Abnormal oral glucose tolerance in genetically obese (fa/fa) rats

Synthesis, Antioxidant, and Antidiabetic Activities of Ketone Derivatives of Succinimide

The prevalence of diabetes mellitus is persistently increasing globally creating a serious public health affliction. Diabetes mellitus is categorized into two major types designated as type I and Type II. Type I diabetes mellitus is characterized by complete lack of secretion of insulin, while Type II diabetes mellitus is the resistance of peripheral tissues to the action of insulin and inadequate compensatory secretion of insulin. Chronic hyperglycemia associated with diabetes causes failure of cardiovascular system, nervous system, kidneys, and eyes. At present, different types of drugs are used for the management of diabetes, but each of them is associated with more or less serious side effects. Therefore, we need to develop new therapeutic agents that have better efficacy and safety profile. In this study, three ketone derivatives of succinimides were synthesized based on Michael addition and characterized using NMR. All the synthesized compounds were checked for their in vitro α-amylase and α-glucosidase inhibitory activities. Further the synthesized compounds were also explored for their antioxidant activities, i.e, DPPH and ABTS assays. Based on the in vitro results, the synthesized compounds were further evaluated for in vivo antidiabetic activity. The synthesized compounds were (2-oxocyclohexyl)-1-phenylpyrrolidine-2,5-dione (BW1), benzyl-3-(2-oxocyclohexyl) pyrrolidine-2,5-dione (BW2), and (4-bromophenyl)-3-(2-oxocyclohexyl) pyrrolidine-2,5-dione (BW3). BW1 showed the highest inhibitory activity for DPPH causing 83.03 ± 0.48 at 500 μg/ml with IC50 value of 10.84 μg/ml and highest inhibitory activity for ABTS causing 78.35 ± 0.23 at 500 μg/ml with IC50 value of 9.40 μg/ml against ascorbic acid used as standard. BW1 also exhibited the highest activity against α-amylase and α-glucosidase inhibition causing 81.60 ± 0.00 at concentrations of 500 μg/ml with IC50 value of 13.90 μg/ml and 89.08 ± 1.04 at concentrations of 500 μg/ml with IC50 value of 10.49 μg/ml, respectively, against the standard drug acarbose.

Leptin resistance before and after obesity: evidence that tissue glucose uptake underlies adipocyte enlargement and liver steatosis/steatohepatitis in Zucker rats from early-life stages

BACKGROUND

Leptin resistance occurs in obese patients, but its independent contribution to adiposity and the accompanying metabolic diseases, i.e., diabetes, liver steatosis, and steatohepatitis, remains to be established. This study was conducted in an extreme model of leptin resistance to investigate mechanisms initiating diabetes, fat expansion, liver steatosis, and inflammatory disease, focusing on the involvement of glucose intolerance and organ-specific glucose uptake in brown and subcutaneous adipose tissues (BAT, SAT) and in the liver.

METHODS

We studied preobese and adult Zucker rats (fa/fa, fa/+ ) during fasting or glucose loading to assess glucose tolerance. Relevant pancreatic and intestinal hormonal levels were measured by Milliplex. Imaging of ¹⁸F-fluorodeoxyglucose by positron emission tomography was used to quantify glucose uptake in SAT, BAT, and liver, and evaluate its relationship with adipocyte size and biopsy-proven nonalcoholic fatty liver disease (NAFLD) and steatohepatitis (NASH).

RESULTS

Preobese fa/fa pups showed impaired glucose tolerance, adipocyte enlargement, hepatic microsteatosis, and lobular inflammation, with elevated hepatic post-glucose load glucose uptake and production. Adult fa/fa rats had more severe glucose intolerance, fasting hyperglycemia, hormonal abnormalities, elevated glucose uptake in SAT and BAT, and more markedly in the liver, together with macrosteatosis, and highly prevalent hepatic inflammation. Organ glucose uptake was proportional to the degree of fat accumulation and tissue inflammation and was able to dissect healthy from NAFLD and NAFLD/NASH livers. Most severe NASH livers showed a decline in glucose uptake and liver enzymes.

CONCLUSIONS

In fa/fa Zucker rats, leptin resistance leads to glucose intolerance, mainly due to hepatic glucose overproduction, preceding obesity, and explaining pancreatic and intestinal hormonal changes and fat accumulation in adipocytes and hepatocytes. Our data support the involvement of liver glucose uptake in the pathogenesis of liver inflammatory disease. Its potential as more generalized biomarker or diagnostic approach remains to be established outside of our leptin-receptor-deficient rat model.

Leptin Receptors Are Not Required for Roux-en-Y Gastric Bypass Surgery to Normalize Energy and Glucose Homeostasis in Rats

Sensitization to the adipokine leptin is a promising therapeutic strategy against obesity and its comorbidities and has been proposed to contribute to the lasting metabolic benefits of Roux-en-Y gastric bypass (RYGB) surgery. We formally tested this idea using Zucker fatty fa/fa rats as an established genetic model of obesity, glucose intolerance, and fatty liver due to leptin receptor deficiency. We show that the changes in body weight in these rats following RYGB largely overlaps with that of diet-induced obese Wistar rats with intact leptin receptors. Further, food intake and oral glucose tolerance were normalized in RYGB-treated Zucker fatty fa/fa rats to the levels of lean Zucker fatty fa/+ controls, in association with increased glucagon-like peptide 1 (GLP-1) and insulin release. In contrast, while fatty liver was also normalized in RYGB-treated Zucker fatty fa/fa rats, their circulating levels of the liver enzyme alanine aminotransferase (ALT) remained elevated at the level of obese Zucker fatty fa/fa controls. These findings suggest that the leptin system is not required for the normalization of energy and glucose homeostasis associated with RYGB, but that its potential contribution to the improvements in liver health postoperatively merits further investigation.

Melatonin Improves Endoplasmic Reticulum Stress-Mediated IRE1α Pathway in Zücker Diabetic Fatty Rat

Obesity and diabetes are linked to an increased prevalence of kidney disease. Endoplasmic reticulum stress has recently gained growing importance in the pathogenesis of obesity and diabetes-related kidney disease. Melatonin, is an important anti-obesogenic natural bioactive compound. Previously, our research group showed that the renoprotective effect of melatonin administration was associated with restoring mitochondrial fission/fusion balance and function in a rat model of diabesity-induced kidney injury. This study was carried out to further investigate whether melatonin could suppress renal endoplasmic reticulum (ER) stress response and the downstream unfolded protein response activation under obese and diabetic conditions. Zücker diabetic fatty (ZDF) rats and lean littermates (ZL) were orally supplemented either with melatonin (10 mg/kg body weight (BW)/day) (M-ZDF and M-ZL) or vehicle (C-ZDF and C-ZL) for 17 weeks. Western blot analysis of ER stress-related markers and renal morphology were assessed. Compared to C-ZL rats, higher ER stress response associated with impaired renal morphology was observed in C-ZDF rats. Melatonin supplementation alleviated renal ER stress response in ZDF rats, by decreasing glucose-regulated protein 78 (GRP78), phosphoinositol-requiring enzyme1α (IRE1α), and ATF6 levels but had no effect on phospho-protein kinase RNA-like endoplasmic reticulum kinase (PERK) level. In addition, melatonin supplementation also restrained the ER stress-mediated apoptotic pathway, as indicated by decreased pro-apoptotic proteins phospho-c-jun amino terminal kinase (JNK), Bax, and cleaved caspase-3, as well as by upregulation of B cell lymphoma (Bcl)-2 protein. These improvements were associated with renal structural recovery. Taken together, our findings revealed that melatonin play a renoprotective role, at least in part, by suppressing ER stress and related pro-apoptotic IRE1α/JNK signaling pathway.

GK-rats respond to gastric bypass surgery with improved glycemia despite unaffected insulin secretion and beta cell mass

Roux-en-Y gastric bypass (RYGB) is the most effective treatment for morbid obesity and results in rapid remission of type 2 diabetes (T2D), before significant weight loss occurs. The underlying mechanisms for T2D remission are not fully understood. To gain insight into these mechanisms we used RYGB-operated diabetic GK-rats and Wistar control rats. Twelve adult male Wistar- and twelve adult male GK-rats were subjected to RYGB- or sham-operation. Oral glucose tolerance tests (OGTT) were performed six weeks after surgery. RYGB normalized fasting glucose levels in GK-rats, without affecting fasting insulin levels. In both rat strains, RYGB caused increased postprandial responses in glucose, GLP-1, and GIP. RYGB caused elevated postprandial insulin secretion in Wistar-rats, but had no effect on insulin secretion in GK-rats. In agreement with this, RYGB improved HOMA-IR in GK-rats, but had no effect on HOMA-β. RYGB-operated GK-rats had an increased number of GIP receptor and GLP-1 receptor immunoreactive islet cells, but RYGB had no major effect on beta or alpha cell mass. Furthermore, in RYGB-operated GK-rats, increased Slc5a1, Pck2 and Pfkfb1 and reduced Fasn hepatic mRNA expression was observed. In summary, our data shows that RYGB induces T2D remission and enhanced postprandial incretin hormone secretion in GK-rats, without affecting insulin secretion or beta cell mass. Thus our data question the dogmatic view of how T2D remission is achieved and instead point at improved insulin sensitivity as the main mechanism of remission.

Insulin-Regulated Aminopeptidase Inhibition Ameliorates Metabolism in Obese Zucker Rats

The aim of our study was to determine the influence of inhibition of insulin-regulated aminopeptidase/oxytocinase (IRAP) on glucose tolerance and metabolism of skeletal muscle and visceral adipose tissue in obese Zucker rats. Obese Zucker rats administered with IRAP inhibitor-HFI-419 at a dose of 29 μg/100 g BW/day by osmotic minipumps implanted subcutaneously for 2 weeks. Two-hour intraperitoneal glucose tolerance test (ipGTT) was performed in fasting rats. Plasma oxytocin levels were measured by enzyme immunoassay after plasma extraction. In the musculus quadriceps and epididymal adipose tissue, the expression of factors affecting tissue oxidative status and metabolism was determined by real-time qPCR and/or Western blot analysys. The plasma and tissue enzymatic activities were determined by colorimetric or fluorometric method. Circulated oxytocin levels in obese animals strongly tended to increase after HFI-419 administration. This was accompanied by significantly improved glucose utilization during ipGTT and decreased area under the curve (AUC) for glucose. In skeletal muscle IRAP inhibitor treatment up-regulated enzymes of antioxidant defense system - superoxide dismutase 1 and 2 and improved insulin signal transduction pathway. HFI-419 increased skeletal muscle aminopeptidase A expression and activity and normalized its plasma levels in obese animals. In epididymal adipose tissue, gene expression of markers of inflammation and adipocyte hypertrophy was down-regulated in obese rats after HFI-419 treatment. Our results demonstrate that IRAP inhibition improves whole-body glucose tolerance in insulin-resistant Zucker fatty rats and that this metabolic effect of HFI-419 involves ameliorated redox balance in skeletal muscle.

Obesity and Cage Environment Modulate Metabolism in the Zucker Rat: A Multiple Biological Matrix Approach to Characterizing Metabolic Phenomena

Obesity and its comorbidities are increasing worldwide imposing a heavy socioeconomic burden. The effects of obesity on the metabolic profiles of tissues (liver, kidney, pancreas), urine, and the systemic circulation were investigated in the Zucker rat model using ¹H NMR spectroscopy coupled to multivariate statistical analysis. The metabolic profiles of the obese ( fa/ fa) animals were clearly differentiated from the two phenotypically lean phenotypes, ((+/+) and ( fa/+)) within each biological compartment studied, and across all matrices combined. No significant differences were observed between the metabolic profiles of the genotypically distinct lean strains. Obese Zucker rats were characterized by higher relative concentrations of blood lipid species, cross-compartmental amino acids (particularly BCAAs), urinary and liver metabolites relating to the TCA cycle and glucose metabolism; and lower amounts of urinary gut microbial-host cometabolites, and intermatrix metabolites associated with creatine metabolism. Further to this, the obese Zucker rat metabotype was defined by significant metabolic alterations relating to disruptions in the metabolism of choline across all compartments analyzed. The cage environment was found to have a significant effect on urinary metabolites related to gut-microbial metabolism, with additional cage-microenvironment trends also observed in liver, kidney, and pancreas. This study emphasizes the value in metabotyping multiple biological matrices simultaneously to gain a better understanding of systemic perturbations in metabolism, and also underscores the need for control or evaluation of cage environment when designing and interpreting data from metabonomic studies in animal models.

Impact of obesity as an independent risk factor for the development of renal injury: implications from rat models of obesity

Diabetes and hypertension are the major causes of chronic kidney disease (CKD). Epidemiological studies within the last few decades have revealed that obesity-associated renal disease is an emerging epidemic and that the increasing prevalence of obesity parallels the increased rate of CKD. This has led to the inclusion of obesity as an independent risk factor for CKD. A major complication when studying the relationship between obesity and renal injury is that cardiovascular and metabolic disorders that may result from obesity including hyperglycemia, hypertension, and dyslipidemia, or the cluster of these disorders [defined as the metabolic syndrome, (MetS)] also contribute to the development and progression of renal disease. The associations between hyperglycemia and hypertension with renal disease have been reported extensively in patients suffering from obesity. Currently, there are several obese rodent models (high-fat diet-induced obesity and leptin signaling dysfunction) that exhibit characteristics of MetS. However, the available obese rodent models currently have not been used to investigate the impact of obesity alone on the development of renal injury before hypertension and/or hyperglycemia. Therefore, the aim of this review is to describe the incidence and severity of renal disease in these rodent models of obesity and determine which models are suitable to study the independent effects obesity on the development and progression of renal disease.

Diabetes-associated microbiota in fa/fa rats is modified by Roux-en-Y gastric bypass

Roux-en-Y gastric bypass (RYGB) and duodenal jejunal bypass (DJB), two different forms of bariatric surgery, are associated with improved glucose tolerance, but it is not clear whether the gut microbiota contributes to this effect. Here we used fa/fa rats as a model of impaired glucose tolerance to investigate whether (i) the microbiota varies between fa/fa and nondiabetic fa/+ rats; (ii) the microbiota of fa/fa rats is affected by RYGB and/or DJB; and (iii) surgically induced microbiota alterations contribute to glucose metabolism. We observed a profound expansion of Firmicutes (specifically, Lactobacillus animalis and Lactobacillus reuteri) in the small intestine of diabetic fa/fa compared with nondiabetic fa/+ rats. RYGB-, but not DJB-, treated fa/fa rats exhibited greater microbiota diversity in the ileum and lower L. animalis and L. reuteri abundance compared with sham-operated fa/fa rats in all intestinal segments, and their microbiota composition resembled that of unoperated fa/+ rats. To investigate the functional role of RYGB-associated microbiota alterations, we transferred microbiota from sham- and RYGB-treated fa/fa rats to germ-free mice. The metabolic phenotype of RYGB-treated rats was not transferred by the transplant of ileal microbiota. In contrast, postprandial peak glucose levels were lower in mice that received cecal microbiota from RYGB- versus sham-operated rats. Thus, diabetes-associated microbiota alterations in fa/fa rats can be modified by RYGB, and modifications in the cecal microbiota may partially contribute to improved glucose tolerance after RYGB.

The glucoregulatory actions of leptin

Background

The hormone leptin is an important regulator of metabolic homeostasis, able to inhibit food intake and increase energy expenditure. Leptin can also independently lower blood glucose levels, particularly in hyperglycemic models of leptin or insulin deficiency. Despite significant efforts and relevance to diabetes, the mechanisms by which leptin acts to regulate blood glucose levels are not fully understood.

Scope of review

Here we assess literature relevant to the glucose lowering effects of leptin. Leptin receptors are widely expressed in multiple cell types, and we describe both peripheral and central effects of leptin that may be involved in lowering blood glucose. In addition, we summarize the potential clinical application of leptin in regulating glucose homeostasis.

Major conclusions

Leptin exerts a plethora of metabolic effects on various tissues including suppressing production of glucagon and corticosterone, increasing glucose uptake, and inhibiting hepatic glucose output. A more in-depth understanding of the mechanisms of the glucose-lowering actions of leptin may reveal new strategies to treat metabolic disorders.

Effects of xanthine oxidase inhibition with febuxostat on the development of nephropathy in experimental type 2 diabetes

BACKGROUND AND PURPOSE

Elevated serum uric acid (UA) is a risk factor for the development of kidney disease. Inhibitors of xanthine oxidase (XOi), an enzyme involved in UA synthesis, have protective effects at early stages of experimental diabetic nephropathy (DN). However, long-term effects of XOi in models of DN remain to be determined.

EXPERIMENTAL APPROACH

The development of albuminuria, renal structure and molecular markers of DN were studied in type 2 diabetic Zucker obese (ZO) rats treated for 18 weeks with the XOi febuxostat and compared with vehicle-treated ZO rats, ZO rats treated with enalapril or a combination of both agents, and lean Zucker rats without metabolic defects.

RESULTS

Febuxostat normalized serum UA and attenuated the development of albuminuria, renal structural changes, with no significant effects on BP, metabolic control or systemic markers of oxidative stress (OS). Most of these actions were comparable with those of enalapril. Combination treatment induced marked decreases in BP and was more effective in ameliorating structural changes, expression of profibrotic genes and systemic OS than either monotherapy. Febuxostat attenuated renal protein expression of TGF-ß, CTGF, collagen 4, mesenchymal markers (FSP1 and vimentin) and a tissue marker of OS nitrotyrosine. Moreover, febuxostat attenuated TGF-ß- and S100B-induced increased expression of fibrogenic molecules in renal tubular cells in vitro in UA-free media in an Akt kinase-dependent manner.

CONCLUSIONS AND IMPLICATIONS

Febuxostat is protective and enhances the actions of enalapril in experimental DN. Multiple mechanisms might be involved, such as a reduction of UA, renal OS and inhibition of profibrotic signalling.

Antioxidant diet and sex interact to regulate NOS isoform expression and glomerular mesangium proliferation in Zucker diabetic rat kidney

Oxidative stress contributes substantially to the pathophysiology of diabetic nephropathy (DN). Consumption of an antioxidant-fortified (AO) diet from an early age prevents or delays later development of DN in the Zucker rat female with type 2 diabetes. We hypothesize this is due to effects on mesangial matrix and renal nitric oxide synthase (NOS) distribution and to sex-specific differences in NOS responses in the diabetic kidney. Total glomerular tuft area (GTA) and PAS-positive tuft area (PTA), endothelial (e), neuronal (n) and inducible (i) NOS were quantified in males and females on AO or regular (REG) diet at 6 and 20 weeks of age. eNOS was observed in glomeruli and tubules. nNOS predominantly localized to tubular epithelium in both cortex and medulla. iNOS was expressed in proximal and distal tubules and collecting ducts. Sex, diabetes duration and AO diet affected the distribution of the three isoforms. GTA and PTA increased with duration of hyperglycemia and showed a negative correlation with renal levels of all NOS isoforms. AO diet in both genders was associated with less PAS-positive staining and less mesangial expansion than the REG diet, an early increase in cortical iNOS in males, and sex-specific changes in cortical eNOS at 20 weeks. These effects of AO diet may contribute to sex-specific preservation of renal function in females.

Effect of bariatric surgery combined with medical therapy versus intensive medical therapy or calorie restriction and weight loss on glycemic control in Zucker diabetic fatty rats

Bariatric surgery rapidly improves Type 2 diabetes mellitus (T2DM). Our objective was to profile and compare the extent and duration of improved glycemic control following Roux-en-Y gastric (RYGB) bypass surgery and vertical sleeve gastrectomy (SG) and compare against calorie restriction/weight loss and medical combination therapy-based approaches using the Zucker diabetic fatty rat (ZDF) rodent model of advanced T2DM. Male ZDF rats underwent RYGB (n = 15) or SG surgery (n = 10) at 18 wk of age and received postsurgical insulin treatment, as required to maintain mid-light-phase glycemia within a predefined range (10-15 mmol/l). In parallel, other groups of animals underwent sham surgery with ad libitum feeding (n = 6), with body weight (n = 8), or glycemic matching (n = 8) to the RYGB group, using food restriction or a combination of insulin, metformin, and liraglutide, respectively. Both bariatric procedures decreased the daily insulin dose required to maintain mid-light-phase blood glucose levels below 15 mmol/l, compared with those required by body weight or glycemia-matched rats (P < 0.001). No difference was noted between RYGB and SG with regard to initial efficacy. SG was, however, associated with higher food intake, weight regain, and higher insulin requirements vs. RYGB at study end (P < 0.05). Severe hypoglycemia occurred in several rats after RYGB. RYGB and SG significantly improved glycemic control in a rodent model of advanced T2DM. While short-term outcomes are similar, long-term efficacy appears marginally better after RYGB, although this is tempered by the increased risk of hypoglycemia.

The higher diabetogenic risk of tacrolimus depends on pre-existing insulin resistance. A study in obese and lean Zucker rats

Insulin resistance may interact with calcineurin inhibitors, enhancing the diabetogenic effect of tacrolimus compared with cyclosporine-A. We studied both drugs in insulin-resistant animals: obese Zucker rats (n = 45), and insulin-sensitive animals: lean Zucker rats (n = 21). During 11 days, animals received saline-buffer, cyclosporine-A (2.5 mg/kg/day) or tacrolimus (0.3 mg/kg/day). At Days 0 and 12 animals underwent intraperitoneal glucose tolerance test (0-30-60-120 min). Islet morphometry, beta-cell proliferation, apoptosis and Ins2 gene expression were analyzed. By Day 12, no lean animal had developed diabetes, while all obese animals on tacrolimus and 40% on cyclosporine-A had. In obese animals, tacrolimus reduced beta-cell proliferation and Ins2 gene expression compared with cyclosporine-A. Five days after treatment discontinuation, partial recovery was observed, with only 10% and 60% of the animals on cyclosporine and tacrolimus remaining diabetic respectively. Beta-cell proliferation increased in animals on tacrolimus while Ins2 gene expression remained unaltered. In conclusion, insulin resistance exacerbated the diabetogenic effect of tacrolimus compared with cyclosporine-A. This may be explained by greater inhibition of Ins2 gene and beta-cell proliferation by tacrolimus in the insulin resistant state. Discontinuation of the drugs may allow the recovery of the metabolic alterations.

Teneligliptin: a DPP-4 inhibitor for the treatment of type 2 diabetes

Dipeptidyl peptidase-4 (DPP-4) inhibitors have recently emerged as a new class of antidiabetic that show favorable results in improving glycemic control with a minimal risk of hypoglycemia and weight gain. Teneligliptin, a novel DPP-4 inhibitor, exhibits a unique structure characterized by five consecutive rings, which produce a potent and long-lasting effect. Teneligliptin is currently used in cases showing insufficient improvement in glycemic control even after diet control and exercise or a combination of diet control, exercise, and sulfonylurea- or thiazolidine-class drugs. In adults, teneligliptin is orally administered at a dosage of 20 mg once daily, which can be increased up to 40 mg per day. Because the metabolites of this drug are eliminated via renal and hepatic excretion, no dose adjustment is necessary in patients with renal impairment. The safety profile of teneligliptin is similar to those of other available DPP-4 inhibitors. However, caution needs to be exercised when administering teneligliptin to patients who are prone to QT prolongation. One study has reported that the postprandial blood glucose-lowering effects of teneligliptin administered prior to breakfast were sustained throughout the day, and the effects observed after dinner were similar to those observed after breakfast or lunch. Thus, although clinical data for this new drug are limited, this drug shows promise in stabilizing glycemic fluctuations throughout the day and consequently suppressing the progression of diabetic complications. However, continued evaluation in long-term studies and clinical trials is required to evaluate the efficacy and safety of the drug as well as to identify additional indications for its clinical use.

Comparison of adipocyte-specific gene expression from WNIN/Ob mutant obese rats, lean control, and parental control

Adipose tissue development is a highly regulated phenomenon orchestrated by several check points (recruitment of mesenchymal stem cells and their lineage commitment) to form mature adipocytes. Once committed to obesity, expansion of adipose tissue occurs either by hypertrophy or hyperplasia or by both resulting in an altered physiological status. This precipitates as inflammatory responses, leading to endoplasmic reticulum and oxidative stress altering the gene expression of adipose tissue in a depot-specific manner. However, such studies reporting a phased gene expression profile in conditions of rodent obesity are not reported so far. WNIN/Ob mutant obese rat, developed at our institute is an excellent model to study the pathophysiological changes underlying obesity. Here, we report the gene expression profile of this mutant rat (obese and lean), compared with the parental control, with reference to markers of embryonic stem cells, adipogenesis, inflammation, and senescence in both subcutaneous (SCAT) and retroperitoneal (RPAT) adipose depots representing abdominal fat. We demonstrate an upregulation of genes such as Sox-2, Pref-1, PPARγ2, LPL, IRS-1, GLUT-4, IL-6, TNFα, and telomerase in SCAT and RPAT depots of the obese rat compared to its lean counterpart indicating no difference in fat depots at different locations. This is suggestive of a similar phenotypic expression of mutant gene. Data form the phased gene expression changes of adipogenesis (embryonic/adipogenic/inflammatory) in the present obese rat model system advocate for inflammatory mediated response(s) associated with obesity-a condition often seen in humans.

AMG 837: a novel GPR40/FFA1 agonist that enhances insulin secretion and lowers glucose levels in rodents

Agonists of GPR40 (FFA1) have been proposed as a means to treat type 2 diabetes. Through lead optimization of a high throughput screening hit, we have identified a novel GPR40 agonist called AMG 837. The objective of these studies was to understand the preclinical pharmacological properties of AMG 837. The activity of AMG 837 on GPR40 was characterized through GTPγS binding, inositol phosphate accumulation and Ca(2+) flux assays. Activity of AMG 837 on insulin release was assessed on isolated primary mouse islets. To determine the anti-diabetic activity of AMG 837 in vivo, we tested AMG 837 using a glucose tolerance test in normal Sprague-Dawley rats and obese Zucker fatty rats. AMG 837 was a potent partial agonist in the calcium flux assay on the GPR40 receptor and potentiated glucose stimulated insulin secretion in vitro and in vivo. Acute administration of AMG 837 lowered glucose excursions and increased glucose stimulated insulin secretion during glucose tolerance tests in both normal and Zucker fatty rats. The improvement in glucose excursions persisted following daily dosing of AMG 837 for 21-days in Zucker fatty rats. Preclinical studies demonstrated that AMG 837 was a potent GPR40 partial agonist which lowered post-prandial glucose levels. These studies support the potential utility of AMG 837 for the treatment of type 2 diabetes.

The prostacyclin analog beraprost sodium ameliorates characteristics of metabolic syndrome in obese Zucker (fatty) rats

OBJECTIVE

The prostacyclin analog, beraprost sodium (BPS), was examined for its potential to improve the symptoms of obesity-type diabetes (i.e., hyperglycemia, hyperinsulinemia, dyslipidemia, histopathologic changes, and diabetic complications).

RESEARCH DESIGN AND METHODS

Obese Zucker rats, an experimental model of genetic obesity-induced type 2 diabetes, were repeatedly administered BPS at oral doses of 0.2 or 0.6 mg x kg(-1) x day(-1) b.i.d. for 12 weeks, and serum chemistry, urinalysis, and histopathologic examination were performed.

RESULTS

BPS dose-dependently suppressed serum glucose, insulin, triglyceride, and cholesterol levels in obese animals. In oral glucose tolerance test, BPS suppressed the post-glucose-loading elevation of serum glucose in a dose-dependent manner. Urinary N-acetyl-beta-D-glucosaminidase was significantly lower in BPS-treated obese animals compared with control animals, although no significant differences were observed in urinary protein levels between the BPS-treated groups and the control group. In addition, histopathologic examination revealed significant protective effects of BPS against renal disorder in obese animals. Histopathologically, BPS also inhibited the progression of hepatic steatosis, hypertrophy of adipose tissue, and pancreatic fibrosis. Furthermore, thermographic analysis of the hind limb sole skin surface indicated a significant increase in temperature in BPS-treated animals, compared with control animals, which was likely due to improved blood circulation by administration of BPS.

CONCLUSIONS

BPS suppressed the pathogenesis and development of diabetes and its complication, nephropathy, which was presumably accompanied by improving glucose intolerance and insulin resistance in obese Zucker rats.

Impaired overload-induced hypertrophy in obese Zucker rat slow-twitch skeletal muscle

The effect of insulin resistance (IR) on the adaptation of skeletal muscle loading is not well understood. Here we examine whether the soleus muscles of the lean Zucker (LZ) and insulin-resistant obese Zucker (OZ) rat exhibit differences in their ability to undergo muscle hypertrophy following 8 wk of mechanical overload. Four-week-old male LZ ( n = 5) and OZ ( n = 5) rats underwent unilateral surgical ablation of the gastrocnemius muscle while the contralateral hindlimb was used as an internal control. Mechanical overload increased soleus muscle wet weight (LZ 57% and OZ 33%, respectively; P < 0 .05) and average type 1 fiber cross-sectional area (LZ 32% and OZ 5%, respectively; P < 0.05) in LZ and OZ rats, while the magnitude of these increases was greater in the LZ animals ( P < 0 .05). The reduced degree of muscle hypertrophy observed in the OZ animals was associated with decreases in the ability of the OZ soleus muscle to phosphorylate p70s6kThr 389 and mTOR, while phosphorylation of p70s6kThr 389 was increased in the LZ overloaded soleus by 83% ( P < 0 .05). The amount of Tuberin/TSC2 phosphorylation, an inhibitor of mTOR, was unchanged in the LZ soleus after overload while it was increased (68.3%, P < 0.05) in OZ animals. Conversely, AMPK phosphorylation was decreased in the LZ (−22.77%, P < 0 .05) but increased (57%, P < 0 .05) in the OZ soleus with overload. Taken together, these data suggest that IR or other related comorbidities may impair the ability of the soleus to activate mTOR signaling and undergo load-induced muscle hypertrophy.

Lean and obese Zucker rats exhibit different patterns of p70s6 kinase regulation in the tibialis anterior muscle in response to high-force muscle contraction

Increased phosphorylation of the 70-kDa ribosomal S6 kinase (p70S6k) signaling is strongly correlated with the degree of muscle adaptation following exercise. Herein we compare the phosphorylation of p70S6k, Akt, and mammalian target of rapamycin (mTOR) in the tibialis anterior (TA) muscles of lean and obese Zucker rats following a bout of eccentric exercise. Exercise increased p70S6k (Thr389) phosphorylation immediately after (33.3+/-7.2%) and during [1 h (24.0+/-14.9%) and 3 h (24.6+/-11.3%)] recovery in the lean TA and at 3 h (33.5+/-8.0%) in the obese TA Zucker rats. mTOR (Ser2448) phosphorylation was elevated in the lean TA immediately after exercise (96.5+/-40.3%) but remained unaltered in the obese TA. Exercise increased Akt (Thr308) and Akt (Ser473) phosphorylation in the lean but not the obese TA. These results suggest that insulin resistance is associated with alterations in the ability of muscle to activate p70S6k signaling following an acute bout of exercise.

Mechanisms of glucose homeostasis after Roux-en-Y gastric bypass surgery in the obese, insulin-resistant Zucker rat

OBJECTIVE

Obesity-related diabetes is caused by insulin resistance and beta-cell dysfunction. The current study examines changes in food intake, weight loss, body fat depots, oxygen consumption, insulin sensitivity, and incretin levels as potential mechanisms for improved glucose tolerance after Roux-en-Y gastric bypass (RYGB).

METHODS

Three groups of genetically obese Zucker rats were studied: RYGB, sham surgery pair-fed (PF), and sham surgery ad libitum (AL) fed rats. Changes in body weight, visceral and subcutaneous fat depots, oral glucose tolerance, insulin sensitivity, and the plasma concentrations of insulin, glucagon, glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic peptide, and peptide YY (PYY) were measured.

RESULTS

Body weight and subcutaneous fat were decreased after RYGB, compared with the PF and AL groups. The reduction in visceral fat after RYGB appeared largely because of food restriction. Glucose tolerance and insulin sensitivity were significantly improved in only the RYGB group (P < 0.05 vs. AL, PF). Euglycemic, hyperinsulinemic clamp studies indicated RYGB improved the ability of insulin to stimulate peripheral (eg, skeletal muscle) glucose uptake. Fasting total GLP-1, glucose-dependent insulinotropic peptide, and PYY levels were similar between the groups, whereas postprandial plasma levels of intact GLP-1 (7-36) amide, total GLP-1, and PYY were increased in the RYGB group compared with PF and AL controls.

CONCLUSIONS

Glucose homeostasis after RYGB is associated with decreased subcutaneous fat, increased postprandial PYY, GLP-1, and insulin, as well as improved insulin sensitivity/action. Changes in food intake and visceral fat do not seem to explain improvements in insulin action after RYGB in the Zucker rat model.

Protective effects of antioxidant-fortified diet on renal function and metabolic profile in obese Zucker rat

Oxidative stress contributes to the pathophysiology of type 2 diabetes mellitus and its complications, including nephropathy. The current study was designed to test the hypothesis that a diet fortified with antioxidants would be beneficial to delay or prevent the progression of this disease. Male and female Zucker fa/fa rats were fed a control or an antioxidant (AO)-fortified diet starting at 4 weeks of age. Metabolic parameters, renal function, and renal histopathology were analyzed at 6, 13, and 20 weeks of age. Females on the AO diet had significantly lower blood glucose at 6 and 13 weeks, less severe renal pathology at 20 weeks, and higher glomerular filtration rates (GFR) at 20 weeks than age-matched females on the regular diet (P < 0.05). Metabolic parameters including blood glucose, insulin resistance, and serum cholesterol, and mean arterial pressure (MAP), worsened with age in both males and females, as expected. GFR decreased and renal pathology also became more severe with age. Finally, females on the AO diet had higher GFRs and lower MAP at 20 weeks than males on the same diet. This may denote a protective effect of the AO diet in females, but not in males. These findings may have implications for the role of antioxidants as therapy in humans with T2DM.

Development of a sleeve gastrectomy weight loss model in obese Zucker rats

BACKGROUND

Obesity promotes the development of diabetes and cardiovascular disease. The most effective weight loss treatment is bariatric surgery, but results greatly vary depending on the procedure. Sleeve gastrectomy (SG) has recently emerged as a reduced risk weight loss procedure for super obese patients. However, the mechanism of weight loss from SG and its effects on obesity-induced complications are yet to be determined. Our goal was to develop an experimental model of SG in genetically obese rats.

MATERIALS AND METHODS

Male obese Zucker rats (400-500 g, leptin-insensitive) were anesthetized with isoflurane. After a midline laparotomy, the stomach was clamped, the greater curvature was excised, and a triple suture line was used to close the gastric remnant. Sham rats underwent laparotomy only. Metabolic parameters were followed for 14 d after surgery.

RESULTS

Caloric intake and body weight decreased in SG rats over 14 d by 98 +/- 10 kcal/d and 74 +/- 14 g, respectively. Blood total cholesterol levels were lower in rats that lost weight. Furthermore, blood glucose levels were lower in rats that lost weight. Active ghrelin levels were unchanged in SG rats 14 d after surgery.

CONCLUSIONS

These results show that SG promotes weight loss in obese Zucker rats. Furthermore, SG-induced weight loss is accompanied by improved plasma cholesterol and glucose profile. However, SG does not promote a prolonged decrease in ghrelin levels. These results suggest that SG is an effective weight loss procedure in leptin insensitivity to improve the lipid profile and decrease insulin resistance and these effects might be independent of changes in ghrelin levels.

Renal activity of Akt kinase in obese Zucker rats

Insulin resistance (IR) and consequent hyperinsulinemia are hallmarks of Type 2 diabetes (DM2). Akt kinase (Akt) is an important molecule in insulin signaling, implicated in regulation of glucose uptake, cell growth, cell survival, protein synthesis, and endothelial nitric oxide (NO) production. Impaired Akt activation in insulin-sensitive tissues contributes to IR. However, Akt activity in other tissues, particularly those affected by complications of DM2, has been less studied. We hypothesized that hyperinsulinemia could have an impact on activity of Akt and its effectors involved in regulation of renal morphology and function in DM2. To address this issue, renal cortical Akt was determined in obese Zucker rats (ZO), a model of DM2, and lean controls (ZL). We also studied expression and phosphorylation of the mammalian target of rapamycin (mTOR) and endothelial NO synthase (eNOS), molecules downstream of Akt in the insulin signaling cascade, and documented modulators of renal injury. Akt activity was measured by a kinase assay with GSK-3 as a substrate. Expression of phosphorylated (active) and total proteins was measured by immunoblotting and immunohistochemistry. Renal Akt activity was increased in ZO as compared to ZL rats, in parallel with progressive hyperinsulinemia. No differences in Akt were observed in the skeletal muscle. Corresponding to increases in Akt activity, ZO rats demonstrated enhanced phosphorylation of renal mTOR. Acute PI3K inhibition with wortmannin (100 mug/kg) attenuated renal Akt and mTOR activities in ZO, but not in ZL rats. In contrast to mTOR, eNOS phosphorylation was similar in ZO and ZL rats, despite higher total eNOS expression. In conclusion, ZO rats demonstrated increases in renal Akt and mTOR activity and expression. However, eNOS phosphorylation did not follow this pattern. These data suggest that DM2 is associated with selective IR in the kidney, allowing pro-growth signaling via mTOR, whereas potentially protective effects mediated by eNOS are blunted.

Metabolic syndrome increases endogenous carbon monoxide production to promote hypertension and endothelial dysfunction in obese Zucker rats

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide (CO). Increased heme-derived CO inhibits nitric oxide synthase and can contribute to hypertension via endothelial dysfunction in Dahl salt-sensitive rats. Obese Zucker rats (ZR) are models of metabolic syndrome. This study tests the hypothesis that endogenous CO formation is increased and contributes to hypertension and endothelial dysfunction in obese ZR. Awake obese ZR showed increased respiratory CO excretion, which was lowered by HO inhibitor administration [zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) 25 micromol.kg(-1).24 h(-1) ip]. In awake obese ZR, chronically instrumented with femoral arterial catheters, blood pressure was elevated but was decreased by the HO inhibitor ZnDPBG. Body weight, blood glucose, glycated hemoglobin, plasma insulin, total and LDL cholesterol, oxidized LDL, and triglyceride levels were elevated in obese ZR, and, except for LDL cholesterol, were unchanged by HO inhibition. Total HO-1 protein levels were not different between lean and obese ZR aortas. In vitro experiments used isolated skeletal muscle arterioles with constant pressure and no flow, or constant midpoint, but altered endpoint pressures to establish graded levels of luminal flow. In obese ZR arterioles, responses to ACh and flow were attenuated. Acute in vitro pretreatment with an HO inhibitor, chromium mesoporphyrin, enhanced ACh and flow-induced dilation and abolished the differences between groups. Furthermore, exogenous CO prevented the restoration of flow-induced dilation by the HO inhibitor in obese ZR arterioles. These results suggest that HO-derived CO production is increased and promotes hypertension and arteriolar endothelial dysfunction in obese ZR with metabolic syndrome independent of affecting metabolic parameters.

Tesaglitazar, a dual PPARα/γ agonist, ameliorates glucose and lipid intolerance in obese Zucker rats

Insulin resistance, impaired glucose tolerance, high circulating levels of free fatty acids (FFA), and postprandial hyperlipidemia are associated with the metabolic syndrome, which has been linked to increased risk of cardiovascular disease. We studied the metabolic responses to an oral glucose/triglyceride (TG) (1.7/2.0 g/kg lean body mass) load in three groups of conscious 7-h fasted Zucker rats: lean healthy controls, obese insulin-resistant/dyslipidemic controls, and obese rats treated with the dual peroxisome proliferator-activated receptor α/γ agonist, tesaglitazar, 3 μmol·kg−1·day−1for 4 wk. Untreated obese Zucker rats displayed marked insulin resistance, as well as glucose and lipid intolerance in response to the glucose/TG load. The 2-h postload area under the curve values were greater for glucose (+19%), insulin (+849%), FFA (+53%), and TG (+413%) compared with untreated lean controls. Treatment with tesaglitazar lowered fasting plasma glucose, improved glucose tolerance, substantially reduced fasting and postload insulin levels, and markedly lowered fasting TG and improved lipid tolerance. Fasting FFA were not affected, but postprandial FFA suppression was restored to levels seen in lean controls. Mechanisms of tesaglitazar-induced lowering of plasma TG were studied separately using the Triton WR1339 method. In anesthetized, 5-h fasted, obese Zucker rats, tesaglitazar reduced hepatic TG secretion by 47%, increased plasma TG clearance by 490%, and reduced very low-density lipoprotein (VLDL) apolipoprotein CIII content by 86%, compared with obese controls. In conclusion, the glucose/lipid tolerance test in obese Zucker rats appears to be a useful model of the metabolic syndrome that can be used to evaluate therapeutic effects on impaired postprandial glucose and lipid metabolism. The present work demonstrates that tesaglitazar ameliorates these abnormalities and enhances insulin sensitivity in this animal model.

Active involvement of PKC for insulin-mediated rates of muscle protein synthesis in Zucker rats

A recent report from our group demonstrated that insulin facilitates muscle protein synthesis in obese Zucker rats. The purpose of this study was to determine whether PKC, a probable modulator of insulin signal transduction and/or mRNA translation, has a role in this insulin-mediated anabolic response. In the first portion of the study, gastrocnemius muscles of lean and obese Zucker rats (n = 5-7 for each phenotype) were bilaterally perfused with or without insulin to assess cytosolic and membrane PKC activity. Limbs perfused with insulin demonstrated greater PKC activity in both lean and obese Zucker rats (P < 0.05) compared with no insulin, but overall activity was greater in obese animals (by approximately 27% compared with lean, P < 0.05). To determine whether PKC plays a role in muscle protein synthesis, hindlimbs (n = 6-8 for each phenotype) were bilaterally perfused with or without insulin and/or GF-109203X (GF; a PKC inhibitor). The presence of GF did not influence the rates of insulin-mediated protein synthesis in gastrocnemius muscle of lean Zucker rats. However, when obese rats were perfused with GF (P < 0.05), the effect of insulin on elevating rates of protein synthesis was not observed. We also used phorbol 12-myristate 13-acetate (TPA, a PKC activator; n = 5-7 for each phenotype) with and without insulin to determine the effect of PKC activation on muscle protein synthesis. TPA alone did not elevate muscle protein synthesis in lean or obese rats. However, TPA plus insulin resulted in elevated rates of protein synthesis in both phenotypes that were similar to rates of insulin alone of obese rats. These results suggest that PKC is a modulator and is necessary, but not sufficient, for insulin-mediated protein anabolic responses in skeletal muscle.

Long-term AICAR administration reduces metabolic disturbances and lowers blood pressure in rats displaying features of the insulin resistance syndrome

The insulin resistance syndrome is characterized by several risk factors for cardiovascular disease. Chronic chemical activation of AMP-activated protein kinase by the adenosine analog 5-aminoimidazole-4-carboxamide-1-beta -D-ribofuranoside (AICAR) has been shown to augment insulin action, upregulate mitochondrial enzymes in skeletal muscles, and decrease the content of intra-abdominal fat. Furthermore, acute AICAR exposure has been found to reduce sterol and fatty acid synthesis in rat hepatocytes incubated in vitro as well as suppress endogenous glucose production in rats under euglycemic clamp conditions. To investigate whether chronic AICAR administration, in addition to the beneficial effects on insulin sensitivity, is capable of improving other phenotypes associated with the insulin resistance syndrome, obese Zucker (fa/fa) rats (n = 6) exhibiting insulin resistance, hyperlipidemia, and hypertension were subcutaneously injected with AICAR (0.5 mg/g body wt) daily for 7 weeks. Obese control rats were either pair-fed (PF) (n = 6) or ad libitum-fed (AL) (n = 6). Lean Zucker rats (fa/-) (n = 8) served as a reference group. AICAR administration significantly reduced plasma triglyceride levels (P < 0.01 for AICAR vs. AL, and P = 0.05 for AICAR vs. PF) and free fatty acids (P < 0.01 for AICAR vs. AL, and P < 0.05 for AICAR vs. PF) and increased HDL cholesterol levels (P < 0.01 for AICAR vs. AL and PF). AICAR treatment also lowered systolic blood pressure by 14.6 +/- 4.3 mmHg (P < 0.05), and AICAR-treated animals exhibited a tendency toward decreased intra-abdominal fat content. Furthermore, AICAR administration normalized the oral glucose tolerance test and decreased fasting concentrations of glucose and insulin close to the level of the lean animals. Finally, in line with previous findings, AICAR treatment was also found to enhance GLUT4 protein expression and to increase maximally insulin-stimulated glucose transport in primarily white fast-twitch muscles. Our data provide strong evidence that long-term administration of AICAR improves glucose tolerance, improves the lipid profile, and reduces systolic blood pressure in an insulin-resistant animal model. The present study gives additional support to the hypothesis that AMPK activation might be a potential future pharmacological strategy for treating the insulin resistance syndrome.

In vivo effects of insulin and bis(maltolato)oxovanadium (IV) on PKB activity in the skeletal muscle and liver of diabetic rats

In this study, the in vivo effects of insulin and chronic treatment with bis(maltolato)oxovanadium (IV) (BMOV) on protein kinase B (PKB) activity were examined in the liver and skeletal muscle from two animal models of diabetes, the STZ-diabetic Wistar rat and the fatty Zucker rat. Animals were treated with BMOV in the drinking water (0.75-1 mg/ml) for 3 (or 8) weeks and sacrificed with or without insulin injection. Insulin (5 U/kg, i.v.) increased PKBalpha activity more than 10-fold and PKBbeta activity more than 3-fold in both animal models. Despite the development of insulin resistance, insulin-induced activation of PKBalpha was not impaired in the STZ-diabetic rats up to 9 weeks of diabetes, excluding a role for PKBalpha in the development of insulin resistance in type 1 diabetes. Insulin-induced PKBalpha activity was markedly reduced in the skeletal muscle of fatty Zucker rats as compared to lean littermates (fatty: 7-fold vs. lean: 14-fold). In contrast, a significant increase in insulin-stimulated PKBalpha activity was observed in the liver of fatty Zucker rats (fatty: 15.7-fold vs. lean: 7.6-fold). Chronic treatment with BMOV normalized plasma glucose levels in STZ-diabetic rats and decreased plasma insulin levels in fatty Zucker rats but did not have any effect on basal or insulin-induced PKBalpha and PKBbeta activities. In conclusion (i) in STZ-diabetic rats PKB activity was normal up to 9 weeks of diabetes; (ii) in fatty Zucker rats insulin-induced activation of PKBalpha (but not PKBbeta) was markedly altered in both tissues; (iii) changes in PKBalpha activity were tissue specific; (iv) the glucoregulatory effects of BMOV were independent of PKB activity.

Insulin stimulation of muscle protein synthesis in obese Zucker rats is not via a rapamycin-sensitive pathway

The obese Zucker rat is resistant to insulin for glucose disposal, but it is unknown whether this insulin resistance is accompanied by alterations of insulin-mediated muscle protein synthesis. We examined rates of muscle protein synthesis either with or without insulin in lean and obese Zucker rats with the use of a bilateral hindlimb preparation. Additional experiments examined insulin's effect on protein synthesis with or without rapamycin, an inhibitor of protein synthesis. Protein synthesis in red and white gastrocnemius was stimulated by insulin compared with control (no insulin) in obese (n = 10, P<0.05) but not in lean (n = 10, P>0.05) Zucker rats. In white gastrocnemius, rapamycin significantly reduced rates of protein synthesis compared with control in lean (n = 6) and obese (n = 6) rats; however, in red gastrocnemius, the attenuating effect of rapamycin occurred only in obese rats. The addition of insulin to rapamycin resulted in rates of synthesis that were similar to those for rapamycin alone for lean rats and to those for insulin alone (augmented) for obese rats in both tissues. Our results demonstrate that insulin enhances protein synthesis in muscle that is otherwise characterized as insulin resistant. Furthermore, rapamycin inhibits protein synthesis in muscle of obese Zucker rats; however, stimulation of protein synthesis by insulin is not via a rapamycin-sensitive pathway.

Decreased skeletal muscle phosphotyrosine phosphatase (PTPase) activity towards insulin receptors in insulin-resistant Zucker rats measured by delayed Europium fluorescence

In order to measure the phosphotyrosine phosphatase (PTPase) activity in small muscle biopsies, a sandwich-immunofluorescence assay was developed using the phosphorylated human insulin receptor as a substrate, a C-terminal insulin receptor antibody as catching antibody and Europium-labelled anti-phosphotyrosine as detecting antibody. Soluble and particulate muscle fractions were prepared from soleus muscle of obese, diabetic (fa/fa) Zucker rats and their lean littermates (Fa/-). In the soluble muscle fractions of the obese (fa/fa) rats PTPase activity was significantly reduced compared to control (Fa/-) rats (45.2 +/- 2.6% vs 61.3 +/- 4.7%, p < 0.02). This reduction was completely prevented by 24 days of metformin treatment which decreased plasma glucose and plasma insulin levels. In particulate muscle fractions, however, no difference in PTPase activity was found among any groups of rats examined. These results show that the alterations in soluble PTPase activity in the insulin-resistant, diabetic Zucker rat vary with the abnormality in glucose homeostasis.

Renal function and hemodynamic study in obese Zucker rats

OBJECTIVES

To investigate the renal function and hemodynamic changes in obesity and hyperinsulinemia which are characteristics of type II diabetes.

METHODS

Studies were carried out in two groups of female Zucker rats. Group 1 rats were obese Zucker rats with hereditary insulin resistance. Group 2 rats were lean Zucker rats and served as controls. In comparison with lean Zucker rats, obese Zucker rats exhibited hyperinsulinemia but normoglycemia. Micropuncture studies and morphologic studies were performed in these rats.

RESULTS

Functional studies showed that obese Zucker rats exhibited increases in kidney weight and GFR(obese Zucker, 1.23 +/- .07)ml/min; lean Zucker, 0.93 +/- .03ml/min). Micropuncture studies revealed that the increase in GFR in obese Zucker rats was attributable to the increases in the single nephron plasma flow rate and glomerular transcapillary hydraulic pressure. The glomerular ultrafiltration coefficient was the same in both groups. Morphologic studies revealed that the increase in GFR in obese Zucker rats was associated with an increase in glomerular volume.

CONCLUSIONS

These results suggest that obesity and hyperinsulinemia, which are the characteristics of type II diabetes, can be associated with glomerular hyperfiltration and glomerular capillary hypertension.

Metformin ameliorates diabetes but does not normalize the decreased GLUT 4 content in skeletal muscle of obese (fa/fa) Zucker rats

We studied the expression of the glucose transporter GLUT 4 in the soleus and red gastrocnemius muscles from obese, diabetic (fa/fa) Zucker rats compared to their lean littermates (Fa/-), with and without treatment with the antidiabetic drug metformin. In the untreated groups of rats, the GLUT 4 content in a crude membrane fraction of both the soleus and the red gastrocnemius muscles were significantly lower in the obese (fa/fa) rats (3.46 +/- 0.28 vs. 6.04 +/- 0.41, p < 0.001 and 6.0 +/- 0.24 vs. 9.1 +/- 0.48, p < 0.0001, respectively). Differences in GLUT 4 expression in soleus muscle from the same rats were confirmed by quantitative immunofluorescence microscopy, and the results were significantly correlated with the results obtained from quantitative immunoblotting (rho = 0.70, p < 0.0005). The decreased expression of GLUT 4 in fa/fa rats could contribute to the well-established insulin resistance in skeletal muscle of these animals. After 4 weeks of treatment with metformin, weight gain was not affected in either the diabetic (fa/fa) rats or the lean (Fa/-) rats. Improvement of glucose homeostasis by metformin was not associated with normalization of the GLUT 4 expression in the skeletal muscles studied, indicating (1) that the decreased GLUT 4 expression is not directly related to hyperinsulinaemia and diabetes mellitus and (2) that metformin does not normalize the expression of GLUT 4 in skeletal muscle of the diabetic (fa/fa) Zucker rats.

Insulin resistance in obese Zucker rat (fa/fa) skeletal muscle is associated with a failure of glucose transporter translocation

The genetically obese Zucker rat (fa/fa) is characterized by a severe resistance to the action of insulin to stimulate skeletal muscle glucose transport. The goal of the present study was to identify whether the defect associated with this insulin resistance involves an alteration of transporter translocation and/or transporter activity. Various components of the muscle glucose transport system were investigated in plasma membranes isolated from basal or maximally insulin-treated skeletal muscle of lean and obese Zucker rats. Measurements of D- and L-glucose uptake by membrane vesicles under equilibrium exchange conditions indicated that insulin treatment resulted in a four-fold increase in the Vmax for carrier-mediated transport for lean animals [from 4.5 to 17.5 nmol/(mg.s)] but only a 2.5-fold increase for obese rats [from 3.6 to 9.1 nmol/(mg.s)]. In the lean animals, this increase in glucose transport function was associated with a 1.8-fold increase in the transporter number as indicated by cytochalasin B binding, a 1.4-fold increase in plasma membrane GLUT4 protein, and a doubling of the average carrier turnover number (intrinsic activity). In the obese animals, there was no change in plasma membrane transporter number measured by cytochalasin B binding, or in GLUT4 or GLUT1 protein. However, there was an increase in carrier turnover number similar to that seen in the lean litter mates. Measurements of GLUT4 mRNA in red gastrocnemius muscle showed no difference between lean and obese rats. We conclude that the insulin resistance of the obese rats involves the failure of translocation of transporters, while the action of insulin to increase the average carrier turnover number is normal.

Mechanism for increased insulin-stimulated glucose metabolism in adipocytes from 13-week-old obese Zucker rats

A study was made on the mechanism of increased glucose metabolism in enlarged adipocytes from 13-week-old obese Zucker rats showing hyperinsulinaemia and hyperglycaemia. Glucose metabolism was assessed by measuring CO2 production from glucose and the concentration of glucose transporters was estimated by immunoblotting. In comparing adipocytes from obese rats with those from lean rats, the basal rates of glucose oxidation at 0.02 mmol/l glucose increased 2.6-fold per unit cellular surface area and the transporters in the plasma membrane increased 1.4-fold per protein, while that in low-density microsome was 67% of the value in lean rats. The increase of glucose oxidation rates observed in basal cells from obese rats could be partly explained by translocation of the transporters from the intracellular site to the plasma membrane. In the presence of insulin, as the basal rates of glucose oxidation increased in obese rats, maximally insulin-stimulated oxidation increased 4-fold in lean rats and 1.7-fold in obese rats. Thus, the rates of insulin-stimulated oxidation on a per unit cellular surface area as well as the transporters on a per protein basis in the plasma membrane became almost similar in cells from both groups of rats. Since protein content per cell increased with cell enlargement, increased glucose metabolism per cell which was observed in adipocytes from the obese rats was mainly due to an increase of glucose transporters accompanied by a similar degree of cellular protein increase.