Hypersecretion of islet amyloid polypeptide from pancreatic islets of ventromedial hypothalamic-lesioned rats and obese Zucker rats

To investigate the possible role of islet amyloid polypeptide (IAPP) in the development of type 2 diabetes mellitus, we examined the IAPP content and secretion in pancreatic islets isolated from ventromedial hypothalamic (VMH)-lesioned rats and genetically obese Zucker rats, using a specific RIA for IAPP. Obesity and hyperinsulinemia were observed in rats 21 days after VMH lesioning. IAPP content was increased in the islets of VMH-lesioned rats compared with findings in the sham-operated controls (100.9 +/- 6.6 vs. 72.8 +/- 3.85 fmol/islet; P less than 0.01). Isolated islets of VMH-lesioned rats secreted larger amounts of IAPP in the presence of 2.8 mM and 16.7 mM glucose (2.99 +/- 0.98 and 11.2 +/- 1.29 fmol.islet(-1).3 h-1) than was noted in sham-operated rats (ND and 6.65 +/- 0.78 fmol.islet(-1).3 h-1). In the obese Zucker rats, aged 14 weeks, IAPP concentrations in the islets were elevated compared with lean rats (133.3 +/- 10.6 vs. 84.4 +/- 8.5 fmol/islet; P less than 0.01). The isolated islets secreted larger amounts of IAPP in response to 2.8 mM and 16.7 mM glucose (2.83 +/- 0.88 and 15.81 +/- 1.35 fmol.islet(-1).3 h-1) than did those from lean control rats (0.36 +/- 0.19 and 12.49 +/- 1.20 fmol.islet(-1).3 h-1). These results strongly suggest that overproduction and hypersecretion of IAPP occur in animals with obesity and hyperinsulinemia.     

Altered pressure-natriuresis in obese Zucker rats.

It has not been examined whether the pressure-natriuresis response is altered in the insulin-resistant condition. Furthermore, despite an important role of nitric oxide (NO) in modulating pressure-natriuresis, no investigations have been conducted assessing the renal interstitial NO production in insulin resistance. The present study examined whether pressure-natriuresis was altered in insulin-resistant obese Zucker rats (OZ) and assessed the cortical and medullary nitrate/nitrite (NOx) levels with the use of the renal microdialysis technique. In OZ, serum insulin/glucose ratio (23.0+/-4.0x10(-8), n=9) and blood pressure (119+/-3 mm Hg) were greater than those in lean Zucker rats (LZ; 7.0+/-1.9x10(-8) and 103+/-4 mm Hg, n=9). The pressure-natriuresis curve in OZ was shifted to higher renal perfusion pressure (RPP), and the slope was blunted compared with that in LZ (0.073+/-0.015 vs 0.217+/-0.047 microEq/min kidney weight/mm Hg, P<0.05). The basal renal NOx level was reduced in OZ (cortex, 4.032+/-0.331 micromol/L; medulla, 4. 329+/-0.515 micromol/L) compared with that in LZ (cortex, 7.315+/-1. 102 micromol/L; medulla: 7.698+/-0.964 micromol/L). Furthermore, elevating RPP increased the medullary NOx in LZ, but this pressure-induced response was lost in OZ. Four-week treatment with troglitazone, an insulin-sensitizing agent, improved hyperinsulinemia, systemic hypertension, and basal renal NOx levels (cortex, 5.639+/-0.286 micromol/L; medulla, 5.978+/-0.284 micromol/L), and partially ameliorated the pressure-natriuresis curves; the slope of pressure-natriuresis curves and elevated RPP-induced NOx, however, were not corrected. In conclusion, our study suggests that insulin resistance is closely associated with abnormal pressure-natriuresis and hypertension. These deranged renal responses to insulin resistance are most likely attributed to impaired medullary NO production within the medulla.     

Metabolic profiling of livers and blood from obese Zucker rats

BACKGROUND/AIMS: Obesity frequently leads to changes in fatty acid metabolism with subsequent fatty infiltration in the liver. METHODS: In this study, metabolic profile of the livers and blood from lean and obese Zucker rats was established based on quantitative nuclear magnetic resonance spectroscopy (NMR) analysis. RESULTS: (1)H NMR on liver lipid extracts indicated significantly increased concentrations of total fatty acids and triglycerides. (31)P NMR on liver extracts revealed that obese livers have a compromised energy balance (low [ATP/ADP]) with decreased mitochondrial activity. Simultaneously, increased glycolytic activity was detected. The most pronounced differences were highly increased methionine and decreased betaine concentrations in obese animals. This suggests a significant alteration in methionine metabolism, which may be in part responsible for the development of steatosis, induction of mitochondrial dysfunction, and increased vulnerability of fatty livers to ischemia/reperfusion injury. A trend towards decreased hepatic glutathione concentrations as well as a reduced [PUFA/MUFA] ratio were present in the obese group, indicating increased oxidative stress and lipid peroxidation. CONCLUSIONS: In conclusion, NMR analysis on blood and liver tissue from obese Zucker rats reveals specific metabolic abnormalities in mitochondrial function and methionine metabolism, which result in a decreased hepatic energy state.     

High sucrose diet increases visceral fat accumulation in VMH-lesioned obese rats

We have previously reported that metabolic abnormalities in obese subjects are closely related to intraabdominal fat accumulation. In order to clarify the causal relationship between high sucrose intake and intraabdominal visceral fat accumulation, the effects of a high sucrose diet on mesenteric fat weight, fasting plasma glucose (FPG) and triglyceride (TG) levels were investigated in bilateral ventromedial hypothalamic-lesioned obese rats (VMH) and control rats (control). The mesenteric fat weight (g)/abdominal subcutaneous fat weight (g) (M/S ratio), a parameter of visceral fat accumulation, was significantly higher in a high sucrose diet fed group (HSD group) (VMH 1.94 +/- 0.71, control 1.63 +/- 0.28) than in a lab chow fed group (LC group) (VMH 1.30 +/- 0.31, control 1.27 +/- 0.33) (P less than 0.01). In addition, the fat cell volume of mesenteric fat deposits was significantly greater in the HSD group (VMH 1.17 +/- 0.26 nl, control 0.24 +/- 0.10 nl) than in the LC group (VMH 0.74 +/- 0.27 nl, control 0.14 +/- 0.07 nl) (P less than 0.001), although the fat cell number did not differ among any experimental rat groups. FPG and TG levels were higher in the HSD group than in the LC group in VMH-lesioned rats, and a significant correlation was observed between mesenteric fat volume and FPG levels (VMH r = 0.58, P less than 0.05; control r = 0.50, P less than 0.05) as well as between mesenteric fat volume and TG levels (VMH r = 0.49, P less than 0.05; control r = 0.56, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)     

Bone structure and calcium metabolism in obese Zucker rats.

Obesity is associated with altered bone mass. However, reports on bone status in obesity are inconsistent. Increased or normal bone mass was reported in obese adults but decreased bone mineral content was described in obese children. Therefore we evaluated the obese fa/fa rat as a possible model to assist in studies of bone metabolism in obesity. Obese and lean 14-week-old male rats underwent 24 h balance studies for calcium, magnesium and phosphate. Plasma calcium, magnesium, phosphate, immunoreactive parathyroid hormone, urinary cAMP (cyclic adenosine monophosphate) and femur bone histomorphometry were also analysed. Obese rats were heavier and had higher plasma insulin, cholesterol and triglycerides levels (P less than 0.05). A comparable positive balance for calcium, magnesium and phosphate was found in obese and lean rats. Total plasma calcium was higher in the obese, but albumin corrected calcium and plasma magnesium, phosphate and glucose were similar to the lean. In contrast to human obesity, obese rats were hypercalciuric, hypermagnisuric and hyperphosphaturic (P less than 0.05). iPTH and urinary cAMP were higher in the obese. Femora of fa/fa rats were shorter and lighter. Their bone osteoid surface and bone calcium content were similar to controls. Femora metaphysis in the obese had increased number of trabeculae, decreased trabecular width and higher erosion surface/bone surface ratio. Their diaphysis had increased cortical area/bone area and cortical width/bone width ratios and decreased medullary area. In summary, obese rats have higher iPTH, are hypercalciuric and have decreased bone mass. These last two observations differ from what is described in adult human obesity. Therefore, the obese fa/fa rat is of limited assistance in studies of bone status in adult human obesity. It might be of help in studies of bone metabolism in juvenile obesity.     

NMDA receptor-mediated modulation of ventilation in obese Zucker rats.

BACKGROUND: Ventilation in response to hypoxia is reduced in some obese humans and is believed to represent part of the pathogenesis of obesity hypoventilation syndrome (OHS). Ventilation in response to hypoxic exposure is closely related to the release of excitatory neurotransmitters, in particular glutamate, acting specifically on N-methyl-D-aspartate (NMDA) receptors. OBJECTIVES: The aim of the present study was to investigate whether NMDA receptor-mediated mechanisms are responsible for the altered ventilatory response to sustained hypoxia observed in obese Zucker (Z) rats. SUBJECTS: Seven lean and seven 15-week-old obese male Z rats were studied. MEASUREMENTS: Ventilation ([V](E)) at rest and during 30 min sustained hypoxic (10% O(2)) exposure was measured by the barometric method. [V](E) was assessed following the blinded-random administration of equal volumes of either saline (vehicle) or dextromethorphan (DM, 10 mg/kg), a non-competitive glutamate NMDA receptor antagonist. RESULTS: DM had no effects on resting [V(E) in both lean and obese rats during room air breathing. Lean rats treated with DM exhibited a significant (P<0.05) depression in [V](E), V(T), and V(T)/T(I) during either the early (5 min) or the late phase (30 min) of ventilatory response to sustained hypoxia. In contrast, DM administration in obese rats did not change [V(E), V(T), or V(T)/T(I) during the early phase of ventilatory response to hypoxia. During the late phase of ventilatory response to hypoxia. obese rats treated with DM exhibited a similar depression in [V](E) and V(T) as observed in lean rats, but had no significant change in V(T)/T(I) during the 30 min hypoxic exposure. CONCLUSION: Our findings indicate that altered glutamatergic mechanisms acting on NMDA receptors are partially responsible for a blunted early phase of ventilatory response to hypoxia noted in obese rats and also contribute to their reduced neural respiratory drive.     

Antiglucocorticoid action of dehydroepiandrosterone in young obese Zucker rats.

Dehydroepiandrosterone (DHEA) reduces weight gain in the hypercorticosteronemic Zucker fatty rat, an animal model of genetic obesity. However, the mechanism of action of DHEA is still unclear. We propose that DHEA acts as an antiglucocorticoid in the Zucker fatty rat. To test this hypothesis we examined DHEA's ability to block the activation of the glucocorticoid-inducible enzymes tyrosine aminotransferase (TAT) and ornithine decarboxylase (ODC) by dexamethasone (i.p. 5 micrograms/100 g body weight) in hepatic tissue of 6-10 week old Zucker rats. Injections of DMSO, the vehicle, served as a control. DHEA alone did not affect TAT, but when DHEA (500 micrograms/100 g b.w.) was administered simultaneously with dexamethasone, activation did not occur. Similar results were seen using a second tissue (kidney). We conclude that DHEA can act acutely as an antiglucocorticoid in the young obese Zucker rat and hypothesize that its chronic anti-obesity effect may reflect, at least in part, a chronic antiglucocorticoid activity.     

Regulation of lipid synthesis in hepatocytes from lean and obese Zucker rats

Fatty acid synthesis and CO₂ production were evaluated in hepatocytes from lean and obese Zucker rats in the presence of ³H₂O, and several carbon precursors. The incorporation of ³H₂O into fatty acids was greater in obese compared to lean rats in both the isolated hepatocyte and in vivo. The rates of incorporation of ³H₂O into fatty acids and cholesterol in hepatocytes of both lean and obese rats were linear for 2 hr, in the absence or presence of 16.7 mM glucose. Rates of fatty acid synthesis were higher in the presence of 16.7 mM glucose compared to the absence of glucose in both lean and obese while rates of cholesterol synthesis were similar. The incorporation of ³H₂O into fatty acids, but not into cholesterol, was correlated with increasing glucose concentration and was 2 to three-fold higher in hepatocytes of obese compared to lean rats in the presence of several carbon precursors. Differences in CO₂ production between lean and obese rats suggested increased pentose phosphate shunt activity, decreased pyruvate dehydrogenase activity, and lower tricarboxylic acid cycle activity in obese rats. Fatty acid synthesis and CO₂ production from ³H₂O and [U-¹⁴C]glucose in hepatocytes of lean and obese rats was similarly elevated by insulin and depressed by glucagon at several concentrations, suggesting that hepatocytes of obese animals respond to these hormones. These data indicate that rates of hepatic fatty acid synthesis although higher in obese rats respond to modulation in a fashion which is similar to the response in lean rats. The present studies suggest that the oxidation of several carbon precursors in the tricarboxylic acid cycle is diminished in obese compared to lean rats, but pentose phosphate shunt activity is greater in the obese Zucker rats.     

Carbohydrate metabolism in lean and obese Zucker rats

Carbohydrate metabolism was evaluated in lean and obese Zucker rats. Plasma glucose concentration, renal and hepatic gluconeogenesis, and hepatic glycogen content and rates of synthesis were investigated in 2-mo and 8-mo-old animals. Mild hyperglycemia was observed in obese Zucker rats compared to lean rats and was more pronounced in males than in females. Rates of glucose disappearance were normal in both female and male rats, although there was a trend toward decreased clearance in the male. Total organ hepatic and kidney PEPCK activity and kidney glucose production were elevated in obese compared to lean rats. Total organ hepatic glycogen levels and rates of glycogen synthesis were increased significantly in obese compared to lean, the increase being greater in males than females. The mild hyperglycemia present in obese Zucker rats is not associated with delayed disappearance of intravenously administered glucose, but may be due to the increased production of glucose by whole kidney and liver.     

Progressive histopathological changes in pancreatic islets of Zucker Diabetic Fatty rats.

Thus far, histopathological changes in the pancreatic islets of Zucker Diabetic Fatty (ZDF) rats, an animal model of type 2 diabetes mellitus (or non-insulin-dependent diabetes mellitus), have only been studied in male rats and in 18-weeks old rats or younger. In this study, we have examined in both male and female ZDF rats the histopathological changes longitudinally, from 6 to 32 weeks of age. We studied islet architecture and cellular distribution of the various islet hormones both in ZDF and control rats. In the ZDF rats, aging was initially associated with an enlargement of the islets. From 18 weeks onwards, no further enlargement was noted but islet boundaries became increasingly irregular, leading to the appearance of projections of endocrine cells into the surrounding exocrine tissue. At the islet boundaries as well as within the islets progressive fibrosis was observed with increasing amounts of collagen and reticular fibers. In the islets, staining intensity of both insulin and islet amyloid polypeptide (IAPP) increased slightly till 10 weeks of age and thereafter decreased rapidly. In contrast, the staining intensities of glucagon, somatostatin, and pancreatic polypeptide (PP) did not change. Even at the age of 32 weeks, just the beta-cells and not the other endocrine islet cells appear to be affected. In control rats, aging evoked only minor changes. Thus, we observed that during prolonged development of diabetes mellitus in both male and female ZDF rats histopathological changes in the pancreatic islets became progressively more severe, eventually leading to disintegration of the islets.     

Dynamics of insulin hypersecretion by obese Zucker rats

The relationship between obesity and hypersecretion of insulin by the pancreas was studied. We found that pancreata from obese Zucker rats secrete significantly more insulin than do pancreata from lean Zucker rats. At a glucose stimulation of 300 mg/dL, the overall dynamic biphasic insulin secretory profiles from obese and lean rats were similar. Further studies to investigate the glucose-insulin dose response relationship in obese and lean rat pancreata demonstrated insulin hypersecretion by pancreata from obese rats which was particularly pronounced at normoglycemic and hypoglycemic levels (by factors as much as 14-fold). This hypersecretion is so striking as to suggest that in the intact state the obese animal may lack the ability to readily "shut off" its insulin secretion under normoglycemic conditions, whereas lean animals possess such an ability. Under hypoglycemic conditions (75 mg/dL), the hypersecretion is transient and insulin secretion returns to normal basal levels after 30 minutes of perfusion. Thus the degree to which this hypersecretory phenomenon may occur in vivo remains to be established.     

Lipogenesis in primary cultures of adipoblasts derived from genetically obese Zucker rats

Adipocyte precursor cultures prepared from the epididymal fat pads of genetically obese ($\text{fa}\text{fa}$) and lean ($\text{Fa}\text{Fa}$) Zucker rats grow similarly in culture. Addition of enriched medium (EM) containing human serum, insulin, and glucose stimulated lipid filling of the adipocyte precursors in both cultures. However, [³H] H₂O incorporation into total lipids, fatty acid synthetase and lipoprotein lipase activities, and cytosolic protein contents are all decreased in the $\text{fa}\text{fa}$ compared with the $\text{Fa}\text{Fa}$ cultures. Substitution of lean or obese rat serum for human serum in the enriched medium does not alter the decreased lipogenic capacity of the $\text{fa}\text{fa}$ adipocyte precursor cultures.     

Intraglomerular fibronectin accumulation and degradation in obese Zucker rats

Summary The obese Zucker rat is a classic model of non-immune mediated spontaneous focal glomerulosclerosis. An important initiating hallmark of glomerulosclerosis in this model is mesangial matrix expansion. Fibronectin, a highly biologically active glycoprotein, is a normal constituent of mesangial extracellular matrix. Using a quantitative method based on enzyme immunoassay we assessed the intraglomerular fibronectin content and its degradation in obese Zucker rats and their lean littermates. In the obese Zucker rats the glomerular fibronectin content was significantly higher in comparison to the controls (88±6 vs 48±4 ng/10³ glomeruli). Furthermore, proteinase activity against fibronectin was significantly reduced in the glomeruli of obese Zucker rats when compared to control animals (at pH 5.4: 186±6 U/mg protein vs 286±14 U/mg protein, at pH 7.4: 152±12 U/mg protein vs 193±12 U/mg protein). These data demonstrate that in obese Zucker rats there is a glomerular accumulation of fibronectin which we propose is at least partly due to diminished proteolytic digestion. Whether accumulation of intraglomerular fibronectin contributes to progressive glomerulosclerosis remains a matter of debate.     

Cholesterol homeostasis in lean and obese male Zucker rats

Studies were performed in male Zucker rats to determine the metabolic effect of genetic obesity on whole body cholesterol homeostasis. Lean and obese mature Zucker rats were studied during intake of either a chow diet or a semisynthetic diet containing 10% corn oil; in addition growing animals were studied during constant body weight gain on a chow diet. Under all conditions the obese Zucker rats had significantly higher levels of total plasma cholesterol and triglyceride; however, measurements of the specific activity of hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase and of the rate of whole body cholesterol synthesis by sterol balance techniques demonstrated that the lean and obese animals did not differ in their endogenous rates of cholesterol synthesis. When sterol balance data were calculated per kilogram body weight, lean male Zucker rats synthesized a greater amount of cholesterol per day than obese animals. These studies demonstrate that the obese male Zucker rat, in many ways a model of human obesity, does not overproduce cholesterol and thus fails to exhibit one of major characteristics of the obese human.     

Altered renal microvascular response in Zucker obese rats

Although available evidence demonstrates that obesity manifests insulin resistance and causes glomerular sclerosis, it has not been determined whether insulin resistance alters the renal microvascular reactivity. This study examined whether insulin- and acetylcholine (ACH)-induced vasodilation was impaired in Zucker obese rats, and attempted to clarify the change in myogenic afferent arteriolar constriction, a determinant of glomerular pressure. Isolated perfused hydronephrotic rat kidneys were used to visualize the renal microcirculation. In Zucker lean rats, insulin (10 to 300 microU/mL) inhibited norepinephrine (NE)-induced afferent and efferent arteriolar constriction in a dose-dependent manner, with 112 % +/- 8% and 98% +/- 8% reversal at 300 microU/mL Similarly, ACH elicited dose-dependent dilation of these vessels. In Zucker obese rats, by contrast, afferent and efferent arterioles failed to dilate in response to insulin, and manifested diminished vasodilator responses to acetylcholine In the presence of nitro-L-arginine methylester (LNAME; 100 micromol/L), ACH (10 micromol/L) induced transient afferent arteriolar dilation (121% +/- 9% reversal) in Zucker lean rats, whereas this response was blunted in obese rats (72% +/- 8% reversal) Furthermore, myogenic afferent arteriolar constriction by elevating renal arterial pressure to 180 mm Hg was diminished in Zucker obese rats (-14% +/- 3% decrement in diameter), compared with that in lean rats (-23% +/- 2% decrement) Finally, the impairment in these vasodilator and vasoconstrictor responses was partially prevented by troglitazone, an insulin-sensitizing agent. Collectively, in insulin resistance, renal microvessels are refractory to the vasodilator action of insulin. Furthermore, "renal insulin resistance" is associated with the impaired vasodilator responses to ACH-induced nitric oxide (NO) and the diminished vasoconstrictor responses to pressure. The blunted myogenic afferent arteriolar constriction would allow glomerular hypertension, and in concert with the impaired endothelium-dependent vasodilation, could be responsible for the development of glomerular injury in obesity.     

Dietary sodium chloride increases blood pressure in obese Zucker rats.

In the rat, elevated arterial pressure is not consistently associated with obesity. The purpose of this study was to compare measurements of blood pressure, cardiac output, and total peripheral resistance in obese and lean Zucker rats on different NaCl intakes. Obese and lean rats drank either water or isotonic NaCl for 18 days. Tail systolic blood pressures of saline-drinking obese rats were higher than all other groups (p less than 0.05). NaCl intake did not affect blood pressure in lean rats, and blood pressures of water-drinking obese rats did not differ from those of lean controls. In a subsequent experiment, direct arterial pressures and cardiac outputs (thermodilution) were measured in separate groups of conscious rats that had been maintained on a 1% or 4% NaCl intake for 12 weeks. Arterial pressure was higher (p less than 0.01) in obese rats fed 4% NaCl (130 +/- 4 mm Hg) than in obese rats fed 1% NaCl (118 +/- 2 mm Hg) or than in lean rats fed either NaCl intake (118 +/- 3 mm Hg and 116 +/- 3 mm Hg, respectively). Cardiac output of obese rats was higher than that of lean rats (p less than 0.01); however, the NaCl-induced increase of blood pressure was accounted for by an increase of peripheral resistance (p less than 0.01). Thus, in contrast to the lean Zucker rat, arterial pressure of the obese Zucker rat is increased by a high dietary intake of NaCl.     

Regional and genotypic differences in stromal-vascular cells from obese and lean Zucker rats

LPL activity, total lipogenesis and rates of growth were determined for stromal-vascular cells derived from epididymal and inguinal depots of 13 1/2-week-old obese and lean Zucker rats. LPL activity, in cells of both depots, was found to increase between days 4 and 6 and decrease by day 8 in the presence of insulin. Inguinal derived fatty cell LPL activity increased between days 4 and 8 in contrast to lean cells which peaked at day 6 under basal growth conditions. LPL activity was elevated in fatty versus lean cells at days 6 and 8 in inguinal derived stromal vascular cells while in epididymal derived stromal vascular cells, LPL activity was elevated in lean versus fatty derived cells at day 4 and 6 but by day 8 the genotypic effect was reversed. Lipogenesis was elevated in lean versus fatty derived epididymal and inguinal cells at all concentrations of insulin and lean cells showed a dose-dependent response to insulin in contrast to fatty cells. There were no effects of genotype on the proliferative capacity of cells from either depot but some regional differences in growth were observed. These data illustrate that fa gene effects can be studied in primary cell culture.     

Insulin sensitivity and response in eviscerated obese Zucker rats

In order to examine the contribution of the skeletal muscles to insulin resistance in obesity, fatty (fa/fa) or lean Zucker rats were functionally hepatectomized/nephrectomized by an evisceration procedure. Glucose loads, adjusted to produce plasma levels of about 500 mg/100 ml, were given IV postoperatively, and during the next four hours the uptake and utilization of glucose by peripheral tissues (muscles) was estimated from the rate of fall in the plasma glucose concentration. Glucose utilization in the fa/fa rats was similar to that in the lean, despite plasma insulin levels that were ten times higher. Furthermore, a marked reduction in uptake was revealed when plasma insulin in the fa/fa rats was normalized (by waiting one hour after evisceration) before the glucose load was administered. Glucose uptake was resistant to insulin over a range of submaximal plasma insulin concentrations, indicating a shift to the right of the dose-response relationship. The maximal response in the fatty rats was markedly depressed compared with that in the lean rats, and even large doses of insulin (up to 1000 mU, IV) failed to restore it. The results suggest that the skeletal muscles isolated by evisceration from fa/fa Zucker rats are less responsive as well as sensitive to insulin than the muscles of lean rats and that the muscles contribute significantly to the insulin resistance found in intact obese animals.