Use of Glycated Hemoglobin to Assess Glycemic Control in Wistar Diabetic Fatty Rats and Zucker Fatty Rats

The Wistar Diabetic Fatty rat (WDF fafa) is a con-genic strain of the Wistar Kyoto rat. Studies using blood glucose reveal that only fatty male (not female) WDF rats spontaneously develop hyperglycemia when fed a stock diet Blood glucose values have not provided consistent results for evaluation of glycemic status in fatty male WDF rats. Zucker fatty (fafa) rats, while sharing the fa gene and the development of hyperinsulinemia and hyperlipemia, do not spontaneously become hyperglycemic. In order to examine strain differences and the effects of age on long-term average glycemic status in WDF and Zucker rats, glycated hemoglobin (GHb) was analyzed. Glycated hemoglobin was measured in male lean and obese WDF and Zucker rats at 2,3,6, and 12 months of age. Nonfasted plasma glucose was measured in male lean and obese WDF rats at 2, 3, 6, and 12 months of age and in lean and obese Zucker rats at 3, 6, and 12 months of age. Plasma insulin was measured in lean and obese WDF and Zucker rats at 3, 6, and 12 months of age. Obese WDF rats had significantly elevated GHb compared to lean controls at 3, 6, and 12 months of age. Glycated hemoglobin was substantially above the normal range (3.8-6.5%) at 3 months of age (14.1%). Glycated hemoglobin significantly declined in the obese WDF rats between 6 and 12 months of age. Nonfasted plasma glucose was significantly elevated in the obese WDF rats at 3 months (14.1 ± 2.1 mM/L) and 6 months of age (16.2 ± 2.3 mM/L) compared to lean controls. At 12 months of age there was no difference in plasma glucose between obese and lean WDF rats. Obese and lean Zucker rats had similar levels of GHb and plasma glucose at all ages. In conclusion, GHb provides more integrated data for classifying disease status of WDF rats and evaluation of potential long-term complications associated with hyperglycemia.     

Alpha 1- and beta-adrenergic receptors in brown adipose tissue of lean (Fa/?) and obese (fa/fa) Zucker rats. Effects of cold-acclimation,sucrose feeding and adrenalectomy.

1. The populations of alpha 1- and beta-adrenergic receptors in brown adipose tissue (BAT) of genetically obese Zucker rats (fa/fa) were studied with [3H]prazosin and [3H]CGP-12177 respectively. 2. The density of alpha 1-adrenergic receptors in BAT was significantly lower in obese than in lean Zucker rats, both at 2-4 months of age and at 6 weeks of age. The density of beta-adrenergic receptors was identical in BAT of lean and obese 6-week-old Zucker rats. 3. Cold-acclimation increased the alpha 1-receptor density significantly in BAT of both lean and obese Zucker rats, and the number of beta-receptors was also somewhat increased. 4. Sucrose feeding did not affect the density of alpha 1-receptors in BAT of lean or obese Zucker rats, but it increased beta-receptor density. 5. Adrenalectomy restored the density of alpha 1-adrenergic receptors in BAT of obese Zucker rats to the value observed in lean rats. 6. It is concluded that there is a direct correlation between alpha 1-receptor density and tissue recruitment, and that alpha 1-receptor density is thus positively correlated with sympathetic activity. beta-Receptor density is apparently better correlated with feeding conditions.     

Mono- and diacylglycerol lipases in spinal cord of lean and obese Zucker rats

1. Genetically obese Zucker rats (fa/fa) contain 2-3 times higher activities mono- and diacylglycerol lipases in their spinal cords than their lean littermates. 2. When rats were exercised (1 hr daily, 5 days/week) on a treadmill for 6 months, there was a decrease of about 30% (P less than 0.05) in the activities of mono- and diacylglycerol lipases in lean rats but not in obese animals. 3. High activities of lipases in Zucker obese rats may be related to the elevated levels of beta-endorphin present in these animals. 4. The activities of arylsulfatase, beta-N-acetylhexosaminidase and alkaline phosphatase, tested to check the stability of spinal cord extracts, were similar in lean and obese rat spinal cords.     

Unexpected regulation of hypothalamic neuropeptide Y by food deprivation and refeeding in the Zucker rat.

Neuropeptide Y strongly stimulates food intake when it is injected in the hypothalamic paraventricular (PVN) and ventromedian (VMN) nuclei. In Sprague-Dawley (SD) rats, NPY synthesis in the arcuate nucleus (ARC) is increased by food deprivation and is normalized by refeeding. We have previously shown that the obese hyperphagic Zucker rat is characterized by higher NPY concentrations in this nucleus. NPY might therefore play an important role in the development of hyperphagia. The aim of the present study was to determine if the regulation by the feeding state works in the obese Zucker rat. For this purpose, 10 weeks-old male lean (n = 30) and obese (n = 30) Zucker rats were either fed ad libitum, either food-deprived (FD) for 48 hours or food-deprived for 48 h and refed (RF) for 6 hours. NPY was measured in several microdissected brain areas involved in the regulation of feeding behavior. NPY concentrations in the ARC was about 50% greater in obese rats than in lean rats (p less than 0.02) whatever the feeding state. In the VMN, NPY concentrations were higher in the lean FD rats than in the obese FD rat (p less than 0.001). Food deprivation or refeeding did not modify NPY in the ARC, in the VMN or in the dorsomedian nucleus whatever the genotype considered. On the other hand, food deprivation induced a significant decrease in NPY concentrations in the PVN of lean rats. This decrease was localized in the parvocellular part of this nucleus (43.0 +/- 1.9 (FD) vs 54.2 +/- 2.1 (Ad lib) ng/mg protein; p less than 0.005). Ad lib levels were restored by 6 hours of refeeding. These variations were not observed in the obese rat. The regulation of NPY by the feeding state in the Zucker rat was therefore very different from that described in the SD rats. Strain or age of the animals used might explain these differences. High NPY levels and absence of regulation in obese Zucker rats could contribute to the abnormal feeding behavior of these rats.     

Gastrin release in obese Zucker rats

In this study, gastrin release in the obese Zucker rat was investigated by in vivo and in vitro experiments. Obese rats exhibited normal plasma gastrin levels at 3 weeks (preobese), were moderately hypergastrinemic at 3 months and severely hypergastrinemic at 5 months, compared to lean littermates. Following oral peptone, plasma gastrin levels doubled in both lean and obese rats. Basal and vagally stimulated gastrin release from perfused stomachs was greater in obese compared to lean rats and atropine had no effect on basal gastrin release in either group. Basal somatostatin release from the perfused stomach was found not to differ in both groups of animals. Morphological studies revealed an increase in the number of gastrin-containing G-cells in adult obese rats compared to lean littermates, but not in 3-week-old pups compared to lean littermates, indicating a strong correlation between cell number and plasma gastrin levels. These data indicate that the obese Zucker rat exhibits fasting hypergastrinemia in vivo, a condition which appears after weaning and increases in severity with age. Gastrin hypersecretion persists from the vascularly perfused stomach preparation. The basal hypergastrinemia of the obese Zucker rat is independent of a hyperactive postganglionic cholinergic drive but is associated with and probably causally related to an increase in antral G-cell numbers.     

Aggregation and thromboxane B2 formation in platelets and vascular prostacyclin production from genetically obese rats

Obesity, diabetes, hyperlipidaemia and age are conditions predisposing to atheroscleorosis and arterial occlusion. Recently it has been claimed that increased synthesis of thromboxane A₂ by platelets and decreased synthesis of prostacyclin (PGI₂) by blood vessels play an important role. The “Zucker” rat, a genetically obese animal with hyperlipidaemia, hyperinsulinaemia and normoglycaemia was used to study platelet aggregation, thromboxane (TXB₂) production and aortic PGI₂ synthesis. Two age groups (6–8 months and 14–16 months old) and their homozygote lean controls were used. In the obese rats no increased aggregation was found with ADP, arachidonic acid and collagen. On the contrary platelets from young fatty rats were less sensitive to ADP than platelets from lean young animals. An increase in platelet sensitivity to aggregating agents with age was observed, especially in the obese rats. TXB₂ measured in platelet rich plasma after exposure to ADP, arachidonic acid, arachidonic acid plus ADP and collagen was similar in the fatty and lean animals.Production of PGI₂ from incubated aortic rings was lowest in young lean animals. No differences existed between the other groups of rats studied. Insulin added to aortic rings had no influence on PGI₂ production. It is concluded that age rather than obesity, hyperlipidaemia or hyperinsulinaemia may cause platelet hyperresponsiveness to aggregating agents. Thromboxane and plateletaggregation do not closely correlate. PGI₂ production is not reduced by metabolic alterations, thought to predispose to atherosclerosis.     

Characteristics of the Obesity Syndrome in Zucker-Brown Norway (ZBN) Hybrid Rats

The existence of a restriction fragment length polymorphism (RFLP) closely linked to the fatty locus between the Zucker (Z) and Brown Norway (BN) rat strains allows evaluation of early effects of the fatty (fa) gene using offspring of back-crosses (N₂) between F₁ females and Zucker obese males. We examined several metabolic characteristics of N₂ animals to determine if these hybrid animals exhibited similar characteristics of the obese syndrome to those of Zucker rats. Females from crosses of obese male Zucker (fd/fa) and lean female BN (+/+) rats were back-crossed to their sires, resulting in twelve N₂ litters. At 9 weeks of age, liver, spleen, interscapular brown fat (IBAT), and gonadal, retroperitoneal (RP), and inguinal fat depots were removed and weighed. Samples of the RP depot were analyzed for cell size and number. Obese N₂ rats were hyperphagic, with body weights in the range of those of obese Zucker rats. Obese N₂ rats were also hyperinsulinemic [mean f SEM, pU/ml: females, 7.9 ± 0.6 vs. 82.1 f 8.4 (lean vs. obese); males, 10.5 ± 1.6 vs. 128.5 ± 13.4 (lean vs. obese)] and mildly hyperglycemic [mean ± SEM, mg/dl: females, 104.1 ± 2.0 vs. 139.0 ± 14.7 (lean vs. obese); males, 100.9 ± 2.6 vs. 132.0 ± 2.8 (lean vs. obese) p ≤ 0.05]. White fat depots in obese tats were 3 to 7 times heavier than those in lean rats; adipocyte numbers in RP depots were 50% greater in obese than in lean rats; and cell size was more than 3 times larger. IBAT, liver, and spleen were also heavier in obese vs. lean rats, while tail lengths were shorter. Percent lean carcass mass and % carcass protein were about 30% greater in lean vs. obese rats, while % carcass fat in obese rats was 5 times greater than that of lean rats. Thus, phenotypic expression of the fa gene in ZBN hybrid animals, with approximately 25% of their genetic background coming from the BN strain, appears to be similar to that in Zucker rats. Given the similarity of phenotypic expression of the fa gene between the Zucker strain and ZBN hybrids, it is plausible to consider using ZBN hybrids for studies of early manifestations of fa gene action prior to onset of detectable obesity .     

Comparison of glycogen stores in 3- and 7-month-old lean and obese Zucker rats under fed and fasted conditions

Glycogen content (mg/g) and stores (mg) were determined in 3- and 7-month-old obese and lean Zucker rats, under fed and fasted (48 hr) conditions. Hepatic content was higher in fed obese than in lean rats (3 months: 90 vs 70; 7 months: 107 vs 74); it was exhausted after fasting in lean but decreased by 56% in obese rats. Muscle content in fed obese and lean animals did not differ; it decreased comparably after fasting. Myocardial content was higher in fed obese than lean rats (3 months: 7.2 vs 3.6; 7 months: 7.5 vs 6.3); it was enhanced with fasting (10.0 vs 7.5). Total glycogen stores were higher in obese than in lean animals (3 months: 2500 vs 1400; 7 months: 4000 vs 2000) because of the hepatic store. The discussion includes a comparison with available data, taking into account methodological aspects, lipid stores and the FFA/carbohydrate interrelationship.     

Hepatic reduction of insulin-like growth factor (IGF)-I and IGF binding protein-3 that results from fasting is not attenuated in genetically obese rats

Fasting or caloric restriction causes substantial reductions in serum IGF-I in normal weight humans and animals, and reductions of liver IGF-I and IGFBP-3 mRNAs in animals. Obese humans, however, have attenuated and delayed decrements in IGF-I in serum when subjected to caloric restriction. Obese Zucker rats show a clear tendency to preserve body protein during fasting. To determine whether obesity opposes the effects of fasting on IGF-I and IGFBP-3, and thereby contributes to preservation of lean tissue, we have examined the effect of 72 h of fasting on IGF-I and IGFBP-3 in lean and obese Zucker rats. We observe that between lean and obese animals, fasting for 72 h produces similar decrements in body weight, serum IGF-I, liver IGF-I mRNA, serum IGFBP-3 and liver IGFBP-3 mRNA. Our finding that the reduction of IGF-I and IGFBP-3 in liver that results from 72 h of fasting is not attenuated in obese Zucker rats raises the possibility that conservation of lean tissue in these animals during fasting is not related to the hepatic production of IGF-I and IGFBP-3.     

Amino acid metabolism in several tissues of the obese Zucker rat as indicated by the tissue accumulation of α-amino[1-14C]isobutyrate

Measurements of the tissue accumulation of α-amino[1-¹⁴C]isobutyrate [1-¹⁴C]AIB) in lean (+/?) and obese (fa/fa) Zucker rats showed an augmented tissue/plasma ratio in the liver of the obese animals. In contrast, brown adipose tissue AIB accumulation was lower in the fa/fa animals. In response to a 24h starvation period AIB accumulation was significantly elevated in the liver and plasma of the lean animals and was unchanged in the liver of the fa/fa animals. The circulating concentration of alanine and branched-chain amino acids was elevated in the fa/fa animals as compared to their lean counterparts. These observations suggest that amino acid uptake is not involved in the impaired muscle development observed in the obese Zucker rat and that the ability of brown adipose tissue for amino acid utilization is decreased in the obese animals suggesting that this may partially explain the impaired thermoregulatory capacity observed in brown adipose tissue of obese Zucker rats.     

Cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in obese Zucker rats

The primary purpose of the study was to test the hypothesis that reduced leptin signaling is necessary to elicit the cardiovascular and metabolic responses to fasting. Lean (Fa/?; normal leptin receptor; n = 7) and obese (fa/fa; mutated leptin receptor; n = 8) Zucker rats were instrumented with telemetry transmitters and housed in metabolic chambers at 23 degrees C (12:12-h light-dark cycle) for continuous (24 h) measurement of metabolic and cardiovascular variables. Before fasting, mean arterial pressure (MAP) was higher (MAP: obese = 103 +/- 3; lean = 94 +/- 1 mmHg), whereas oxygen consumption (VO(2): obese = 16.5 +/- 0.3; lean = 18.6 +/- 0.2 ml. min(-1). kg(-0.75)) was lower in obese Zucker rats compared with their lean controls. Two days of fasting had no effect on MAP in either lean or obese Zucker rats, whereas VO(2) (obese = -3.1 +/- 0.3; lean = -2.9 +/- 0.1 ml. min(-1). kg(-0.75)) and heart rate (HR: obese = -56 +/- 4; lean = -42 +/- 4 beats/min) were decreased markedly in both groups. Fasting increased HR variability both in lean (+1.8 +/- 0.4 ms) and obese (+2.6 +/- 0.3 ms) Zucker rats. After a 6-day period of ad libitum refeeding, when all parameters had returned to near baseline levels, the cardiovascular and metabolic responses to 2 days of thermoneutrality (ambient temperature 29 degrees C) were determined. Thermoneutrality reduced VO(2) (obese = -2.4 +/- 0.2; lean = -3.3 +/- 0.2 ml. min(-1). kg(-0.75)), HR (obese = -46 +/- 5; lean = -55 +/- 4 beats/min), and MAP (obese = -13 +/- 6; lean = -10 +/- 1 mmHg) similarly in lean and obese Zucker rats. The results indicate that the cardiovascular and metabolic responses to fasting and thermoneutrality are conserved in Zucker rats and suggest that intact leptin signaling may not be requisite for the metabolic and cardiovascular responses to reduced energy intake.     

Pulmonary arterial dysfunction in insulin resistant obese Zucker rats

Background

Insulin resistance and obesity are strongly associated with systemic cardiovascular diseases. Recent reports have also suggested a link between insulin resistance with pulmonary arterial hypertension. The aim of this study was to analyze pulmonary vascular function in the insulin resistant obese Zucker rat.

Methods

Large and small pulmonary arteries from obese Zucker rat and their lean counterparts were mounted for isometric tension recording. mRNA and protein expression was measured by RT-PCR or Western blot, respectively. K_V currents were recorded in isolated pulmonary artery smooth muscle cells using the patch clamp technique.

Results

Right ventricular wall thickness was similar in obese and lean Zucker rats. Lung BMPR2, K_V1.5 and 5-HT_2A receptor mRNA and protein expression and K_V current density were also similar in the two rat strains. In conductance and resistance pulmonary arteries, the similar relaxant responses to acetylcholine and nitroprusside and unchanged lung eNOS expression revealed a preserved endothelial function. However, in resistance (but not in conductance) pulmonary arteries from obese rats a reduced response to several vasoconstrictor agents (hypoxia, phenylephrine and 5-HT) was observed. The hyporesponsiveness to vasoconstrictors was reversed by L-NAME and prevented by the iNOS inhibitor 1400W.

Conclusions

In contrast to rat models of type 1 diabetes or other mice models of insulin resistance, the obese Zucker rats did not show any of the characteristic features of pulmonary hypertension but rather a reduced vasoconstrictor response which could be prevented by inhibition of iNOS.     

Age-related decrease in sensitivity to glucagon and dibutyryl cyclic AMP inhibition of fatty acid synthesis in hepatocytes isolated from obese female Zucker rats

Hepatocytes were isolated from 3 and 5 month old female genetically obese Zucker rats and their lean littermate controls. An age-dependent loss in sensitivity of fatty acid synthesis to inhibition by both glucagon and dibutyryl cyclic AMP was observed with hepatocytes from the obese rats. Hepatocytes from lean animals were much more sensitive to these agents, regardless of age. Low concentrations of glucagon and dibutyryl cyclic AMP actually produced some stimulation of fatty acid synthesis with hepatocytes prepared from the older obese rats. 5-Tetradecyloxy-2-furoic acid, a compound which inhibits fatty acid synthesis, was a very effective inhibitor of fatty acid synthesis by hepatocytes isolated from all rats used in the study. An inhibition of lactate plus pyruvate accumulation and a strong stimulation of glycogenolysis occurred in response to both glucagon and dibutyryl cyclic AMP with hepatocytes from both age groups of lean and obese rats. The results suggest that with aging of the obese female Zucker rat some step of hepatic fatty acid synthesis becomes progressively less sensitive to inhibition by glucagon and dibutyryl cyclic AMP. This may play an important role in maintenance of obesity in these animals.     

Activation of the central melanocortin system contributes to the increased arterial pressure in obese Zucker rats

We have previously demonstrated that leptin-mediated activation of the central nervous system (CNS) melanocortin system reduces appetite and increases sympathetic activity and blood pressure (BP). In the present study we examined whether endogenous melanocortin system activation, independent of leptin's actions, contributes to the regulation of BP and metabolic functions in obese Zucker rats, which have mutated leptin receptors. The long-term cardiovascular and metabolic effects of central melanocortin-3/4 receptor (MC3/4R) antagonism with SHU-9119 were assessed in lean (n = 6) and obese (n = 8) Zucker rats. BP and heart rate (HR) were measured 24-h/day by telemetry and an intracerebroventricular cannula was placed in the brain lateral ventricle. After stable control measurements, SHU-9119 was infused intracerebroventricularlly (1 nmol/h) for 10 days followed by a 10-day recovery period. Chronic CNS MC3/4R antagonism significantly increased food intake and body weight in lean (20 ± 1 to 45 ± 2 g and 373 ± 11 to 432 ± 14 g) and obese (25 ± 2 to 35 ± 2 g and 547 ± 10 to 604 ± 11 g) rats. No significant changes were observed in plasma glucose levels in lean or obese rats, whereas plasma leptin and insulin levels markedly increased in lean Zucker rats during CNS MC3/4R antagonism. Chronic SHU-9119 infusion in obese Zucker rats reduced mean arterial pressure (MAP) and HR by 6 ± 1 mmHg and 24 ± 5 beats/min, whereas in lean rats SHU-9119 infusion reduced HR by 31 ± 9 beats/min while causing only a transient decrease in MAP. These results suggest that in obese Zucker rats the CNS melanocortin system contributes to elevated BP independent of leptin receptor activation.     

The CB1 Antagonist Rimonabant Decreases Insulin Hypersecretion in Rat Pancreatic Islets

Type 2 diabetes and obesity are characterized by elevated nocturnal circulating free fatty acids, elevated basal insulin secretion, and blunted glucose‐stimulated insulin secretion (GSIS). The CB1 receptor antagonist, Rimonabant, has been shown to improve glucose tolerance and insulin sensitivity in vivo but its direct effect on islets has been unclear. Islets from lean littermates and obese Zucker (ZF) and Zucker Diabetic Fatty (ZDF) rats were incubated for 24 h in vitro and exposed to 11 mmol/l glucose and 0.3 mmol/l palmitate (GL) with or without Rimonabant. Insulin secretion was determined at basal (3 mmol/l) or stimulatory (15 mmol/l) glucose concentrations. As expected, basal secretion was significantly elevated in islets from obese or GL‐treated lean rats whereas the fold increase in GSIS was diminished. Rimonabant decreased basal hypersecretion in islets from obese rats and GL‐treated lean rats without decreasing the fold increase in GSIS. However, it decreased GSIS in islets from lean rats without affecting basal secretion. These findings indicate that Rimonabant has direct effects on islets to reduce insulin secretion when secretion is elevated above normal levels by diet or in obesity. In contrast, it appears to decrease stimulated secretion in islets from lean animals but not in obese or GL‐exposed islets.     

Chronic rosiglitazone treatment restores AMPKalpha2 activity in insulin-resistant rat skeletal muscle

Rosiglitazone (RSG) is an insulin-sensitizing thiazolidinedione (TZD) that exerts peroxisome proliferator-activated receptor-gamma (PPARgamma)-dependent and -independent effects. We tested the hypothesis that part of the insulin-sensitizing effect of RSG is mediated through the action of AMP-activated protein kinase (AMPK). First, we determined the effect of acute (30-60 min) incubation of L6 myotubes with RSG on AMPK regulation and palmitate oxidation. Compared with control (DMSO), 200 microM RSG increased (P < 0.05) AMPKalpha1 activity and phosphorylation of AMPK (Thr172). In addition, acetyl-CoA carboxylase (Ser218) phosphorylation and palmitate oxidation were increased (P < 0.05) in these cells. To investigate the effects of chronic RSG treatment on AMPK regulation in skeletal muscle in vivo, obese Zucker rats were randomly allocated into two experimental groups: control and RSG. Lean Zucker rats were treated with vehicle and acted as a control group for obese Zucker rats. Rats were dosed daily for 6 wk with either vehicle (0.5% carboxymethylcellulose, 100 microl/100 g body mass), or 3 mg/kg RSG. AMPKalpha1 activity was similar in muscle from lean and obese animals and was unaffected by RSG treatment. AMPKalpha2 activity was approximately 25% lower in obese vs. lean animals (P < 0.05) but was normalized to control values after RSG treatment. ACC phosphorylation was decreased with obesity (P < 0.05) but restored to the level of lean controls with RSG treatment. Our data demonstrate that RSG restores AMPK signaling in skeletal muscle of insulin-resistant obese Zucker rats.     

Diazoxide down-regulates leptin and lipid metabolizing enzymes in adipose tissue of Zucker rats.

We have previously reported that attenuation of hyperinsulinemia by diazoxide (DZ), an inhibitor of glucose-mediated insulin secretion, increased insulin sensitivity and reduced body weight in obese Zucker rats. These findings prompted us to investigate the effects of DZ on key insulin-sensitive enzymes regulating adipose tissue metabolism, fatty acid synthase (FAS), and lipoprotein lipase (LPL), as well as on circulating levels of leptin. We also determined the direct effects of diazoxide on FAS in 3T3-L1 adipocytes. Seven-week-old female obese and lean Zucker rats were treated with DZ (150 mg/kg/d) or vehicle (C, control) for a period of 6 wk. Changes in plasma parameters by DZ include significant decreases in triglycerides, free fatty acids, glucose, and insulin, consistent with our previous reports. DZ obese rats exhibited lower plasma leptin levels (P<0.03) compared to their C animals. DZ significantly reduced adipose tissue FAS activity in both lean (P<0.0001) and obese (P<0.01) animals. LPL mRNA content was also decreased significantly in DZ-treated obese animals (P<0.009) as compared to their respective controls without a significant effect on lean animals. The possibility that DZ exerted a direct effect on adipocytes was further tested in cultured 3T3-L1 adipocytes. Although diazoxide (5 microM) alone did not change FAS activity in cultured 3T3-L1 adipocytes, it significantly attenuated insulin's effect on FAS activity (P<0.001). We demonstrate that DZ regulates key insulin-sensitive enzymes involved in regulation of adipose tissue metabolism. These findings suggest that modification of insulin-sensitive pathways can be therapeutically beneficial in obesity management.     

Effect of adrenalectomy on in vivo glucose metabolism in insulin resistant Zucker obese rats

Lean (Fa/?) and obese (fa/fa) Zucker rats were adrenalectomized (ADX) in order to assess the contribution of adrenal hormones to insulin resistance of the obese Zucker rat. Glucose utilization was measured using an insulin suppression test. Sham-operated obese rats gained almost twice as much weight as sham-operated lean littermates. However, body weight gain of ADX animals was comparable in both genotypes. It was significantly less than that of the respective sham-operated controls. Body weight differences can be accounted for almost entirely by a marked loss of adipose tissue. Although insulin resistance may be attributable to obesity in part, steroid hormones are thought to be directly antagonistic to insulin for glucose metabolism. Adrenalectomy resulted in a decrease in serum glucose concentrations for both lean and obese Zucker rats compared with their respective sham-operated groups. Serum insulin concentration of lean ADX rats was 23% of sham-operated controls; in obese ADX rats, it was 9% of controls. Elevated levels of steady state serum glucose (SSSG) levels in sham-operated obese rats demonstrate a marked resistance to insulin induced glucose uptake compared with sham-operated lean animals. Adrenalectomy caused a marked improvement in insulin sensitivity of obese rats. The hyperglycemic SSSG levels of the obese rats were reduced 2.5 times by ADX. These results indicate that insulin resistance of Zucker obese rats can be ameliorated by ADX, suggesting adrenal hormones contribute to insulin resistance in these animals.     

Adaptation of ghrelin levels to limit body weight gain in the obese Zucker rat

In this study, we measured the ghrelin, leptin, and insulin variations in lean and obese Zucker fa/fa rats during the acute phase of body weight gain. At 2 months of age, plasma insulin and leptin concentrations in fa/fa rats were, respectively, 470% and 3700% higher than in lean rats (p <0.0001). Plasma ghrelin was significantly lower (-24.6%; p <0.02) than in lean rats. At 6 months of age, ghrelin increased in both genotypes but the difference was no more significant. The inverse correlations existing between ghrelin and either body weight (BW), insulin or leptin at 2 months of age were no more observable in 6-month-old rats. At 6 months of age, the lean rats had the same body weight as the 2-month-old obese rats. In these body weight-matched rats, ghrelin was not correlated with BW but it remained negatively correlated with insulin and leptin. At the same body weight, obese rats had a much lower plasma ghrelin than lean rats (717+/-42 vs. 1754+/-83 pg/ml; p <0.0001). These data indicate that body composition rather than body weight is the primary factor for the down-regulation of the ghrelin system. This down-regulation constitutes a mechanism of defense of the organism against the development of obesity at least during the first part of life.