rainbow trout (Oncorhynchus mykiss) by a single meal with glucose

The low dietary starch utilisation by rainbow trout (Oncorhynchus mykiss) may be attributed to a dysfunction of the nutritional regulation of the hepatic glucose/glucose-6-phosphate cycle. The present study was initiated to analyse the regulation of activity and gene expression of hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) by dietary carbohydrates in this species. We found that even a single meal containing 24% of glucose is sufficient to induce the GK expression (mRNA and activity) as in mammals. In contrast, although the inhibitory effect of dietary glucose on G6Pase expression is observed at the molecular level, the G6Pase activity is not significantly inhibited by dietary glucose. Thus, in contrast to the gluconeogenic G6Pase enzyme, a rapid adaptation of the hepatic glycolytic GK enzyme to dietary glucose seems effective in rainbow trout. These results suggest that in carnivorous rainbow trout, the liver is capable to strongly regulate the utilisation of glucose but not the synthesis of glucose.     

Antihyperglycemic effect of aqueous and ethanolic extracts of Gongronema latifolium leaves on glucose and glycogen metabolism in livers of normal and streptozotocin-induced diabetic rats

The present study was designed to investigate the antihyperglycemic effects of aqueous and ethanolic extracts from Gongronema latifolium leaves on glucose and glycogen metabolism in livers of non-diabetic and streptozotocin-induced diabetic rats. To investigate the effects of aqueous or ethanolic leaf extracts of G. latifolium, non-diabetic and STZ diabetic rats were treated twice daily (100 mg/Kg) for two weeks. Diabetic rats showed a significant decrease in the activities of hepatic hexokinase (HK), phosphofructokinase (PFK) and glucose-6-phosphate dehydrogenase (G6PDH) and an increase in glucokinase (GK) activity. The levels of hepatic glycogen and glucose were also increased in diabetic rats. However, there were no significant differences in the activities of glucose-6-phosphatase (G6Pase) in treated and untreated diabetic rats. The ethanolic extract significantly increased the activities of HK (p<0.01), PFK (p<0.001) and G6PDH (p<0.01) in diabetic rats, decreased the activity of GK (p<0.05) and the levels of hepatic glycogen (p<0.01) and both hepatic (p<0.001) and blood glucose (40%). The aqueous extract of G. latifolium was only able to significantly increase the activities of HK and decrease the activities of GK but did not produce any significant change in the hepatic glycogen and both hepatic and blood glucose content of diabetic rats. Our data show that the ethanolic extract from G. latifolium leaves has antihyperglycemic potency, which is thought to be mediated through the activation of HK, PFK, G6PDH and inhibition of GK in the liver. The ethanolic extract is under further investigation to determine the chemical structure of the active compound(s) and its/their mechanism of action.     

Does overfeeding enhance genotype effects on liver ability for lipogenesis and lipid secretion in ducks?

We evaluated the effects of genotype (Muscovy, Pekin and their crossbreed hinny and mule ducks) and feeding levels (overfeeding between 12 and 14 weeks of age vs ad libitum feeding) on liver ability for lipogenesis and lipid secretion in ducks. Samples of liver and blood were collected at 14 weeks of age from 8 birds per group. Plasma levels of insulin was considerably increased in overfed ducks (1.9-fold), stimulating the hepatic activity of the main enzymes involved in lipogenesis from glucose (glucokinase, GK, glucose-6-phosphate dehydrogenase, G6PDH, malic enzyme, ME, acetyl CoA carboxylase, ACX), while cytochrome-c oxidase (COX) activity, indicating overall oxidation ability of energy-yielding substrates, remained unchanged. Plasma levels of triglycerides, phospholipids and total cholesterol were therefore increased (1.9, 3.7, 1.6 and 1.6-fold, respectively). Glycaemia also significantly increased (+8%). Pekin ducks exhibited higher levels of GK and G6PDH activity in the liver than Muscovy ducks, suggesting a greater ability to use glucose consistent with their lower glycaemia. Muscovy ducks had greater ACX activity, suggesting greater ability to synthesise lipids. However, plasma lipid levels were much higher in Pekin ducks than in Muscovy ducks, suggesting a greater ability to export lipids from the liver. Values for the different criteria measured in this study were intermediate or similar in hinny and mule ducks to those of parental species. The high values for GK, G6PDH, ME and ACX activity in hybrid ducks enabled them to produce heavy fatty livers with the same chemical and lipid composition as Muscovy ducks and characterised by high amounts of triglycerides (around 96% of total lipids), and saturated and mono-unsaturated fatty acids.     

Molecular cloning of hepatic glucose-6-phosphatase catalytic subunit from gilthead sea bream (Sparus aurata): response of its mRNA levels and glucokinase expression to refeeding and diet composition

To examine the relationship between structure and function of glucose-6-phosphatase (G6Pase) in fish, we undertook molecular cloning and modulation of G6Pase expression by starvation and refeeding on diets with different nutrient composition in the liver of the carnivorous fish, Sparus aurata. A cDNA encoding the full-length G6Pase catalytic subunit from the liver of S. aurata was isolated. This cDNA encodes a 350-amino acid protein, with low homology to the mammalian G6Pase, although it contains most of the key residues required for catalysis. Based on hydrophobicity and membrane structure prediction, we propose a model containing nine-transmembrane regions for S. aurata G6Pase. Northern blots showed that refeeding after a prolonged starvation rapidly reverses the glucose/glucose-6-phosphate substrate cycle flux in the fish liver through decreased G6Pase expression and strong glucokinase (GK) induction. The effect of refeeding different diets on G6Pase and GK expression, indicated that hepatic intermediary metabolism of fish fed diets with low protein/high carbohydrate diets is impelled towards utilization of dietary carbohydrates, by means of modulation of GK mRNA levels rather than G6Pase expression. These findings challenge the role attributed to dysregulation of G6Pase or GK expression in the low ability of carnivorous fish to metabolise glucose.     

Rearing temperature enhances hepatic glucokinase but not glucose-6-phosphatase activities in European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) juveniles fed with the same level of glucose

The aim of this work was to elucidate if the previous results observed in hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) activities in European sea bass and gilthead sea bream are due to temperature per se or to differences in feed intake at different water temperatures. For that purpose triplicate groups of fish (30 g initial body weight) were kept at 18 degrees C or 25 degrees C during two weeks and fed a fixed daily ration of a glucose-free or 20% glucose diet. At the end of the experimental period, plasma glucose levels in both species were not influenced by water temperature but were higher in fish fed the glucose diet. Higher hepatic GK activity was observed in the two fish species fed the glucose diet than the glucose-free diet. In the glucose fed groups, GK activity was higher at 25 degrees C than at 18 degrees C. Glucose-6-phosphatase activities in both species were not influenced by water temperature. In European sea bass and in contrast to gilthead sea bream it was observed an effect of dietary composition on G6Pase activities with surprising higher activities recorded in fish fed the glucose diet than in fish fed the glucose-free diet. Overall, our data strongly suggest that European sea bass and gilthead sea bream are apparently capable to strongly regulate glucose uptake by the liver but not glucose synthesis, which is even enhanced by dietary glucose in European sea bass. Within limits, increasing water temperature enhances liver GK but not G6Pase activities, suggesting that both species are more able to use dietary carbohydrates at higher rearing temperatures.     

Evidence for the expression of both the hydrolase and translocase components of hepatic glucose-6-phosphatase in murine pancreatic islets

Glucose-6-phosphatase (G6Pase) is a multicomponent enzyme system which regulates the catalysis of glucose-6-phosphate (G6P) to glucose and inorganic phosphate. G6Pase can antagonize glucose phosphorylation, a step prerequisite in the regulation of insulin secretion from pancreatic beta cells, and G6Pase activity is increased in islets isolated from animal models of type II diabetes. Using RT-PCR with hepatic G6Pase catalytic subunit primers, we demonstrate that the sizes of amplified products from ob/ob mouse islets are identical to those from liver cDNA. This was confirmed by PCR-based cloning and sequencing of the hepatic G6Pase catalytic subunit open reading frame from islet cDNA. The expression in islets of the G6P transporter, G6PT1, was also demonstrated, suggesting that all of the identified hepatic G6Pase system genes are expressed in pancreatic islets. Finally, the expression of islet-specific G6Pase-related protein (IGRP) in pancreatic islets was confirmed and its expression in liver was also observed.     

槟榔碱对2型糖尿病大鼠肝脏胰岛素抵抗的作用

目的：探讨槟榔碱对2型糖尿病大鼠肝脏胰岛素抵抗的影响及其机制。方法：采用高果糖饲料饲养Wistar大鼠12周制备2型糖尿病大鼠模型,实验动物随机分为5组（n=8）：对照组、模型组、模型＋不同浓度的槟榔碱（0,0．5,1,5mg／kg）组。4周后通过检测血糖、血脂、胰岛素、RT-PCR检测肝脏组成型雄甾烷受体（CAR）、孕甾烷x受体（PXR）、糖代谢相关基因：葡萄糖-6-磷酸酶（G6Pase）、磷酸烯醇式丙酮酸羧激酶（PEPCK）和炎症相关因子：白细胞介素-6（IL-6）、肿瘤坏死因子α（TNF-α）mRNA表达,Western blot检测大鼠肝内p-AKT和葡萄糖转运体4（GLUT4）蛋白表达。结果：1,5mg／kg槟榔碱显著降低糖尿病大鼠体重、空腹血糖、空腹胰岛素、血脂和糖代谢相关基因及炎症相关因子mRNA水平,提高CAR、PXR mRNA水平及p-AKT、GLUT4蛋白水平。结论：槟榔碱可能通过提高CAR和PXR的表达,导致肝脏糖代谢关键酶PEPCK、G6Pase基因表达或者炎性因子肿瘤坏死因子-α（TNF-α）、白介素-6（n-6）表达降低,改善2型糖尿病大鼠肝脏胰岛素抵抗。     

Hepatic glucokinase and glucose-6-phosphatase responses to dietary glucose and starch in gilthead sea bream (Sparus aurata) juveniles reared at two temperatures

The effects of carbohydrate sources/complexity and rearing temperature on hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) activities and gene expression were studied in gilthead sea bream juveniles. Two isonitrogenous (50% crude protein) and isolipidic (19% crude lipids) diets were formulated to contain 20% waxy maize starch or 20% glucose. Triplicate groups of fish (63.5 g initial body weight) were fed each diet to near satiation during four weeks at 18 degrees C or 25 degrees C. Growth, feed intake, feed efficiency and protein efficiency ratio, were higher at the higher water temperature. At each water temperatures fish growth and feed efficiency were higher with the glucose diet. Plasma glucose levels were not influenced by water temperature but were higher in fish fed the glucose diet. Hepatosomatic index and liver glycogen were higher at the lower water temperature and within each water temperature in fish fed the glucose diet. No effect of water temperature on enzymes activities was observed, except for hexokinase and GK which were higher at 25 degrees C. Hepatic hexokinase and pyruvate kinase activities were not influenced by diet composition, whereas glucose-6-phosphate dehydrogenase activity was higher in fish fed the glucose diet. Higher GK activity was observed in fish fed the glucose diet. GK gene expression was higher at 25 degrees C in fish fed the waxy maize starch diet while in fish fed the glucose diet, no temperature effect on GK gene expression was observed. Hepatic G6Pase activities and gene expression were neither influenced by dietary carbohydrates nor water temperature. Overall, our data suggest that in gilthead sea bream juveniles hepatocytes dietary carbohydrate source and temperature affect more intensively GK, the enzyme responsible for the first step of glucose uptake, than G6Pase the enzyme involved in the last step of glucose hepatic release.     

Hepatic glucokinase and glucose-6-phosphatase responses to dietary glucose and starch in gilthead sea bream (Sparus aurata) juveniles reared at two temperatures.

The effects of carbohydrate sources/complexity and rearing temperature on hepatic glucokinase (GK) and glucose-6-phosphatase (G6Pase) activities and gene expression were studied in gilthead sea bream juveniles. Two isonitrogenous (50% crude protein) and isolipidic (19% crude lipids) diets were formulated to contain 20% waxy maize starch or 20% glucose. Triplicate groups of fish (63.5 g initial body weight) were fed each diet to near satiation during four weeks at 18 degrees C or 25 degrees C. Growth, feed intake, feed efficiency and protein efficiency ratio, were higher at the higher water temperature. At each water temperatures fish growth and feed efficiency were higher with the glucose diet. Plasma glucose levels were not influenced by water temperature but were higher in fish fed the glucose diet. Hepatosomatic index and liver glycogen were higher at the lower water temperature and within each water temperature in fish fed the glucose diet. No effect of water temperature on enzymes activities was observed, except for hexokinase and GK which were higher at 25 degrees C. Hepatic hexokinase and pyruvate kinase activities were not influenced by diet composition, whereas glucose-6-phosphate dehydrogenase activity was higher in fish fed the glucose diet. Higher GK activity was observed in fish fed the glucose diet. GK gene expression was higher at 25 degrees C in fish fed the waxy maize starch diet while in fish fed the glucose diet, no temperature effect on GK gene expression was observed. Hepatic G6Pase activities and gene expression were neither influenced by dietary carbohydrates nor water temperature. Overall, our data suggest that in gilthead sea bream juveniles hepatocytes dietary carbohydrate source and temperature affect more intensively GK, the enzyme responsible for the first step of glucose uptake, than G6Pase the enzyme involved in the last step of glucose hepatic release.     

Dietary induction of hepatic glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, and malic enzyme in lean and obese female Zucker rats

Responses of the hepatic lipogenic enzymes, glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH), and malic enzyme (ME) to starvation refeeding and diet shifting were determined in lean and obese female Zucker rats. Rats were either fed nonpurified diet, starved 48 hr, and then refed nonpurified diet or one of the refined carbohydrate diets containing either glucose, fructose, cornstarch, or sucrose for 72 hr, or shifted from nonpurified diet directly to one of the refined carbohydrate diets for 72 hr. Initial activities were greater in obese than lean rats for all three enzymes studied. Similar to other strains of female rats, lean Zucker rats failed to demonstrate a starve-refeed response when refed nonpurified diet. Obese female littermates showed a statistically significant increase in enzymes when refed a nonpurified diet. Both lean and obese female Zucker rats demonstrated increases in enzyme activities above controls when starved and refed any of the refined carbohydrate diets. The greatest responses were observed when female rats were starved and refed sucrose; activities increased 2.6- to 3.5-fold in lean and 3.0- to 4.3-fold in obese Zuckers. In lean females 50-70% of the starve-refeed response observed with G6PDH and ME can be accounted for by simply shifting from a nonpurified diet to the respective refined carbohydrate diet, whereas in obese females only 33-55% of the increase could be attributed to diet shifting. Plasma testosterone/estrogen ratios were consistently 1.5 times higher in obese than in lean female rats. This phenotypic difference may potentiate the heightened starve-refeed overshoot response observed in obese rats.     

不同糖源及糖水平对大菱鲆糖代谢酶活性的影响

采用34双因素实验设计, 以初始质量为(8.060.08) g的大菱鲆幼鱼(Scophthalmus maximus L.)为对象, 研究在饲料中添加3种糖源(葡萄糖、蔗糖和糊精)及4个水平(0、5%、15%、28%)对大菱鲆肝脏糖酵解关键酶己糖激酶(HK)、葡萄糖激酶(GK)、磷酸果糖激酶(PFK)、丙酮酸激酶(PK)和糖异生关键酶磷酸烯醇式丙酮酸羧激酶(PEPCK)、1, 6-二磷酸果糖酶(FBPase)活性的影响。结果表明: 饲料糖添加量从0升高到15%时, 大菱鲆的糖酵解酶GK和PK活性随饲料葡萄糖或糊精含量的增加而增加; 当饲料中葡萄糖或糊精含量为28%时, GK和PK活性有下降的趋势。3种糖源的4个添加水平对HK和PFK活性均无显著影响(P 0.05)。添加不同水平的葡萄糖对大菱鲆糖异生途径的PEPCK活性无显著影响(P 0.05), 但在饲料中葡萄糖添加量为5%时显著促进了FBPase活性(P 0.05), 当葡萄糖添加量升高为15%或28%时, FBPase活性与对照组无显著差异(P 0.05)。糊精作为饲料糖源时抑制了大菱鲆肝脏FBPase和PEPCK的活性, 而添加不同水平的蔗糖对FBPase和PEPCK活性的影响均不显著(P 0.05)。总的来说, 从大菱鲆幼鱼肝脏糖代谢角度而言, 在饲料中添加15%的葡萄糖或糊精时, 可以有效促进大菱鲆肝脏糖酵解能力; 较添加葡萄糖, 糊精在促进大菱鲆肝脏糖酵解的同时对糖异生存在一定程度的抑制。蔗糖作为饲料糖源时, 仅在添加量为28%时显著促进糖酵解酶GK活性, 糖酵解其他酶活性以及糖异生酶活性均不受蔗糖水平的显著影响。       

Dietary protein level and circadian variation of enzyme activities for glucose metabolism and lipogenesis in male rats (author's transl)]

One hundred and seven Wistar rats, 8 weeks old and weighing 180-200 g, were housed under conditions of controlled temperature (22 plus or minus 2 degrees) and lighting (light on from 07:00 to 19:00). They were divided into 2 groups and fed diets containing either 15 per cent cas-protein for 23 days. Food consumption was recorded every 2 hours for each animal during 48 hours. Four or five rats from each group were killed every 2 hours for 24 hours and the hepatic activities of PK (EC.2.7.1.40),G6P-DH (EC1.1.1.49), ME (EC1.1.1.40), Acetyl-CoA-carbox (EC.6.4.1.2.),PC(EC.6.4.1.1.), PEP-CK(EC.4.1.1.32), G6Pase (EC.3.1.3.9) and GPT (EC.2.6.1.2.) were measured...     

Expression of Protein Kinase C Isoforms in Pancreatic Islets and Liver of Male Goto-Kakizaki Rats,a Model of Type 2 Diabetes

Protein kinase C (PKC) is a family of protein kinases controlling protein phosphorylation and playing important roles in the regulation of metabolism. We have investigated expression levels of PKC isoforms in pancreatic islets and liver of diabetic Goto-Kakizaki (GK) rats with and without insulin treatment to evaluate their association with glucose homeostasis. mRNA and protein expression levels of PKC isoforms were assessed in pancreatic islets and liver of Wistar rats and GK rats with or without insulin treatment. PKCα and PKCζ mRNA expressions were down-regulated in islets of GK compared with Wistar rats. PKCα and phosphorylated PKCα (p-PKCα) protein expressions were decreased in islets of GK compared with insulin-treated GK and Wistar rats. PKCζ protein expression in islets was reduced in GK and insulin-treated GK compared with Wistar rats, but p-PKCζ was decreased only in GK rats. Islet PKCε mRNA and protein expressions were lower in GK compared with insulin-treated GK and Wistar rats. In liver, PKCδ and PKCζ mRNA expressions were decreased in both GK and insulin-treated GK compared with Wistar rats. Hepatic PKCζ protein expression was diminished in both GK rats with and without insulin treatment compared with Wistar rats. Hepatic PKCε mRNA expression was down-regulated in insulin-treated GK compared with GK and Wistar rats. PKCα, PKCε, and p-PKCζ expressions were secondary to hyperglycaemia in GK rat islets. Hepatic PKCδ and PKCζ mRNA expressions were primarily linked to hyperglycaemia. Additionally, hepatic PKCε mRNA expression could be under control of insulin.     

Follow-up of GK rats during prediabetes highlights increased insulin action and fat deposition despite low insulin secretion

The adult Goto-Kakizaki (GK) rat is characterized by impaired glucose-induced insulin secretion in vivo and in vitro, decreased beta-cell mass, decreased insulin sensitivity in the liver, and moderate insulin resistance in muscles and adipose tissue. GK rats do not exhibit basal hyperglycemia during the first 3 wk after birth and therefore could be considered prediabetic during this period. Our aim was to identify the initial pathophysiological changes occurring during the prediabetes period in this model of type 2 diabetes (T2DM). To address this, we investigated beta-cell function, insulin sensitivity, and body composition in normoglycemic prediabetic GK rats. Our results revealed that the in vivo secretory response of GK beta-cells to glucose is markedly reduced and the whole body insulin sensitivity is increased in the prediabetic GK rats in vivo. Moreover, the body composition of suckling GK rats is altered compared with age-matched Wistar rats, with an increase of the number of adipocytes before weaning despite a decreased body weight and lean mass in the GK rats. None of these changes appeared to be due to the postnatal nutritional environment of GK pups as demonstrated by cross-fostering GK pups with nondiabetic Wistar dams. In conclusion, in the GK model of T2DM, beta-cell dysfunction associated with increased insulin sensitivity and the alteration of body composition are proximal events that might contribute to the establishment of overt diabetes in adult GK rats.     

Impaired glucose-stimulated insulin secretion in the GK rat is associated with abnormalities in islet nitric oxide production

We investigated implications of nitric oxide (NO) derived from islet neuronal constitutive NO synthase (ncNOS) and inducible NOS (iNOS) on insulin secretory mechanisms in the mildly diabetic GK rat. Islets from GK rats and Wistar controls were analysed for ncNOS and iNOS by HPLC, immunoblotting and immunocytochemistry in relation to insulin secretion stimulated by glucose or l-arginine in vitro and in vivo. No obvious difference in ncNOS fluorescence in GK vs control islets was seen but freshly isolated GK islets displayed a marked iNOS expression and activity. After incubation at low glucose GK islets showed an abnormal increase in both iNOS and ncNOS activities. At high glucose the impaired glucose-stimulated insulin release was associated with an increased iNOS expression and activity and NOS inhibition dose-dependently amplified insulin secretion in both GK and control islets. This effect by NOS inhibition was also evident in depolarized islets at low glucose, where forskolin had a further amplifying effect in GK but not in control islets. NOS inhibition increased basal insulin release in perfused GK pancreata and amplified insulin release after glucose stimulation in both GK and control pancreata, almost abrogating the nadir separating first and second phase in controls. A defective insulin response to l-arginine was seen in GK rats in vitro and in vivo, being partially restored by NOS inhibition. The results suggest that increased islet NOS activities might contribute to the defective insulin response to glucose and l-arginine in the GK rat. Excessive iNOS expression and activity might be deleterious for the beta-cells over time.