Is exercise training effective in preventing diabetes mellitus in the Otsuka-Long-Evans-Tokushima fatty rat, a model of spontaneous non-insulin-dependent diabetes mellitus?

Investigation of the effects of swimming exercises in rats given acrylamide

BACKGROUND

Acrylamide is a toxic substance used in industrial and laboratory processes. Acrylamide exposure has a toxic effect on many systems. Protective mechanisms should be developed against the effects caused by acrylamide.

OBJECTIVE

In our study, we investigated whether exercise has a protective effect against the changes that acrylamide will cause in pancreas.

METHODS

32 adult Sprague-Dawley male rats were used. Control group was given only saline. Exercise group was applied swimming exercise for 1hour daily for 4 weeks. Acrylamide group was given 50mg/kg acrylamide by gavage for 4 weeks. Acrylamide+exercise group was applied 50mg/kg acrylamide for 4 weeks and swimming exercise for 1hour daily. After the experiment, fasting blood glucose and oral glucose tolerance test measurements were performed. Then, blood and pancreas samples were taken.

RESULTS

Acrylamide exposure caused an increase in fasting blood glucose and oral glucose tolerance, a decrease in insulin levels and oxidative stress in acrylamide group. In exercise group, these values were similar to control group and no significant change was observed in acrylamide+exercise group. While there was an increase in the number of alpha cells in acrylamide group compared to the other groups, here was a decrease in the number of beta cells compared to control group.

CONCLUSION

We can say that acrylamide causes changes in the islets of Langerhans by affecting alpha and beta cell numbers. The protective effect of exercise on beta and alpha cell mass was not statistically significant in the acrylamide+exercise group. When the results were examined, the decrease in oxidative stress and the higher number of beta and alpha cells in the acrylamide+exercise group compared to the acrylamide group suggested that 4 weeks of swimming exercise may have an effect on acrylamide exposure.

Long-term exercise preserves pancreatic islet structure and β-cell mass through attenuation of islet inflammation and fibrosis

Islet fibrosis is associated with the disruption of islet structure and contributes to β-cell dysfunction, playing an essential role in the pathogenesis of type 2 diabetes. Physical exercise has been shown to attenuate fibrosis in various organs; however, the effect of exercise on islet fibrosis has not been defined. Male Sprague-Dawley rats were divided into four groups: normal diet sedentary [N-Sed], normal diet + exercise [N-Ex], high-fat diet sedentary [H-Sed], and high-fat diet + exercise [H-Ex]. After 60 weeks of exercise, 4452 islets from Masson-stained slides were analyzed. Exercise led to a 68% and 45% reduction in islet fibrosis in the normal and high-fat diet groups and was correlated with a lower serum blood glucose. Fibrotic islets were characterized by irregular shapes and substantial loss of β-cell mass, which were significantly reduced in the exercise groups. Remarkably, the islets from exercised rats at week 60 were morphologically comparable to those of sedentary rats at 26 weeks. In addition, the protein and RNA levels of collagen and fibronectin, and the protein levels of hydroxyproline in the islets were also attenuated by exercise. This was accompanied by a significant reduction in inflammatory markers in the circulation Interleukin-1 beta (IL-1β)] and pancreas [IL-1β, Tumor Necrosis Factor-alpha, Transforming Growth Factor-β, and Phosphorylated Nuclear Factor Kappa-B p65 subunit], lower macrophage infiltration, and stellate cell activation in the islets of exercised rats. In conclusion, we have demonstrated that long-term exercise preserves pancreatic islet structure and β-cell mass through anti-inflammatory and anti-fibrotic actions, suggesting additional rationales for the success of exercise training in the prevention and treatment of type 2 diabetes that should be further explored.

Long-term wheel-running prevents reduction of grip strength in type 2 diabetic rats

Diabetic skeletal muscles show reduced contractile force and increased fatigability. Hands are a target for several diabetes-induced complications. Therefore, reduced handgrip strength often occurs as a consequence of diabetes. The aim of this study was to examine whether long-term exercise can prevent reduction of grip strength in type 2 diabetes mellitus (T2DM) model OLETF rats, and to explore the mechanisms underlying diabetes-induced grip strength reduction. Ten 5-week-old OLETF rats were used as experimental animals, and five non-diabetic LETO rats as controls of OLETF rats. Half OLETF rats performed daily voluntary wheel-running for 17 months (OLETF + EXE), and the rest of OLETF and LETO rats were sedentary. Grip strength was higher in OLETF + EXE and LETO groups than in OLETF group. OLETF group with hyperglycemia showed an increase in HbA1c, serum TNF-α, and muscle SERCA activity, but a decrease in circulating insulin. Each fiber area, total fiber area, and % total fiber area in type IIb fibers of extensor digitorum longus muscles were larger in OLETF + EXE and LETO groups than in OLETF group. There was a positive correlation between grip strength and the above three parameters concerning type IIb fiber area. Therefore, type IIb fiber atrophy may be the major direct cause of grip strength reduction in OLETF group, although there seems multiple etiological mechanisms. Long-term wheel-running may have blocked the diabetes-induced reduction of grip strength by preventing type IIb fiber atrophy. Regular exercise may be a potent modality for preventing not only the progression of diabetes but muscle dysfunction in T2DM patients.

Beta cell dysfunction in diabetes: the islet microenvironment as an unusual suspect

Cells in different tissues, including endocrine cells in the pancreas, live in complex microenvironments that are rich in cellular and acellular components. Intricate interactions with their microenvironment dictate most cellular properties, such as their function, structure and size, and maintain tissue homeostasis. Pancreatic islets are populated by endocrine, vascular and immune cells that are immersed in the extracellular matrix. While the intrinsic properties of beta cells have been vastly investigated, our understanding of their interactions with their surroundings has only recently begun to unveil. Here, we review current research on the interplay between the islet cellular and acellular components, and the role these components play in beta cell physiology and pathophysiology. Although beta cell failure is a key pathomechanism in diabetes, its causes are far from being fully elucidated. We, thus, propose deleterious alterations of the islet niche as potential underlying mechanisms contributing to beta cell failure. In sum, this review emphasises that the function of the pancreatic islet depends on all of its components. Graphical abstract.

Islet pericytes convert into profibrotic myofibroblasts in a mouse model of islet vascular fibrosis

AIMS/HYPOTHESIS

Islet vascular fibrosis may play an important role in the progression of type 2 diabetes, but there are no mouse models allowing detailed mechanistic studies to understand how a dysfunctional islet microvasculature contributes to diabetes pathogenesis. Here we report that the transgenic AktTg mouse, unlike other mouse strains, shows an increased deposition of extracellular matrix (ECM) proteins in perivascular regions, allowing us to study the cellular mechanisms that lead to islet vascular fibrosis.

METHODS

Using immunohistochemistry, we labelled the islet microvasculature and ECM in pancreas sections of AktTg mice and human donors and performed lineage tracing to follow the fate of islet pericytes. We compared islet microvascular responses in living pancreas slices from wild-type and AktTg mice.

RESULTS

We found that vascular pericytes proliferate extensively, convert into profibrotic myofibroblasts and substantially contribute to vascular fibrosis in the AktTg mouse model. The increased deposition of collagen I, fibronectin and periostin within the islet is associated with diminished islet perfusion as well as impaired capillary responses to noradrenaline (norepinephrine) and to high glucose in living pancreas slices.

CONCLUSIONS/INTERPRETATION

Our study thus illustrates how the AktTg mouse serves to elucidate a cellular mechanism in the development of islet vascular fibrosis, namely a change in pericyte phenotype that leads to vascular dysfunction. Because beta cells in the AktTg mouse are more numerous and larger, and secrete more insulin, in future studies we will test the role beta cell secretory products play in determining the phenotype of pericytes and other cells residing in the islet microenvironment under physiological and pathophysiological conditions. Graphical abstract.

Differential regulation of thyrotropin-releasing hormone mRNA expression in the paraventricular nucleus and dorsomedial hypothalamus in OLETF rats

Thyrotropin-releasing hormone (TRH) plays an important role in the regulation of energy balance. While the regulation of TRH in the paraventricular nucleus (PVN) in response to changes of energy balance has been well studied, how TRH is regulated in the dorsomedial hypothalamus (DMH) in maintaining energy homeostasis remains unclear. Here, we assessed the effects of food restriction and exercise on hypothalamic Trh expression using Otsuka Long-Evens Tokushima Fatty (OLETF) rats. Sedentary ad lib fed OLETF rats (OLETF-SED) became hyperphagic and obese. These alterations were prevented in OLETF rats with running wheel access (OLETF-RW) or food restriction in which their food was pair-fed (OLETF-PF) to the intake of lean control rats (LETO-SED). Evaluation of hypothalamic gene expression revealed that Trh mRNA expression was increased in the PVN of OLETF-SED rats and normalized in OLETF-RW and OLETF-PF rats compared to LETO-SED rats. In contrast, the expression of Trh in the DMH was decreased in OLETF-SED rats relative to LETO-SED rats. This alteration was reversed in OLETF-RW rats as seen in LETO-SED rats, but food restriction resulted in a significant increase in DMH Trh expression in OLETF-PF rats compared to LETO-SED rats. Strikingly, while Trh mRNA expression was decreased in the PVN of intact rats in response to acute food deprivation, food deprivation resulted in increased expression of Trh in the DMH. Together, these results demonstrate the differential regulation of Trh expression in the PVN and DMH in OLETF rats and suggest that DMH TRH also contributes to hypothalamic regulation of energy balance.

The effects of scheduled running wheel access on binge-like eating behavior and its consequences

Binge eating disorder (BED) is an eating disorder involving repeated, intermittent over consumption of food in brief periods of time, usually with no compensatory behaviors. There are few successful treatments and the underlying neural mechanisms remain unclear. In the current study, we hypothesized that voluntary running wheel (RW) activity could reduce binge-like eating behavior in a rat model. Rats were given intermittent (3 times/wk) limited (1hr) access to a high-fat food (Crisco), in addition to continuously available chow. Crisco was available every Mon, Wed, and Fri for 1hr before dark onset. Rats were divided into 2 groups: those with RW access during the first half of the experiment and sedentary during the second half (RW-SED) and those that were sedentary during the first half of the experiment and had RW access during the second half (SED-RW). Crisco intake was significantly less in both groups during the period of time with a RW present. Within the bingeing RW-SED rats, the gene expression of the orexigenic neuropeptides AgRP and NPY were similar to a non-bingeing sedentary control (CON) group, while the expression of the anorexigenic neuropeptide POMC was significantly increased relative to the SED-RW and CON groups. Despite elevated POMC, the rats continued to binge. Additionally, within both groups, the gene expression of the D2R and Oprm1 in the NAc and the VTA were altered suggesting that the reward system was stimulated by both the bingeing behavior and the running wheel activity. Overall, access to a RW and the resulting activity significantly reduced binge-like behavior as well as modulated the effects of binging on brain appetite and reward systems.

Changes in lipid metabolism and capillary density of the skeletal muscle following low-intensity exercise training in a rat model of obesity with hyperinsulinemia

Although exercise is effective in improving obesity and hyperinsulinemia, the exact influence of exercise on the capillary density of skeletal muscles remains unknown. The aim of this study was to investigate the effects of low-intensity exercise training on metabolism in obesity with hyperinsulinemia, focusing specifically on the capillary density within the skeletal muscle. Otsuka Long-Evans Tokushima fatty (OLETF) rats were used as animal models of obesity with hyperinsulinemia, whereas Long-Evans Tokushima Otsuka (LETO) rats served as controls (no obesity, no hyperinsulinemia). The animals were randomly assigned to either non-exercise or exercise groups (treadmill running for 60 min/day, for 4 weeks). The exercise groups were further divided into subgroups according to training mode: single bout (60 min, daily) vs. multiple bout (three bouts of 20 min, daily). Fasting insulin levels were significantly higher in OLETF than in LETO rats. Among OLETF rats, there were no significant differences in fasting glucose levels between the exercise and the non-exercise groups, but the fasting insulin levels were significantly lower in the exercise group. Body weight and triacylglycerol levels in the liver were significantly higher in OLETF than in LETO rats; however, among OLETF rats, these levels were significantly lower in the exercise than in the non-exercise group. The capillary-to-fiber ratio of the soleus muscle was significantly higher in OLETF than in LETO rats; however, among OLETF rats, the ratio was lower in the exercise group than in the non-exercise group. No significant differences in any of the studied parameters were noted between the single-bout and multiple-bout exercise training modes among either OLETF or LETO rats. These results suggest that low-intensity exercise training improves insulin sensitivity and fatty liver. Additionally, the fact that attenuation of excessive capillarization in the skeletal muscle of OLETF rats was accompanied by improvement in increased body weight.

Effects of voluntary running exercise on bone histology in type 2 diabetic rats

The incidence of obesity in children and adolescents, which may lead to type 2 diabetes, is increasing. Exercise is recommended to prevent and improve diabetes. However, little is known about the bone marrow environment at the onset of diabetes in the young, and it is unclear whether exercise training is useful for maintaining bone homeostasis, such as mechanical and histological properties. Thus, this study clarified the histological properties of bone and whether exercise contributes to maintaining bone homeostasis at the onset of type 2 diabetes in rats. Four-week-old male Otsuka Long-Evans Tokushima Fatty (OLETF; n = 21) rats as a diabetic model and Long-Evans Tokushima Otsuka (LETO; n = 18) rats as a control were assigned randomly to four groups: the OLETF sedentary group (O-Sed; n = 11), OLETF exercise group (O-Ex; n = 10), LETO sedentary group (L-Sed; n = 9), and LETO exercise group (L-Ex; n = 9). All rats in the exercise group were allowed free access to a steel running wheel for 20 weeks (5-25 weeks of age). In the glucose tolerance test, blood glucose level was higher in the O-Sed group than that in the L-Sed and L-Ex groups, and was markedly suppressed by the voluntary running exercise of O-Ex rats. The energy to fracture and the two-dimensional bone volume at 25 weeks of age did not differ significantly among the groups, though the maximum breaking force and stiffness were lower in OLETF rats. However, bone marrow fat volume was greater in O-Sed than that in L-Sed and L-Ex rats, and was markedly suppressed by wheel running in the O-Ex rats. Our results indicate that exercise has beneficial effects not only for preventing diabetes but also on normal bone remodeling at an early age.

Royal jelly improves hyperglycemia in obese/diabetic KK-Ay mice

The study examined whether royal jelly (RJ) can prevent obesity and ameliorate hyperglycemia in type 2 diabetes. This study utilized obese/diabetic KK-Ay mice. RJ (10 mg/kg) was administered by oral gavage. Body weight, plasma glucose and insulin levels were measured. mRNA and protein levels were determined using quantitative reverse transcription polymerase chain reaction and western blotting, respectively. Four weeks of RJ administration improved hyperglycemia and partially suppressed body weight gain, although the latter effect did not reach statistical significance. In addition, RJ administration did not improve insulin resistance. RJ administration suppressed the mRNA expression of glucose-6-phosphatase (G6Pase), a key enzyme of gluconeogenesis, in the liver. Simultaneously, RJ administration induced adiponectin (AdipoQ) expression in abdominal fat, adiponectin receptor-1 (AdipoR1) expression in the liver and phosphorylated AMP-activated protein kinase (pAMPK) expression, which suppressed G6Pase levels in the livers of KK-Ay mice. pAMPK levels were also increased in skeletal muscle, but glucose transporter-4 (Glut4) translocation was not increased in the RJ supplementation group. The improvement in hyperglycemia due to long-term RJ administration may be because of the suppression of G6Pase expression through the upregulation of AdipoQ and AdipoR1 mRNA and pAMPK protein expressions.

Voluntary Exercise Can Ameliorate Insulin Resistance by Reducing iNOS-Mediated S-Nitrosylation of Akt in the Liver in Obese Rats

Voluntary exercise can ameliorate insulin resistance. The underlying mechanism, however, remains to be elucidated. We previously demonstrated that inducible nitric oxide synthase (iNOS) in the liver plays an important role in hepatic insulin resistance in the setting of obesity. In this study, we tried to verify our hypothesis that voluntary exercise improves insulin resistance by reducing the expression of iNOS and subsequent S-nitrosylation of key molecules of glucose metabolism in the liver. Twenty-one Otsuka Long-Evans Tokushima Fatty (OLETF) rats, a model of type 2 diabetes mellitus, and 18 non-diabetic control Long-Evans Tokushima Otsuka (LETO) rats were randomly assigned to a sedentary group or exercise group subjected to voluntary wheel running for 20 weeks. The voluntary exercise significantly reduced the fasting blood glucose and HOMA-IR in the OLETF rats. In addition, the exercise decreased the amount of iNOS mRNA in the liver in the OLETF rats. Moreover, exercise reduced the levels of S-nitrosylated Akt in the liver, which were increased in the OLETF rats, to those observed in the LETO rats. These findings support our hypothesis that voluntary exercise improves insulin resistance, at least partly, by suppressing the iNOS expression and subsequent S-nitrosylation of Akt, a key molecule of the signal transduction pathways in glucose metabolism in the liver.

Swimming training prevents metabolic imprinting induced by hypernutrition during lactation

BACKGROUND & AIMS

Reduction in litter size during lactation induces hypernutrition of the offspring culminating with altered metabolic programming during adult life. Overnourished rats present alterations in the endocrine pancreas and major predisposition to the development of type 2 diabetes. Our study evaluated the impact of swimming training on insulin secretion control in overnourished rats.

METHODS

At postnatal day 3 male rat pup litters were redistributed randomly into Small Litters (SL, 3 pups) or Normal Litters (NL, 9 pups) to induce early overfeeding during lactation. Both groups were subjected to swimming training (3 times/week/30 min) post-weaning (21 days) for 72 days. At 92 days of life pancreatic islets were isolated using collagenase technique and incubated with glucose in the presence or absence of acetylcholine (Ach, 0.1-1000 μM) or glucagon-like peptide 1 (GLP1, 10 nM). Adipose tissue depots (white and brown) and endocrine pancreas samples were examined by histological analysis. Food intake and body weight were measured. Blood biochemical parameters were also evaluated.

RESULTS

Swimming training prevented metabolic program alteration by hypernutrition during lactation. Exercise reduced obesity and hyperglycemia in overnourished rats. Pancreatic islets isolated from overnourished rats showed a reduction in glucose-induced insulin secretion and cholinergic responses while the insulinotropic action of GLP1 was increased. Physical training effectively restored glucose-induced insulin secretion and GLP1-stimulated action in pancreatic islets from overnourished rats. However, swimming training did not correct the weak cholinergic response in pancreatic islets isolated from overnourished rats.

CONCLUSIONS

Swimming training avoids obesity development, corrects glucose-induced insulin secretion, as well as, GLP1 insulinotropic response in overnourished rats.

Lack of exercise is a major cause of chronic diseases

Chronic diseases are major killers in the modern era. Physical inactivity is a primary cause of most chronic diseases. The initial third of the article considers: activity and prevention definitions; historical evidence showing physical inactivity is detrimental to health and normal organ functional capacities; cause versus treatment; physical activity and inactivity mechanisms differ; gene-environment interaction (including aerobic training adaptations, personalized medicine, and co-twin physical activity); and specificity of adaptations to type of training. Next, physical activity/exercise is examined as primary prevention against 35 chronic conditions [accelerated biological aging/premature death, low cardiorespiratory fitness (VO2max), sarcopenia, metabolic syndrome, obesity, insulin resistance, prediabetes, type 2 diabetes, nonalcoholic fatty liver disease, coronary heart disease, peripheral artery disease, hypertension, stroke, congestive heart failure, endothelial dysfunction, arterial dyslipidemia, hemostasis, deep vein thrombosis, cognitive dysfunction, depression and anxiety, osteoporosis, osteoarthritis, balance, bone fracture/falls, rheumatoid arthritis, colon cancer, breast cancer, endometrial cancer, gestational diabetes, pre-eclampsia, polycystic ovary syndrome, erectile dysfunction, pain, diverticulitis, constipation, and gallbladder diseases]. The article ends with consideration of deterioration of risk factors in longer-term sedentary groups; clinical consequences of inactive childhood/adolescence; and public policy. In summary, the body rapidly maladapts to insufficient physical activity, and if continued, results in substantial decreases in both total and quality years of life. Taken together, conclusive evidence exists that physical inactivity is one important cause of most chronic diseases. In addition, physical activity primarily prevents, or delays, chronic diseases, implying that chronic disease need not be an inevitable outcome during life.

Liraglutide prevents diabetes progression in prediabetic OLETF rats

One of human GLP-1 analogues, liraglutide has been approved as adjuvant therapy to oral medication in T2DM. It was also shown to prevent diabetes in obese subjects and rats. However, it is unknown whether liraglutide can effectively mitigate the effects of prediabetes. We therefore investigate this by treating 12-weeks old Otsuka-Long-Evans-Tokushima fatty (OLETF) rats with liraglutide 50, 100, and 200 μg/kg, respectively twice a day for 12 weeks. Eight Long-Evans-Tokushima-Otsuka (LETO) rats with saline injection served as normal controls. Body weight, food intake, lipid profiles, inflammatory markers (fibrinogen, Hs-CRP, IL-6, TNFα, and PAI-1), glycemic metabolism and insulin sensitivity, and apoptotic factors (Bcl-2 and Bax) expression were monitored. We found that 12-week old OLETF rats had significantly increased body weight, food intake, serum levels of lipid profiles, inflammatory markers, and insulin compared to LETO rats. FPG level was significantly increased but still lower than 7mmol/L without impaired glucose tolerance (IGT). After 12 weeks, vehicle-treated OLETF rats had further deterioration in IFG, IGT, insulin resistance, lipid profiles, and inflammatory state. Pancreatic islets were hypertrophic with distorted structure, scarring, and inflammatory cell infiltration. However, in the three liraglutide-treated groups, IFG, IGT, the increased lipid profiles and inflammatory markers were reversed. Insulin resistance was similar to the level before the treatment. Moreover, liraglutide restored the islet structure, up-regulated Bcl-2 expression and down-regulated Bax expression. It indicated that liraglutide could suppress diabetes onset in OLETF rats with prediabetes, probably by reserving β cell function via regulating apoptotic factors as well as ameliorating lipid metabolism and inflammatory reactions.

Advances in murine models of diabetic nephropathy

Diabetic nephropathy (DN) is one of the microvascular complications of both type 1 and type 2 diabetes, which is also associated with a poor life expectancy of diabetic patients. However, the pathogenesis of DN is still unclear. Thus, it is of great use to establish appropriate animal models of DN for doing research on pathogenesis and developing novel therapeutic strategies. Although a large number of murine models of DN including artificially induced, spontaneous, and genetically engineered (knockout and transgenic) animal models have been developed, none of them develops renal changes sufficiently reflecting those seen in humans. Here we review the identified murine models of DN from the aspects of genetic background, type of diabetes, method of induction, gene deficiency, animal age and gender, kidney histopathology, and phenotypic alterations in the hope of enhancing our comprehension of genetic susceptibility and molecular mechanisms responsible for this disease and providing new clues as to how to choose appropriate animal models of DN.

Genetic dissection of complex genetic factor involved in NIDDM of OLETF rat

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat is an animal model for obese-type, noninsulin-dependent diabetes mellitus (NIDDM) in humans. NIDDM in this rat model was shown to be regulated by multiple genes. We have identified 14 quantitative trait loci (QTLs) responsible for NIDDM (Nidd1-14/of) on chromosomes 1, 5, 7, 8, 9, 11, 12, 14, 16, and 17 by a whole genome search in 160 F2 progenies obtained by mating the OLETF and the F344 rats. Among these loci, two QTLs, Nidd1 and 2/of, were declared significant loci at a genome-wide level. Nidd3, 8, 9, and 13/of exhibited heterosis: heterozygotes showing significantly higher glucose levels than OLETF or F344 homozygotes. We also found evidence for interaction (epistasis) between Nidd1/of and Nidd2/of, between Nidd1/of and Nidd10/of, between Nidd2/of and Nidd8/of, and between Nidd2/of and Nidd14/of. Furthermore, Nidd6 and 11/of showed linkage with body weight, and Nidd1, 2, 8, 9, 10, and 12/of had an interaction with body weight. These indicated that NIDDM in the OLETF would have a higher degree of genetic complexity. We suggest several interesting candidate genes located in rat genomic regions for Nidd1-14/of or the syntenic regions in human genome.

Effect of Voluntary Exercise on Genetically ObeseCpefat/fatMice: Quantitative Proteomics of Serum

OBJECTIVE

To compare the effect of voluntary exercise on body weight, food consumption, and levels of serum proteins between wild-type and carboxypeptidase E-deficient (Cpefat/fat) mice.

RESEARCH METHODS AND PROCEDURES

Study 1 consisted of three groups of female mice: Cpefat/fat mice with continuous access to exercise wheels for 3 weeks (n = 4); wild-type C57BKS mice with access to exercise wheels for 3 weeks (n = 4); and sedentary Cpefat/fat mice (n = 3). Activity, body weight, and food consumption were monitored for this period and a subsequent 9-week period without exercise wheels. Study 2 consisted of four groups of male mice (n = 6 to 7 each): Cpefat/fat mice with exercise wheels, wild-type mice with exercise wheels, and Cpefat/fat and wild-type mice without exercise wheels. Body weight and food consumption were measured over 4 weeks. Sera were collected, and the protein profile was determined by 2-dimensional gel electrophoresis and mass spectrometry.

RESULTS

Cpefat/fat mice were moderately hyperphagic but lost weight during the initial exercise period because of greater energy expenditure. The effect of exercise was temporary, and the mice gained weight after the second week. Several serum proteins were found to be altered by exercise: haptoglobin was decreased by exercise in Cpefat/fat mice, and several kallikreins were increased by exercise in wild-type mice.

DISCUSSION

The access to exercise wheels provided an initial weight loss in Cpefat/fat mice, but this effect was offset by elevated food consumption. The serum proteomics results indicated that Cpefat/fat and wild-type mice differed in their response to exercise.

Glycosphingolipid modification: structural diversity, functional and mechanistic integration of diabetes

Glycosphingolipids (GSLs) are present in all mammalian cell plasma membranes and intracellular membrane structures. They are especially concentrated in plasma membrane lipid domains that are specialized for cell signaling. Plasma membranes have typical structures called rafts and caveola domain structures, with large amounts of sphingolipids, cholesterol, and sphingomyelin. GSLs are usually observed in many organs ubiquitously. However, GSLs, including over 400 derivatives, participate in diverse cellular functions. Several studies indicate that GSLs might have an effect on signal transduction related to insulin receptors and epidermal growth factor receptors. GSLs may modulate immune responses by transmitting signals from the exterior to the interior of the cell. Guillain-Barré syndrome is one of the autoimmune disorders characterized by symmetrical weakness in the muscles of the legs. The targets of the immune response are thought to be gangliosides, which are one group of GSLs. Other GSLs may serve as second messengers in several signaling pathways that are important to cell survival or programmed cell death. In the search for clear evidence that GSLs may play critical roles in various biological functions, many researchers have made genetically engineered mice. Before the era of gene manipulation, spontaneous animal models or chemical-induced disease models were used.

Changes in skeletal muscle mitochondria in response to the development of type 2 diabetes or prevention by daily wheel running in hyperphagic OLETF rats

The temporal changes in skeletal muscle mitochondrial content and lipid metabolism that precede type 2 diabetes are largely unknown. Here we examined skeletal muscle mitochondrial fatty acid oxidation (MitoFAOX) and markers of mitochondrial gene expression and protein content in sedentary 20- and 40-wk-old hyperphagic, obese Otsuka Long-Evans Tokushima fatty (OLETF-SED) rats. Changes in OLETF-SED rats were compared with two groups of rats who maintained insulin sensitivity: age-matched OLETF rats given access to voluntary running wheels (OLETF-EX) and sedentary, nonobese Long-Evans Tokushima Otsuka (LETO-SED) rats. As expected, glucose tolerance tests revealed insulin resistance at 20 wk that progressed to type 2 diabetes at 40 wk in the OLETF-SED, whereas both the OLETF-EX and LETO-SED maintained whole body insulin sensitivity. At 40 wk, complete MitoFAOX (to CO(2)), beta-hydroxyacyl-CoA dehydrogenase activity, and citrate synthase activity did not differ between OLETF-SED and LETO-SED but were significantly (P < 0.05) higher in OLETF-EX compared with OLETF-SED rats. Genes controlling skeletal muscle MitoFAOX (PGC-1alpha, PPARdelta, mtTFA, cytochrome c) were not different between OLETF-SED and LETO-SED at any age. Compared with the OLETF-SED, the OLETF-EX rats had significantly (P < 0.05) higher skeletal muscle PGC-1alpha, cytochrome c, and mtTFA mRNA levels at 20 and 40 wk and PPARdelta at 40 wk; however, protein content for each of these markers did not differ between groups at 40 wk. Limited changes in skeletal muscle mitochondria were observed during the transition from insulin resistance to type 2 diabetes in the hyperphagic OLETF rat. However, diabetes prevention through increased physical activity appears to be mediated in part through maintenance of skeletal muscle mitochondrial function.

Hyperbaric oxygen exposure improves blood glucose level and muscle oxidative capacity in rats with type 2 diabetes

BACKGROUND

The effects of exposure to hyperbaric oxygen on blood glucose level and muscle oxidative capacity in rats with type 2 diabetes were investigated.

METHODS

Five-week-old male Goto-Kakizaki rats were divided into four groups: normobaric (NN; exposed to 21% oxygen at 760 mm Hg for 8 weeks), hyperbaric to normobaric (HN; exposed to 36% oxygen at 950 mm Hg for 4 weeks, followed by 21% oxygen at 760 mm Hg for 4 weeks), normobaric to hyperbaric (NH; exposed to 21% oxygen at 760 mm Hg for 4 weeks, followed by 36% oxygen at 950 mm Hg for 4 weeks), and hyperbaric (HH; exposed to 36% oxygen at 950 mm Hg for 8 weeks).

RESULTS

Blood glucose levels were lower in the HN, NH, and HH groups than in the NN group. Up-regulated mRNA expression levels of peroxisome proliferator-activated receptor-gamma co-activator-1alpha were observed in the soleus muscles of the HN, NH, and HH groups and in the plantaris muscles of the HN and HH groups. The soleus muscles of the NN group contained only type I fibers, whereas those of the HN, NH, and HH groups contained type I, type IIA, and type IIC fibers. An increased percentage of type I fibers and a decreased percentage of type IIB fibers were observed in the plantaris muscles of the NH, HN, and HH groups.

CONCLUSIONS

Exposure to hyperbaric oxygen reduces high blood glucose levels and improves oxidative capacities in the skeletal muscles of rats with diabetes, and these effects are maintained under normobaric conditions even after exposure to hyperbaric oxygen.

Changes in visceral adipose tissue mitochondrial content with type 2 diabetes and daily voluntary wheel running in OLETF rats

Using the hyperphagic, obese, Otsuka Long-Evans Tokushima Fatty (OLETF) rat, we sought to determine if progression to type 2 diabetes alters visceral white adipose tissue (WAT) mitochondrial content and if these changes are modified through prevention of type 2 diabetes with daily exercise. At 4 weeks of age, OLETF rats began voluntary wheel running (OLETF-EX) while additional OLETF rats (OLETF-SED) and Long-Evans Tokushima Otsuka (LETO-SED) rats served as obese and lean sedentary controls, respectively, for 13, 20 and 40 weeks of age (n = 6-8 for each group at each age). OLETF-SED animals displayed insulin resistance at 13 and 20 weeks and type 2 diabetes by 40 weeks. OLETF-SED animals gained significantly (P < 0.001) more weight and omental fat mass compared with OLETF-EX and LETO-SED. Markers of WAT mitochondrial protein content (cytochrome c, COXIV-subunit I, and citrate synthase activity) significantly increased (P < 0.05) from 13 to 40 weeks in the LETO-SED, but were significantly attenuated in the OLETF-SED rats. Daily exercise normalized WAT cytochrome c and COXIV-subunit I protein content in the OLETF-EX to the healthy LETO-SED animals. In conclusion, increases in omental WAT mitochondrial content between 20 and 40 weeks of age in LETO control animals are attenuated in the hyperphagic, obese OLETF rat. These alterations occurred in conjunction with the progression from insulin resistance to type 2 diabetes and were prevented with daily exercise. Reduced ability to increase WAT mitochondrial content does not appear to be a primary cause of insulin resistance, but may play a key role in the worsening of the disease condition.

Cessation of daily exercise dramatically alters precursors of hepatic steatosis in Otsuka Long-Evans Tokushima Fatty (OLETF) rats

The purpose of this study was to delineate potential mechanisms initiating the onset of hepatic steatosis following the cessation of daily physical activity. Four-week-old, hyperphagic/obese Otsuka Long-Evans Tokushima Fatty rats were given access to voluntary running wheels for 16 weeks to prevent the development of hepatic steatosis. The animals were then suddenly transitioned to a sedentary condition as wheels were locked (wheel lock; WL) for 5 h (WL5), 53 h (WL53) or 173 h (WL173). Importantly after the cessation of daily exercise (5-173 h), no changes occurred in body weight, fat pad mass (omental and retroperitoneal), food intake, serum insulin, hepatic triglycerides or in the exercise-suppressed hepatic stearoyl-CoA desaturase-1 and peroxisome proliferator-activated receptor-gamma protein content. However, complete hepatic fatty acid oxidation and mitochondrial enzyme activities were highest at WL5 and WL53 and dropped significantly to SED levels by WL173. In addition, cessation of daily exercise quickly increased the hepatic protein contents of fatty acid synthase and acetyl-coenzyme A carboxylase (ACC), reduced ACC phosphorylation status, and dramatically increased hepatic malonyl-CoA concentration. This study is the first to show that the sudden cessation of daily exercise in a hyperphagic/obese model activates a subgroup of precursors and processes known to initiate hepatic steatosis, including decreased hepatic mitochondrial oxidative capacity, increased hepatic expression of de novo lipogenesis proteins, and increased hepatic malonyl CoA levels; each probably increasing the susceptibility to non-alcoholic fatty liver disease.

Daily exercise increases hepatic fatty acid oxidation and prevents steatosis in Otsuka Long-Evans Tokushima Fatty rats

Exercise training is commonly prescribed for treatment of nonalcoholic fatty liver disease (NAFLD). We sought to determine whether exercise training prevents the development of NAFLD in Otsuka Long-Evans Tokushima Fatty (OLETF) rats and to elucidate the molecular mechanisms underlying the effects of exercise on hepatic steatosis. Four-week-old OLETF rats were randomly assigned to either a sedentary control group (Sed) or a group given access to voluntary running wheels for 16 wk (Ex). Wheels were locked 2 days before euthanasia in the Ex animals, and both groups were euthanized at 20 wk old. Voluntary wheel running attenuated weight gain and reduced serum glucose, insulin, free fatty acids, and triglycerides in Ex animals compared with Sed (P < 0.001). Ex animals exhibited significantly reduced hepatic triglyceride levels and displayed fewer lipid droplets (Oil Red O staining) and reduced lipid droplet size compared with Sed. Wheel running increased by threefold the percent of palmitate oxidized completely to CO(2) in the Ex animals but did not alter AMP-activated protein kinase-alpha (AMPKalpha) or AMPK phosphorylation status. However, fatty acid synthase and acetyl-coenzyme A carboxylase (ACC) content were significantly reduced (approximately 70 and approximately 35%, respectively), and ACC phosphorylation and cytochrome c content were significantly elevated (approximately 35 and approximately 30%, respectively) in the Ex animals. These results unequivocally demonstrate that daily physical activity attenuates hepatic steatosis and NAFLD in an obese rodent model and suggest that this effect is likely mediated, in part, through enhancement of hepatic fatty acid oxidation and reductions in key protein intermediates of fatty acid synthesis.

S-adenosyl-L-methionine increases skeletal muscle mitochondrial DNA density and whole body insulin sensitivity in OLETF rats

Both mitochondrial dysfunction and alterations in mitochondrial DNA (mtDNA) are implicated in type 2 diabetes mellitus and insulin resistance. Evidence also suggests that metabolism of S-adenosyl-L-methionine (SAM), the universal methyl donor for biological methylation, is associated with mitochondrial dysfunction and insulin resistance. We investigated the effect of SAM on mtDNA density and insulin sensitivity using the Otsuka Long-Evans Tokushima Fatty (OLETF) rat, an animal model of type 2 diabetes mellitus and insulin resistance. To determine the short-term effect on mtDNA density, SAM (15 mg.kg-1.d-1) was administered intraperitoneally for 7 d to 6 male, 57-wk-old OLETF rats and 6 Long-Evans Tokushima Otsuka (LETO) rats of the same age as a nondiabetic control. To determine the long-term effect, the same dose of SAM was administered daily to 5 male, 6-wk-old OLETF rats until the age of 25 wk; 7 control OLETF rats received vehicle and 7 LETO rats were untreated. Skeletal muscle mtDNA density was measured by either competitive or multiplex PCR and insulin sensitivity was measured by hyperinsulinemic-euglycemic clamp. SAM treatment for 1 wk increased skeletal muscle mtDNA density of both OLETF and LETO rats. The long-term SAM treatment significantly reduced body weight gain as well as increased skeletal muscle mtDNA density and whole body insulin sensitivity in OLETF rats compared with their vehicle-treated controls. Furthermore, in all 3 groups, skeletal muscle mtDNA density correlated with insulin sensitivity (r=0.752, P<0.001). In conclusion, SAM treatment increased mtDNA density in the skeletal muscle, improved whole body insulin sensitivity, and prevented body weight gain in OLETF rats.

Role of dorsomedial hypothalamic neuropeptide Y in energy homeostasis

Hypothalamic neuropeptide Y (NPY) is primarily expressed in the arcuate nucleus (Arc) and the dorsomedial hypothalamus (DMH). Although Arc NPY gene expression is responsive to circulating leptin, the regulation of DMH NPY expression is leptin-independent. DMH NPY expression is increased in response to chronic food restriction, but not acute food deprivation. DMH NPY expression is elevated in pair-fed OLETF rats lacking cholecystokinin (CCK)-1 receptors. A role for CCK in controlling DMH NPY expression is demonstrated by the down-regulation of DMH NPY by parenchymal DMH CCK administration in intact rats. Moreover, access to running wheels normalizes body weight and prevents altered DMH NPY expression of OLETF rats. Together, these data suggest that DMH NPY plays an important role in feeding and body weight control.

Fibre type distribution and gene expression levels of both succinate dehydrogenase and peroxisome proliferator-activated receptor-gamma coactivator-1alpha of fibres in the soleus muscle of Zucker diabetic fatty rats

We have reported that a change in muscle fibre type distribution is present in two strains of diabetic rats (Otsuka Long-Evans Tokushima Fatty and Goto-Kakizaki rats). In this study, we determined whether the change in soleus muscle fibre type distribution was caused by diabetes, using obese, diabetic (Zucker diabetic fatty, ZDF), obese, non-diabetic (Zucker fatty, ZF) and non-diabetic, non-obese rats (Zucker lean, ZL). Moreover, we investigated whether the gene expression levels of metabolic key molecules, namely the transcriptional factors of metabolic genes, exemplified by peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha), and the oxidative enzymes in mitochondria, exemplified by succinate dehydrogenase (SDH), were changed in type I and II muscle fibres in each type of rat, using the new technique of laser capture microdissection (LCM). Both plasma glucose and glucosylated haemoglobin levels were significantly higher in ZDF than in ZL and ZF rats. A lower percentage of type IIA fibres was observed in the muscles of ZDF rats than in those of ZL and ZF rats. The mRNA expression levels of SDH in type II fibres and of PGC-1alpha in type I fibres were significantly lower in ZDF than in ZL and ZF rats as assessed by LCM and real-time PCR analysis. We have shown, for the first time, that a lower percentage of type IIA fibres was observed in ZDF rats. We have also discovered that the expression levels of the oxidative metabolism-related genes, PGC-1alpha and SDH, decreased in type I and type II fibres, respectively, of ZDF rats.

Effects of running exercise on fibre-type distribution of soleus and plantaris muscles in diabetic Otsuka Long-Evans Tokushima fatty rats

AIM

Effect of running exercise on fibre-type distributions of the slow soleus and fast plantaris muscles was investigated in male Otsuka Long-Evans Tokushima fatty rats (OLETF) as an animal model of spontaneous type 2 diabetes mellitus.

METHODS

Five-week-old OLETF rats were allowed to exercise voluntarily in running wheels for 32 days and the data were compared with those of age-matched non-exercised OLETF and non-diabetic Long-Evans Tokushima Otsuka rats (LETO).

RESULTS

In the soleus muscle, a higher percentage of type I fibres was observed in non-exercised OLETF rats compared with LETO rats, and there were no type IIA fibres in non-exercised OLETF rats. In the plantaris muscle, a higher percentage of type IIB fibres and a lower percentage of type I and type IIA fibres were observed in non-exercised OLETF rats compared with LETO rats. In contrast, there were no differences in the fibre-type distribution of soleus and plantaris muscles between exercised OLETF and LETO rats. The body weight and type I fibre percentage of the soleus muscle were related to the running distance in exercised OLETF rats. White adipose tissue weight, HbA(1c) and blood insulin and glucose concentrations were lower in exercised OLETF rats than in non-exercised OLETF rats, irrespective of the running distance. There was a difference in the gene-expression pattern of the soleus muscle among LETO rats, non-exercised OLETF and exercised OLETF rats.

CONCLUSION

Running exercise can inhibit diabetes-associated type shifting of fibres, which is more apparent with postnatal growth, in skeletal muscles of diabetic OLETF rats, as a result of mRNA expression change in muscle.

Altered taste sensitivity in obese, prediabetic OLETF rats lacking CCK-1 receptors

Otsuka Long-Evans Tokushima fatty (OLETF) rats lack the CCK-1 receptor, are hyperphagic, progressively become obese, and develop type-2 diabetes. We recently demonstrated an increased preference for both real and sham feeding of sucrose in this strain, suggesting altered orosensory sensitivity. To investigate taste functions, we used an automated gustometer with 10-s access to different concentrations of various sapid stimuli. Tests were repeated at 10 and 18 wk of age to assess the early and advanced stages of prediabetes, respectively. Compared with age-matched, nonmutant controls, the OLETF rats showed higher avidity for sucrose at both ages. This difference increased as a function of age and tastant concentration. An exaggerated response also occurred for saccharin, alanine, and fructose, but not for Polycose. Similarly, OLETF rats consumed monosodium-glutamate more at the lower concentrations compared with controls, an effect that age also accentuated. In contrast, there was no statistical strain or age differences in responses to NaCl, MgCl2, citric acid, quinine-HCl, and the trigeminal stimulus capsaicin. These findings demonstrate that compared with controls, OLETF rats differ in their gustatory functions with an overall augmented sensitivity for sweet that progresses during prediabetes. This effect explains their overconsumption of sweet solutions and may contribute to the overall hyperphagia and obesity in this strain.

Development and application of rodent models for type 2 diabetes

The increasing worldwide incidence of diabetes in adults constitutes a global public health burden. It is predicted that by 2025, India, China and the United States will have the largest number of people with diabetes. According to the 2003 estimates of the International Diabetes Federation, the diabetes mellitus prevalence in the USA is 8.0% and approximately 90-95% of diabetic Americans have type 2 diabetes - about 16 million people. Type 2 diabetes is a complex, heterogeneous, polygenic disease characterized mainly by insulin resistance and pancreatic beta-cell dysfunction. Appropriate experimental models are essential tools for understanding the molecular basis, pathogenesis of the vascular and neural lesions, actions of therapeutic agents and genetic or environmental influences that increase the risks of type 2 diabetes. Among the animal models available, those developed in rodents have been studied most thoroughly for reasons such as short generation time, inherited hyperglycaemia and/or obesity in certain strains and economic considerations. In this article, we review the current status of most commonly used rodent diabetic models developed spontaneously, through means of genetic engineering or artificial manipulation. In addition to these models, the Psammomys obesus, rhesus monkeys and many other species are studied intensively and reviewed by Shafrir, Bailey and Flatt and Hansen.

Exercise training improves acetylcholine-induced endothelium-dependent hyperpolarization in type 2 diabetic rats, Otsuka Long-Evans Tokushima fatty rats

We investigated whether endothelium-derived relaxing (EDRF) and hyperpolarizing factor (EDHF) is impaired in type 2 diabetic rats (Otsuka Long-Evans Tokushima Fatty (OLETF) rat) and whether the exercise training improves impaired EDRF and EDHF. Diabetic rats were divided into the sedentary and exercise-trained groups at the age of 16 weeks. Long-Evans Tokushima Otsuka (LETO) rats were used as age-matched non-diabetic controls. EDRF as well as EDHF induced by acetylcholine in the presence of indomethacine and L-nitro N-arginine was significantly attenuated in the diabetic rats, and was further impaired with age. Exercise training significantly improved it. Both insulin resistance and abdominal fat accumulation were significantly greater in the diabetic rats, compared with the non-diabetic rats, but were decreased in exercise-trained rats. Urinary NO(2) secretion was decrease in the diabetic rats at each age, and it was improved by exercise training. The results of the study indicated that exercise training prevented impairment of EDHF, as well as EDRF in type 2 diabetic rats, presumably due to improvement of hyperglycemia and insulin resistance and increase in the production of nitric oxide by exercise training.

Physical exercise prevents the development of type 2 diabetes mellitus in Psammomys obesus

We hypothesized that exercise training might prevent diabetes mellitus in Psammomys obesus. Animals were assigned to three groups: high-energy diet (CH), high-energy diet and exercise (EH), and low-energy diet (CL). The EH group ran on a treadmill 5 days/wk, twice a day. After 4 wk, 93% of the CH group were diabetic compared with only 20% of the EH group. There was no difference in weight gain among the groups. Both EH and CH groups were hyperinsulinemic. Epididymal fat (% of body weight) was higher in the CH group than in either the EH and or the CL group. Protein kinase C (PKC)-delta activity and serine phosphorylation were higher in the EH group. No differences were found in tyrosine phosphorylation of the insulin receptor, insulin receptor substrate-1, and phosphatidylinositol 3-kinase among the groups. We demonstrate for the first time that exercise training effectively prevents the progression of diabetes mellitus type 2 in Psammomys obesus. PKC-delta may be involved in the adaptive effects of exercise in skeletal muscles that lead to the prevention of type 2 diabetes mellitus.

Disease model: hyperinsulinemia and insulin resistance. Part B--polygenic and other animal models

Deficiency of leptin or its receptor produces hyperinsulinemia with marked obesity. Paradoxically, severe insulin resistance also accompanies lipodystrophy. Animal models of these contrasting conditions have enabled us to observe the profound and complicated aspects of the underlying pathologies. In addition, conventional polygenic rodents with known genetic backgrounds, such as the spontaneously hypertensive rat and the Goto-Kakisaki rat, have also been used to investigate these abnormalities.

Metabolic and functional studies on isolated islets in a new rat model of type 2 diabetes

In a new experimental type 2 diabetic syndrome, a 40% reduction of pancreatic beta cells was observed by morphometric analysis. In diabetic islets, as compared to control islets, insulin release was decreased in response to high glucose but not to other stimuli, and total glucose oxidation and utilization were unchanged or slightly reduced. The extent of metabolic and functional impairment appeared proportional to the beta-cell loss. However, a substantial decrease was found in protein level and activity (by 77 and 60%, respectively, versus controls) of mitochondrial FAD-glycerophosphate dehydrogenase (mGDH), the key enzyme of the glycerophosphate shuttle. Interestingly, in diabetic islets, as recently reported for mGDH-deficient transgenic mice, definite functional alterations (mainly in response to D-glyceraldehyde) were only obtained upon pharmacological blockade of the second shuttle (i.e. malate-aspartate) responsible for mitochondrial transfer of reducing equivalents. In conclusion, in this diabetes model with reduction of beta-cell mass, the islets, despite decreased mGDH amount and activity, appear metabolically and functionally active in vitro, likely through the intervention of adaptive mechanisms, yet prone to failure in challenging situations.

Effect of obesity and troglitazone on expression of two glutathione peroxidases: cellular and extracellular types in serum, kidney and adipose tissue

To determine the effect of obesity on expression of cellular- (C-) and extracellular (EC-) glutathione peroxidase (GPX) in serum, kidney and adipose tissue, we measured GPX in serum, kidneys and adipose tissue of the obese Otsuka-Long-Evans-Tokushima Fatty (OLETF) rat and its lean counterpart (LETO). We also investigated the effect of troglitazone. Five each of OLETF and LETO rats were fed diet with or without 0.2% troglitazone for 10 days. Final body weight, kidney weight, blood glucose and serum tumor necrosis factor-alpha (TNF-alpha) level were higher in OLETF rats than in LETO rats. Serum and kidney GPX activities were higher, but adipose tissue GPX activity was lower, in OLETF rats than in LETO rats. Troglitazone treatment decreased adipose tissue GPX activity and abolished overproduction of TNF-alpha in OLETF rats. Immunoblot analysis, for the first time, revealed that both obesity and troglitazone suppressed the protein signals for C-GPX and EC-GPX in adipose tissue. Serum protein carbonyl groups were increased in OLETF rats and troglitazone completely blocked this increase. Increased serum GPX activity in obese rat was due to the increased secretion of EC-GPX from the kidney. Troglitazone protected against the enhanced oxidative stress induced by obesity independently of the serum GPX concentration.

Taurine improves insulin sensitivity in the Otsuka Long-Evans Tokushima Fatty rat, a model of spontaneous type 2 diabetes

BACKGROUND

Taurine, a potent antioxidant, has been reported to improve streptozotocin-induced diabetes mellitus, in which the development of diabetes results from an attack by oxygen free radicals on pancreatic beta cells. However, taurine also increases the excretion of cholesterol via conversion to bile acid and would be expected to improve insulin resistance.

OBJECTIVE

The effects of taurine on insulin sensitivity were examined in a model rat of insulin resistance and type 2 diabetes-the Otsuka Long-Evans Tokushima Fatty (OLETF) rat.

DESIGN

Male OLETF rats were divided into 2 groups at the age of 16 wk: a taurine-supplemented group and an unsupplemented group. As a nondiabetic control, Long-Evans-Tokushima-Otsuka rats were used. An oral-glucose-tolerance test and hyperinsulinemic euglycemic clamp were performed at the ages of 23 and 25 wk.

RESULTS

The OLETF rats had hyperglycemia and insulin resistance and they had a greater accumulation of abdominal fat than did control rats. Abdominal fat accumulation, hyperglycemia, and insulin resistance were significantly lower in the taurine-supplemented group than in the unsupplemented group. Serum and liver concentrations of triacylglycerol and cholesterol were significantly higher in the OLETF rats than in the control rats and were significantly lower in the taurine-supplemented group than in the unsupplemented group, presumably because of the increased secretion of cholesterol into bile acid. Taurine-supplemented rats also showed higher nitric oxide secretion, evidenced by increased urinary excretion of nitrite.

CONCLUSION

Taurine effectively improves metabolism in OLETF rats by decreasing serum cholesterol and triacylglycerol, presumably via increased secretion of cholesterol into bile acid and decreased production of cholesterol because of increased nitric oxide production.

Decreased responsiveness to dietary fat in Otsuka Long-Evans Tokushima fatty rats lacking CCK-A receptors

Adult Otsuka Long-Evans Tokushima fatty (OLETF) rats lack functional cholecystokinin A (CCK-A) receptors, are diabetic, hyperphagic, and obese, and have patterns of ingestion consistent with a satiety deficit secondary to CCK insensitivity. Because dietary fat potently stimulates CCK release, we examined how dietary fat modulates feeding in adult male OLETF rats and their lean [Long-Evans Tokushima (LETO)] controls. High-fat feeding produced sustained overconsumption of high-fat diet (30% corn oil in powdered chow) over a 3-wk period in OLETF but not LETO rats. We then assessed the ability of gastric gavage (5 ml, 1-2 kcal/ml x 15 s) or duodenal preloads (1 kcal/ml, 0.44 ml/min x 10 min) of liquid carbohydrate (glucose), protein (peptone), or fat (Intralipid) to suppress subsequent 30-min 12.5% glucose intake in both strains. In OLETF rats, gastric and duodenal fat preloads were significantly less effective in suppressing subsequent intake than were equicaloric peptone or glucose. These results demonstrate that OLETF rats fail to compensate for fat calories and suggest that their hyperphagia and obesity may stem from a reduced ability to process nutrient-elicited gastrointestinal satiety signals.

Cilostazol, a phosphodiesterase inhibitor, improves insulin sensitivity in the Otsuka Long-Evans Tokushima Fatty Rat, a model of spontaneous NIDDM

AIM

Angiotensin converting enzyme inhibitors and alpha1-adrenergic blockers improve insulin sensitivity, the mechanism of which was considered, at least in part, to be due to the increased blood flow to muscle. The present study aimed to clarify whether cilostazol, a phosphodiesterase inhibitor, improves insulin sensitivity in a model of spontaneous non-insulin dependent diabetes mellitus (NIDDM), Otsuka Long-Evans Tokushima Fatty (OLETF) rat.

METHODS

OLETF rats were divided into the two groups at the age of 16 weeks: the cilostazol-supplemented group (cilostazol 40 mg/kg/day) and the normal-diet group. As a non-diabetic control, we used Long-Evans-Tokushima-Otsuka rats (non-diabetic rats). Oral glucose tolerance test and hyperinsulinemic euglycemic clamp was performed at the ages of 23 and 25 weeks, respectively. Serum levels of lipids and leptin were measured.

RESULTS

Body weight and abdominal fat was increased in OLETF rats but cilostazol supplementation did not alter them. Insulin sensitivity, as measured by the hyperinsulinemic euglycemic clamp technique, was significantly decreased in OLETF rats (glucose infusion rate: 73.5 +/- 10.0 vs. 41.5 +/- 9.8 micromol/min/kg body weight, p < 0.01). Cilostazol supplementation improved insulin sensitivity partially but significantly 51.0 +/- 5.7 micromol/min/kg body weight, p < 0.05) in OLETF rats at 25 weeks of age, although it did not decrease serum levels of glucose, lipids or leptin. However, this effect was not observed in non-diabetic rats.

CONCLUSION

Cilostazol, which is used in diabetic patients for the treatment of obstructive disease of artery, is expected to have a beneficial effect on insulin sensitivity in NIDDM.

Increased hepatic glucose production and decreased hepatic glucose uptake at the prediabetic phase in the Otsuka Long-Evans Tokushima fatty rat model

To investigate the time course of the hepatic glucose metabolism in non-insulin-dependent diabetes (NIDDM), we measured hepatic glucose production (HGP) and first-pass uptake of portal glucose infusion by the liver (HGU) using dual-tracer methods in a NIDDM model, Otsuka Long-Evans Tokushima Fatty (OLETF) rats, and in normal controls, Long-Evans Tokushima Otsuka (LETO) rats, at 8, 14, and 28 weeks of age (n = 5, respectively). The fasting plasma glucose level in OLETF rats was significantly higher than in LETO rats at 28 weeks of age (8.9 +/- 1.7 v 6.3 +/- 0.4 mmol/L, P < .01), while there was no significant difference at 8 and 14 weeks. Hyperinsulinemia in OLETF rats appeared at > or = 8 weeks of age. Basal HGP was significantly higher in OLETF than in LETO rats at 8 and 28 weeks (8 weeks, 12.7 +/- 1.7 v 9.4 +/- 1.8 mg x kg(-1) x min(-1), P < .05; 28 weeks, 10.9 +/- 1.6 v 7.1 +/- 1.3 mg x kg(-1) x min(-1), P < .01). At 14 weeks, basal HGP was not significantly different between OLETF and LETO rats. However, at all study points, HGU during a portal glucose infusion was significantly lower in OLETF than in LETO rats (8 weeks, 0.9 +/- 0.2 v 2.3 +/- 0.5, P < .01; 14 weeks, 0.8 +/- 0.3 v 1.4 +/- 0.3, P < .05; 28 weeks, 0.7 +/- 0.2 v 1.4 +/- 0.3 mg x kg(-1) x min(-1), P < .01). Fasting plasma free fatty acid (FFA) levels were not significantly different between OLETF and LETO, except at 8 weeks. Suppression of plasma FFA levels by endogenous insulin during a portal glucose infusion was impaired in OLETF rats compared with LETO rats. In summary, this study demonstrates that derangement of hepatic glucose handling, such as increased basal HGP and decreased HGU, is observed in obese NIDDM model OLETF rats at the prediabetic phase when hyperglycemia is still not apparent. Furthermore, these derangements may be accompanied by impaired lipid metabolism.

Centrally administered TRH-induced insulin secretion is impaired in the Otsuka-Long-Evans-Tokushima Fatty rats, a model of spontaneous non-insulin-dependent diabetes mellitus

To investigate whether insulin secretion induced by stimulation of the vagus nerve is preserved or impaired in Otsuka-Long-Evans-Tokushima Fatty (OLETF) rats, we injected 10(-8) mol of thyrotropin-releasing hormone (TRH) into the third cerebral ventricle and determined the serum level of insulin in the unanesthetized, unrestrained rats. Intracerebroventricular (i.c.v.) injection increased the serum levels of glucose and insulin in both OLETF and Long-Evans-Tokushima-Otsuka (LETO) rats, a nondiabetic control strain, at 8-12 weeks of age. At 24-28 weeks of age, the increased level of glucose in OLETF rats was comparable to LETO rats but that of insulin was lower than control after the i.c.v. injection of TRH. Pretreatment with i.v. atropine had no significant effect on such hyperglycemia. However, the increases in the serum levels of insulin were suppressed in both OLETF and LETO rats. The plasma levels of epinephrine, norepinephrine, and glucagon rose significantly after TRH. There was no significant difference in the levels of any hormones between the two groups. In OLETF rats at 24-28 weeks of age, i.v. glucose load induced significantly higher serum levels of glucose and insulin than LETO rats. The results suggest that the vagus nerve-mediated insulin secretion is impaired in OLETF rats, similar to an autonomic diabetic neuropathy in the early stage of diabetes. This impairment may play some role in deteriorating glucose tolerance in this spontaneously developed diabetes model.

Alteration of the properties of gastric smooth muscle in the genetically hyperglycemic OLETF rat

Membrane responses were recorded from isolated gastric smooth muscle of Otsuka Long-Evans Tokushima Fatty (OLETF) and Long-Evans Tokushima Otsuka (LETO) rats, using microelectrode techniques. At the age of 68-76 weeks, the blood sugar level was 181 mg/dl in LETO rats and 350 mg/dL in OLETF rats. In both rats, the membrane potential was stable in fundus muscle and spontaneously active with generation of slow waves in antrum muscle. The resting membrane potential was about - 46 mV in fundus and - 55 mV in antrum muscles of LETO rats, and the values were 3-7 mV lower in OLETF rats. The slow waves were generated regularly in LETO rats, while they were irregular and of small amplitude in OLETF rats. Transmural nerve stimulation evoked a cholinergic excitatory junction potential and following inhibitory junction potential in LETO rats, and only an inhibitory junction potential of smaller size was generated in most of OLETF rats. The acetylcholine-induced depolarization was greater in OLETF than in LETO rats. The level of hyperpolarization produced by noradrenaline was similar between OLETF and LETO rats. Thus, the reduction of the resting membrane potential, weakening of spontaneous activity, impairment of cholinergic transmission and cholinergic supersensitivity were associated with hyperglycemia. These alterations were considered due to the development of diabetes mellitus.

Enhanced expression of hepatic acyl-coenzyme A synthetase and microsomal triglyceride transfer protein messenger RNAs in the obese and hypertriglyceridemic rat with visceral fat accumulation

The liver plays a central role in lipoprotein metabolism. In particular, very-low density lipoprotein (VLDL) is assembled in the hepatocytes and secreted into the blood circulation. The VLDL is then catabolized to low-density lipoprotein by lipoprotein lipase and hepatic triglyceride lipase. Obese subjects, especially those with visceral fat accumulation, are frequently associated with hyperlipidemia, non-insulin-dependent diabetes mellitus (NIDDM), and hypertension. The mechanism of hyperlipidemia in visceral fat obesity has not yet been elucidated. Otsuka Long-Evans Tokushima Fatty (OLETF) rat is an animal model of NIDDM, characterized by obesity with visceral fat accumulation, hyperlipidemia, and late-onset insulin resistance. To elucidate the mechanism of hyperlipidemia observed in OLETF rats, we focused on the production of VLDL by the liver and investigated hepatic messenger RNA (mRNA) levels of microsomal triglyceride transfer protein (MTP), acyl-coenzyme A synthetase (ACS), and apolipoprotein B (apo B), which play important roles in VLDL synthesis and secretion. In 6-week-old OLETF rats, in which insulin resistance had not been manifested, visceral fat weight was already higher and portal free fatty acid (FFA) and VLDL-triglyceride levels were elevated compared with the control rats. Hepatic ACS activity and mRNA levels, and MTP mRNA levels were also increased in OLETF rats, whereas apo B mRNA levels were similar; these results suggest that the enhanced expression of both ACS and MTP genes associated with visceral fat accumulation before developing insulin resistance may be involved in the pathogenesis of hyperlipidemia in obese animal models with NIDDM.

Measurement of abdominal fat by magnetic resonance imaging of OLETF rats, an animal model of NIDDM

We measured abdominal fat masses (intra-abdominal visceral fat summing retroperitoneal, mesenteric, and epididymal fat and subcutaneous fat) and analyzed abdominal fat distribution of Otsuka Long-Evans Tokushima Fatty (OLETF) rats with non-insulin-dependent diabetes mellitus and control strain Long-Evans Tokushima Otsuka (LETO) rats using magnetic resonance imaging. Intra-abdominal visceral and subcutaneous fat were highly correlated with body weight both in OLETF and in LETO rats. Both intra-abdominal visceral and subcutaneous fat of OLETF rats significantly accumulated compared with those of LETO rats. Intra-abdominal visceral fat mass correlated positively with subcutaneous fat mass, and the accumulation of intra-abdominal visceral fat mass was about 3.5 times that of subcutaneous fat. Thus, obesity of OLETF rats was characterized by marked accumulation of intra-abdominal visceral fat compared with that of subcutaneous fat. Body weight and abdominal fat of OLETF rats were closely correlated with the level of total plasma glucose measured by oral glucose tolerance test. However the ratio of intra-abdominal visceral to subcutaneous fat of OLETF rats was not correlated with the level of total plasma glucose.

Alterations of insulin and glucagon secretion from the perfused pancreas before, at the onset and after the development of diabetes in male Otsuka Long-Evans Tokushima Fatty (OLETF) rats

The Otsuka Long-Evans Tokushima Fatty (OLETF) rat is an obese non-insulin dependent diabetes mellitus (NIDDM) model of an inbred strain. In this study, basal (2.8 mM glucose) insulin and glucagon and their responses to glucose (16.7 mM) were examined at the age of 9 weeks (n = 3) before the onset of diabetes, at 23 weeks (n = 6) at the onset of diabetes, and at 48 weeks (n = 5) after the development of diabetes by pancreatic perfusion. In Long-Evans Tokushima Otsuka (LETO, control) rats, insulin responses to glucose showed a biphasic pattern at all three ages, while in OLETF rats, basal insulin concentrations were significantly increased compared to those in controls at the age of 9 and 48 weeks. Insulin responses to glucose showed no difference from controls at 9 and 23 weeks, however, at 48 weeks the response was significantly decreased. In controls, high basal glucagon concentrations showed significant decrease in response to glucose at all ages. In OLETF rats, basal glucagon concentrations showed significant decrease compared to those in control rats at 23 and 48 weeks. Glucagon response to glucose significantly decreased at 9 and 23 weeks, but at 48 weeks there was no change in concentration in response to glucose. Pancreatic insulin content was lower at 48 weeks in OLETF rats than in LETO rats, although no differences were observed at other ages. There were no significant differences in pancreatic glucagon content between the two groups at any age. Morphologically, in OLETF rats the number of pancreatic B cells were decreased and A cells migrated into the center of islets at 48 weeks. The results suggested that one of the causes of diabetes in OLETF rats is impaired insulin response to glucose.

Exercise training in Otsuka Long-Evans Tokushima Fatty rat, a model of spontaneous non-insulin-dependent diabetes mellitus: effects on the B-cell mass, insulin content and fibrosis in the pancreas

The effects of exercise on alterations in the amount of B-cell mass, insulin content and fibrous tissue present in the pancreas were examined for a diabetic state induced by a 70% pancreatectomy and a prediabetic state in Otsuka Long-Evans Tokushima Fatty (OLETF) rat, a model for the spontaneous development of non-insulin-dependent diabetes mellitus (NIDDM). The rats (5-weeks old) were trained either by a 6-week running program or sedentary controls, and at 6-weeks of age, received either a 70% pancreatectomy or a sham-pancreatectomy (sham). As in our previous report, persistent hyperglycemia was detected after surgery for both trained pancreatectomized (Px) and sedentary Px groups. In the nondiabetic sham rats, exercise training resulted in a significantly smaller increase in body weight and beneficial effects on the pancreas as reflected by an increase in pancreatic volume, accompanied by increases in B-cell mass and insulin content as well as less connective tissue in the pancreas compared with the sedentary nondiabetic sham rats. The effect was not sufficient to improve sustained hyperglycemia in the trained diabetic Px rats. This is probably due to a decreased capacity for B-cell proliferation in response to an increased demand for insulin. Although exercise failed to improve this inherent defect in B-cell proliferation, it ameliorated the further deterioration of the pancreas which occurred with hyperglycemia, and resulted in a higher quantity of insulin stored per milligram of B-cell mass (as function of B-cell mass) and less fibrosis in the pancreas, compared with the sedentary diabetic Px rats. The findings of the present study suggest that exercise training has a beneficial effect on the pancreas in the nondiabetic state, and also exerts some positive effects in the diabetic state in this model rat.

Diabetic neuropathy in sucrose-fed Otsuka Long-Evans Tokushima fatty rats: effect of an aldose reductase inhibitor, TAT

In an animal model of human non-insulin dependent diabetes mellitus (NIDDM), Otsuka Long-Evans Tokushima Fatty (OLETF) rats were fed with sucrose for 8 weeks to obtain severe hyperglycemia. The effects of sucrose administration on peripheral nerve functions, motor nerve conduction velocity (MNCV) and coefficient of variance of R-R interval (CVR-R), were investigated with concomitant measuring of sciatic nerve blood flow (SNBF), ADP-induced platelet aggregation and polyol content in the sciatic nerves. The effects of an aldose reductase inhibitor, TAT, on these parameters were also studied. Administration of sucrose to OLETF rats caused significant body weight reduction and remarkable hyperglycemia. Sucrose-fed OLETF rats demonstrated significantly delayed MNCV, decreased CVR-R, reduced SNBF and increased platelet aggregation activity to ADP. Sorbitol and fructose accumulation, and myo-inositol depletion in sciatic nerves were observed only in sucrose-fed OLETF rats. These abnormalities were all ameliorated by the treatment with TAT. These observations suggest that the sucrose-fed OLETF rat is a useful animal model for studying the pathogenesis of diabetic neuropathy in human NIDDM, and that an aldose reductase inhibitor is a useful therapeutic agent for the treatment of diabetic neuropathy.