Leptin responses to glucose infusions in obesity-prone rats

The secretion of leptin is dually regulated. In fasting animals, plasma leptin concentrations reflect body fat stores, whereas the incremental leptin response to fasting or refeeding most likely reflects insulin-mediated energy flux and metabolism within adipocytes. Impaired secretion of leptin in either pathway could result in obesity. We therefore measured plasma leptin concentrations in fasted animals and plasma leptin concentrations after an intravenous glucose infusion in a rat model of obesity. Young Sprague-Dawley (S-D) and Fischer 344 (F344) rats had similar percent body fat and fasting glucose and fasting leptin concentrations. However, F344 animals had higher insulin concentrations and leptin responses to intravenous glucose than did the S-D animals. The animals were then fed a control or high-fat diet for 6 wk. High-fat fed animals gained more weight and body fat than did the control fed animals. Control and high-fat fed F344 animals gained approximately 40% (P < 0.0001) more weight and >100% (P < 0.01) more body fat than did the S-D animals. Fasting leptin concentrations and leptin concentrations after intravenous glucose infusions and feeding were more than double (P < 0.05) in F344 animals compared with S-D animals. Whether an animal is fed a control or high-fat diet had little effect on the leptin response to intravenous glucose. In conclusion, young, lean F344 animals, before the onset of obesity, demonstrated a greater acute leptin response to intravenous glucose than similarly lean S-D animals. After a 6-wk diet, F344 animals had a greater percent increase in body weight and insulin resistance and exhibited higher fasting leptin concentrations and a greater absolute leptin response to intravenous glucose compared with the S-D animals. The chronic diet (control or high fat) had little impact on the acute leptin response to intravenous glucose. F344 animals exhibit leptin resistance in young, lean animals and after aging and fat accumulation.     

Effect of leptin on insulin resistance of muscle--direct or indirect?

We examined the effect of leptin on the insulin resistance in skeletal muscles by measuring glucose transport. Male Wistar rats were fed rat chow or high-fat diets for 30 days. Before sacrifice, rats fed high-fat diet were subcutaneously injected with leptin (1 mg/kg b.w.) for 3 days. The glucose transport in epitrochlearis and soleus muscles did not differ in the experimental groups under basal conditions, however these values decreased significantly in the rats fed high-fat diet under insulin stimulation (p<0.01). Leptin treatment recovered the decreased glucose transport in epitrochlearis (p<0.05) and soleus muscles (p=0.08). Triglyceride concentrations in soleus muscles were increased significantly in the rats fed high-fat diet as compared to rats fed chow diet (p<0.01), and were decreased significantly by leptin treatment (p<0.01). The glucose transport was measured under basal conditions and after 60 microU/ml of insulin treatment with or without 50 ng/ml of leptin. Leptin had no direct stimulatory effect on glucose transport under both basal and insulin-stimulated conditions in vitro. These results demonstrate that leptin injection to rats fed high-fat diet recovered impaired insulin responsiveness of skeletal muscles and muscle triglyceride concentrations. However, there was no direct stimulatory effect of leptin on insulin sensitivity of skeletal muscles in vitro.     

Beneficial effects of hydro-alcoholic extract of Caralluma fimbriata against high-fat diet-induced insulin resistance and oxidative stress in Wistar male rats

The main aim of this study was to investigate the beneficial effects of hydro-alcoholic extract of Caralluma fimbriata (CFE) on the effects of high-fat diet feeding on insulin resistance and oxidative stress in Wistar rats. High-fat diet (60 % of fat) and CFE (200 mg/kg body weight/day) were given concurrently to the rats for a period of 90 days. Feeding with high-fat diet resulted in the development of hyperglycemia, hyperinsulinemia, hyperleptinemia, and hypertriglyceridemia and impaired insulin sensitivity (P?<?0.05). Administration of CFE to high-fat diet-fed rats for 90 days resulted in a significant improvement in plasma glucose, insulin, leptin, and triglycerides. Regarding liver antioxidant status, high-fat fed rats showed higher levels of lipid peroxidation, protein oxidation and lower GSH levels and lower activities of enzymatic antioxidants, while CFE treatment prevented all these observed abnormalities. In conclusion, intake of CFE may be beneficial for the suppression of high-fat diet-induced insulin resistance and oxidative stress.     

Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats

In addition to suppressing appetite, leptin may also modulate insulin secretion and action. Leptin was administered here to insulin-resistant rats to determine its effects on secretagogue-stimulated insulin release, whole body glucose disposal, and insulin-stimulated skeletal muscle glucose uptake and transport. Male Wistar rats were fed either a normal (Con) or a high-fat (HF) diet for 3 or 6 mo. HF rats were then treated with either vehicle (HF), leptin (HF-Lep, 10 mg. kg(-1). day(-1) sc), or food restriction (HF-FR) for 12-15 days. Glucose tolerance and skeletal muscle glucose uptake and transport were significantly impaired in HF compared with Con. Whole body glucose tolerance and rates of insulin-stimulated skeletal muscle glucose uptake and transport in HF-Lep were similar to those of Con and greater than those of HF and HF-FR. The insulin secretory response to either glucose or tolbutamide (a pancreatic beta-cell secretagogue) was not significantly diminished in HF-Lep. Total and plasma membrane skeletal muscle GLUT-4 protein concentrations were similar in Con and HF-Lep and greater than those in HF and HF-FR. The findings suggest that chronic leptin administration reversed a high-fat diet-induced insulin-resistant state, without compromising insulin secretion.     

下丘脑Orexin 对肥胖大鼠能量代谢的影响

目的:探讨下丘脑注射OXR-1选择性受体拮抗剂ACT-335827对肥胖大鼠代谢的效果。方法:通过高脂饮食建立肥胖大鼠模型,采用CODA 8通道高通量非侵入性血压系统(EMKA)测量血压;所有脂类都使用商品酶试剂盒和TOSHIBA-40FR全自动分析仪测量;空腹血糖采用葡萄糖氧化酶法;空腹胰岛素采用放射免疫法测定。肥胖大鼠出现代谢紊乱后,给予ACT-335827处理,检测大鼠体重、血压、脂肪、甘油三酯、总胆固醇、高密度脂蛋白、低密度脂蛋白、游离脂肪酸(NEFA)、瘦素、空腹血糖及空腹胰岛素等的变化。结果:与普通饮食组相比,经过10周高脂饮食,高脂饮食组大鼠体重显著升高(P0.05),给予ACT-335827处理后,普通大鼠的体重、血压、脂肪含量、脂代谢等均无明显变化;与高脂饮食和高脂饮食加生理盐水处理组大鼠比较,高脂饮食加ACT-335827处理组肥胖大鼠的体重显著下降(P0.05),腹部和附睾脂肪含量下降(P0.05),低密度脂蛋白、甘油三酯、总胆固醇、瘦素水平下降(P0.05),空腹血糖及空腹胰岛素也显著降低(P0.05),但血压、肠系膜脂肪和肩胛棕色脂肪、高密度脂蛋白和NEFA无明显变化(P0.05)。结论:ACT-335827对肥胖大鼠的代谢紊乱具有改善作用,对肥胖大鼠有一定的减肥作用。     

Oleanolic acid,a natural triterpenoid improves blood glucose tolerance in normal mice and ameliorates visceral obesity in mice fed a high-fat diet

Excess visceral adiposity may predispose to chronic diseases like hypertension and type 2 diabetes with a high risk for coronary artery disease. Adipose tissue secreted cytokines and oxidative stress play an important role in chronic disease progression. To combat adiposity, plant-derived triterpenes are currently receiving much attention as they possess antioxidant and anti-inflammatory properties and the ability to regulate glucose and lipid metabolism. In the search for potential antiobese compounds from natural sources, this study evaluated the effects of oleanolic acid (OA), a pentacyclic triterpene commonly present in fruits and vegetables, in glucose tolerance test and on high-fat diet (HFD)-induced obesity in mice. Adult male Swiss mice treated or not with OA (10 mg/kg) were fed a HFD during 15 weeks. Sibutramine (SIB) treated group (10 mg/kg) was included for comparison. Weekly body weights, food and water consumption were measured, and at the end of study period, the levels of blood glucose and lipids, plasma hormone levels of insulin, ghrelin and leptin, and the visceral abdominal fat content were analysed. Mice treated with OA and fed a HFD showed significantly (p < 0.05) improved glucose tolerance, decreased body weights, visceral adiposity, blood glucose, plasma lipids relative to their respective controls fed no OA. Additionally, OA treatment, while significantly elevating the plasma hormone level of leptin, decreased the level of ghrelin. However, it caused a greater decrease in plasma amylase activity than lipase. Sibutramine-treated group also manifested similar effects like OA except for blood glucose level that was not different from HFD control. These findings suggest that OA ameliorates visceral adiposity and improves glucose tolerance in mice and thus has an antiobese potential through modulation of carbohydrate and fat metabolism.     

The effect of fasting and physical exercise on plasma leptin concentrations in high-fat fed rats.

The aim of our study was to estimate the effect of fasting and physical exercise on a treadmill on plasma leptin concentrations in high-fat fed rats. Male Wistar rats were injected a low dose of streptozotocin (STZ) or buffer at 2 days of age and later fed a standard or high-fat diet (HFD). Plasma leptin was measured by RIA method in all the groups studied in basal conditions, after 48h fasting, a single bout of exhaustive exercise, and 4 weeks of exercise training. Plasma leptin concentrations were markedly elevated in the HFD and STZ/HFD groups compared to the control group. The significant correlation between plasma leptin and body weight was noted. Fasting and exercise training decreased plasma leptin in similar percentage in all the groups studied. The observed decrease was greater than expected from changes in body weight. We conclude that high-fat feeding results in an increase in plasma leptin levels in rats independently of plasma insulin or daily calorie intake. High-fat fed rats have maintained leptin response to fasting and exercise training. The reduction in plasma leptin after exercise training is partly independent on changes in body weight or plasma insulin.     

Acute changes in the response to peripheral leptin with alteration in the diet composition

Dietary induced obesity in rodents is associated with a resistance to leptin. We have investigated the hypothesis that dietary fat per se alters the feeding response to peripheral leptin in rats that were fed either their habitual high- or low-fat diet or were naively exposed to the alternative diet. Osborne-Mendel rats were adapted to either high- or low-fat diet. Food-deprived rats were given either leptin (0.5 mg/kg body wt ip) or saline, after which they were provided with either their familiar diet or the alternative diet. Food intake of rats adapted and tested with the low-fat diet was reduced 4 h after leptin injection, whereas rats adapted and tested with a high-fat diet did not respond to leptin. Leptin was injected again 1 and 5 days after the high-fat diet-adapted rats were switched to the low-fat diet. Leptin reduced the food intake on both days. In contrast, when low-fat diet-adapted rats were switched to a high-fat diet, the leptin inhibitory response was present on day 1 but not observed on day 5. Peripheral injection of leptin increased serum corticosterone level and decreased hypothalamic neuropeptide Y mRNA expression in rats fed the low-fat but not the high-fat diet for 20 days. The data suggest that dietary fat itself, rather than obesity, may induce leptin resistance within a short time of exposure to a high-fat diet.     

Variation in Gene Expression of Inflammatory Cytokines in Leukocyte-Derived Cells of High-Fat-Diet-Induced Insulin-Resistant Rats

A high-fat diet is thought to enhance inflammation in various tissues by increasing insulin resistance. In this study, we determined the mRNA levels of inflammatory cytokines in leukocyte-derived cells in the blood of rats with high-fat-diet-induced insulin resistance. Feeding rats a high-fat diet for 77 d induced moderate insulin resistance, which was determined by increased plasma glucose and insulin concentrations, following an oral glucose tolerance test. The interleukin (IL)-1β mRNA level was higher in the insulin-resistant rats than in control rats at the fasting stage, whereas the tumor necrosis factor (TNF)-α mRNA level was greatly elevated at 180 min after glucose administration in the insulin-resistant rats. The results suggest that feeding rats a high-fat diet enhances the expression of fasting IL-1β and postprandial TNF-α genes in leukocyte-derived cells.     

Variation in gene expression of inflammatory cytokines in leukocyte-derived cells of high-fat-diet-induced insulin-resistant rats

A high-fat diet is thought to enhance inflammation in various tissues by increasing insulin resistance. In this study, we determined the mRNA levels of inflammatory cytokines in leukocyte-derived cells in the blood of rats with high-fat-diet-induced insulin resistance. Feeding rats a high-fat diet for 77 d induced moderate insulin resistance, which was determined by increased plasma glucose and insulin concentrations, following an oral glucose tolerance test. The interleukin (IL)-1beta mRNA level was higher in the insulin-resistant rats than in control rats at the fasting stage, whereas the tumor necrosis factor (TNF)-alpha mRNA level was greatly elevated at 180 min after glucose administration in the insulin-resistant rats. The results suggest that feeding rats a high-fat diet enhances the expression of fasting IL-1beta and postprandial TNF-alpha genes in leukocyte-derived cells.     

Intermittent fasting,adipokines, insulin sensitivity,and hypothalamic neuropeptides in a dietary overload with high-fat or high-fructose diet in mice

The intermittent fasting (IF) might have benefits on metabolism and food intake. Twelve-week old C57BL/6 J mice were fed a control diet (C, 10% kcal fat), a high-fat diet (HF, 50% kcal fat) or a high-fructose diet (HFru, 50% kcal fructose) for 8 weeks, then half of the animals in each group underwent IF (24 h fed, 24 h fasting) for an additional 4 weeks. Although food intake on the fed day remained the same for all groups, all fasting groups showed a reduction in body mass compared to their counterparts. IF reduced total cholesterol, triacylglycerol, fasting glucose, fasting insulin resistance index, and plasma leptin, but increased plasma adiponectin. IF reduced Leptin gene expression in the HF-IF group, but increased proinflammatory markers in the hypothalamus, also in the C-IF group. Both groups HFru-IF and C-IF, showed alterations in the leptin signaling pathway (Leptin, OBRb, and SOCS3), mainly in the HFru-IF group, suggesting leptin resistance. NPY and POMC neuropeptides labeled the neurons of the hypothalamus by immunofluorescence, corroborating qualitatively other quantitative findings of the study. In conclusion, current results are convincing in demonstrating the IF effect on central regulation of food intake control, as shown by NPY and POMC neuropeptide expressions, resulting in a lower weight gain. Besides, IF improves glycemia, lipid metabolism, and consequently insulin and leptin resistance. However, there is increased expression of inflammatory markers in mouse hypothalamus challenged by the HF and HFru diets, which in the long term may induce adverse effects.     

Correction of diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance by moderate hyperleptinemia

Human obesity and high fat feeding in rats are associated with the development of insulin resistance and perturbed carbohydrate and lipid metabolism. It has been proposed that these metabolic abnormalities may be reversible by interventions that increase plasma leptin. Up to now, studies in nongenetic animal models of obesity and in human obesity have concentrated on multiple injection therapy with mixed results. Our study sought to determine whether a sustained, moderate increase in plasma leptin, achieved by administration of a recombinant adenovirus containing the leptin cDNA (AdCMV-leptin) would be effective in reversing the metabolic abnormalities of the obese phenotype. Wistar rats fed a high-fat diet (HF) were heavier (P < 0.05), had increased fat mass and intramuscular triglycerides (mTG), and had elevated plasma glucose, insulin, triglyceride, and free fatty acids compared with standard chow-fed (SC) control animals (all P < 0.01). HF rats also had impaired glucose tolerance and were markedly insulin resistant, as demonstrated by a 40% reduction in insulin-stimulated muscle glucose uptake (P < 0.001). Increasing plasma leptin levels to 29.0 +/- 1.5 ng/ml (from 7.0 +/- 1.4 ng/ml, P < 0.001) for a period of 6 days decreased adipose mass by 40% and normalized plasma glucose and insulin levels. In addition, insulin-stimulated skeletal muscle glucose uptake was normalized in hyperleptinemic rats, an effect that correlated closely with a 60% (P < 0.001) decrease in mTG. Importantly, HF rats that received a control adenovirus containing the beta-galactosidase cDNA and were calorically matched to AdCMV-leptin-treated animals remained hyperglycemic, hyperinsulinemic, insulin resistant, and maintained elevated mTG. We conclude that a gene-therapeutic intervention that elevates plasma leptin moderately for a sustained period reverses diet-induced hyperglycemia, hyperinsulinemia, and skeletal muscle insulin resistance, and that these improvements are tightly linked to leptin-induced reductions in mTG.     

Chronic sleep disorder induced by psychophysiological stress induces glucose intolerance without adipose inflammation in mice

Sleep disturbances are associated with various metabolic diseases such as hypertension and diabetes. We had previously established a mouse model of a psychophysiological stress-induced chronic sleep disorder (CSD) characterized by disrupted circadian rhythms of wheel-running activity, core body temperature, and sleep-wake cycles. To evaluate the underlying mechanisms of metabolic disorders induced by CSD, we created mice with CSD for six weeks and fed them with a high-fat diet. Glucose intolerance with hyperglycemia resulted, although plasma insulin levels and body weight increases were identical between control and CSD mice. Gluconeogenesis and glycolysis were enhanced and suppressed, respectively, in the livers of CSD mice, because the mRNA expression of Pck1 was significantly increased, whereas that of Gck and Pklr were significantly decreased in the CSD mice. Adipose inflammation induced by the high-fat diet seemed suppressed by the CSD, because the mRNA expression levels of Adgre1, Ccl2, and Tnf were significantly downregulated in the adipose tissues of CSD mice. These findings suggest that CSD impair glucose tolerance by inducing gluconeogenesis and suppressing glycolysis. Hyperphasia with hypoleptinemia, hypercorticosteronemia, and increased plasma free fatty acids might be involved in the impaired glucose metabolism under a CSD. Further studies are needed to elucidate the endocrine and molecular mechanisms underlying the associations between sleep disorders and impaired glucose homeostasis that consequently causes diabetes.     

高脂饮食和地塞米松联合诱导胰岛素抵抗大鼠模型

目的用高脂饲料＋地塞米松（dexamethasone,DEX）隔日腹腔注射建立实验性胰岛素抵抗大鼠模型,研究该模型糖代谢、脂代谢和激素水平等方面的变化。方法采用Wistar雄性大鼠,分为正常对照组、高脂组、DEX组（1mg/kg,i.p.）和高脂＋DEX组（1mg/kg,i.p.）,连续观察8周,每周测定大鼠空腹血糖,分别于造模第2周和第8周测糖耐量,8周后处死大鼠,测定胸腺、脾脏、肝脏等脏器重量。结果高脂饲料能加重腹腔注射DEX造成的空腹血糖升高,造模第8周空腹血糖（7.7±0.7）较空白组（6.5±0.6）显著升高。使模型动物糖耐量明显异常,肝糖原、肌糖原含量显著增加,血浆胰岛素及游离脂肪酸水平显著升高,各脏器指数明显增加。结论高脂＋DEX隔日腹腔注射能成功诱导胰岛素抵抗大鼠模型,这种造模方法较单纯注射DEX或单纯高脂饲养成模率高,造模周期短。     

Effect of fasting on carbohydrate metabolism in frugivorous bats (Artibeus lituratus and Artibeus jamaicensis)

The compensatory changes of carbohydrate metabolism induced by fasting were investigated in frugivorous bats, Artibeus lituratus and Artibeus jamaicensis. For this purpose, plasma levels of glucose and lactate, liver and muscle glycogen content, rates of liver gluconeogenesis and the activity of related enzymes were determined in male bats. After a decrease during the first 48 h of fasting, plasma glucose levels remained constant until the end of the experimental period. Plasma lactate levels, extremely high in fed bats, decreased after 48 h of fasting. Similarly, liver glycogen content, markedly high in fed animals, was reduced to low levels after 24 h without food. Muscle glycogen was also reduced in fasted bats. The expected increase in liver gluconeogenesis during fasting was observed after 48 h of fasting. The activities of liver glucose-6-phosphatase and fructose-1,6-bisphosphatase were not affected by food withdrawn. On the other hand, fasting for 24 h induced an increase in the activity of liver cytosolic phosphoenolpyruvate carboxykinase. The data indicate that liver gluconeogenesis has an important role in the glucose homeostasis in frugivorous bats during prolonged periods of food deprivation. During short periods of fasting liver glycogenolysis seems to be the main responsible for the maintenance of glycemia.     

Kinetic Modeling of Human Hepatic Glucose Metabolism in Type 2 Diabetes Mellitus Predicts Higher Risk of Hypoglycemic Events in Rigorous Insulin Therapy

A major problem in the insulin therapy of patients with diabetes type 2 (T2DM) is the increased occurrence of hypoglycemic events which, if left untreated, may cause confusion or fainting and in severe cases seizures, coma, and even death. To elucidate the potential contribution of the liver to hypoglycemia in T2DM we applied a detailed kinetic model of human hepatic glucose metabolism to simulate changes in glycolysis, gluconeogenesis, and glycogen metabolism induced by deviations of the hormones insulin, glucagon, and epinephrine from their normal plasma profiles. Our simulations reveal in line with experimental and clinical data from a multitude of studies in T2DM, (i) significant changes in the relative contribution of glycolysis, gluconeogenesis, and glycogen metabolism to hepatic glucose production and hepatic glucose utilization; (ii) decreased postprandial glycogen storage as well as increased glycogen depletion in overnight fasting and short term fasting; and (iii) a shift of the set point defining the switch between hepatic glucose production and hepatic glucose utilization to elevated plasma glucose levels, respectively, in T2DM relative to normal, healthy subjects. Intriguingly, our model simulations predict a restricted gluconeogenic response of the liver under impaired hormonal signals observed in T2DM, resulting in an increased risk of hypoglycemia. The inability of hepatic glucose metabolism to effectively counterbalance a decline of the blood glucose level becomes even more pronounced in case of tightly controlled insulin treatment. Given this Janus face mode of action of insulin, our model simulations underline the great potential that normalization of the plasma glucagon profile may have for the treatment of T2DM.     

Early Neuroendocrine Alterations in Female Rats Following a Diet Moderately Enriched in Fat

1. High-fat diets disrupt metabolic equilibrium and hypothalamic-pituitary-adrenal axis function and may lead to the development of metabolic and endocrine dysfunctions. The early neuroendocrine responses elicited by a combination of short-term metabolic and emotional stressors is not fully elucidated. 2. The purpose of the present study was to determine the impact on female rats, of a short-term enriched in fat diet, combined with an acute stressor. 3. Adult female Wistar rats were fed a fat diet for 7 days and subsequently exposed to 5 min swimming stress. Plasma leptin, insulin, glucose, luteinizing hormone (LH) and corticosterone, along with brain corticosteroid receptors' mRNAs were measured at 1 h post stress. 4. Diet, compared to chow, reduced food intake and body weight gain, increased leptin and LH, and decreased glucose in the periphery. The diet increased plasma corticosterone and reduced GR mRNA in the hippocampus, similarly to swim stress. 5. The diet significantly modified the animals' response to the subsequent swim stress, by blocking further corticosterone rise and GR mRNA reduction. In addition, exposure of diet-fed rats to stress, altered their endocrine response, in terms of leptin and LH. 6. These observations suggest that even short, moderately unbalanced diets can affect peripheral and central components of energy balance, reproduction and stress response.     

Influence of a long-term high-fat diet on ghrelin secretion and ghrelin-induced food intake in rats

The aims of this study were: (1) to define the extent to which a high-fat (HF) diet given on a long-term basis reduces resting plasma ghrelin (total [acyl+des-acyl]) levels and the plasma ghrelin (total) response to fasting, (2) to determine whether a chronic HF diet modifies the orexigenic activity of acyl-ghrelin, (3) whether insulin pretreatment inhibits the plasma ghrelin (total) response to fasting, and (4) the extent to which pioglitazone (PIO) treatment will increase stomach and plasma ghrelin (total) levels in rats fed a HF diet. PIO is a drug given to diabetics which improves insulin resistance. Our findings show that a chronic HF diet given for either 10 or 60 weeks exerts a persistent inhibitory effect on resting plasma ghrelin (total) levels. Additionally, the plasma ghrelin (total) elevation to overnight fasting is not altered in rats fed a HF diet on a long-term basis. A HF diet does not impair the ingestive response to acyl-ghrelin. Together, these results suggest that acyl-ghrelin serves as an important orexigenic factor. Results show that insulin pretreatment does not inhibit the plasma ghrelin (total) response to fasting suggesting that meal-induced insulin secretion does not have a role in reducing ghrelin (total) secretion. In rats fed a HF diet, PIO administration increases stomach ghrelin (total) levels. Because PIO can reduce systemic glucose and lipid levels, our findings suggest that elevated glucose and lipid levels are part of the inhibitory mechanism behind reduced ghrelin (total) secretion in rats fed a HF diet.