Data on lipid metabolism in the genetically obese Zucker rat

We assayed various lipid fractions as well as some intermediate metabolites and adenine nucleotides in the liver and the blood of the genetically obese Zucker rat (fa/fa) and compared their values with the corresponding ones obtained for nonobese controls and for Sprague-Dawley rats.Results for the two latter groups were similar, and differed greatly from those for the obese rats. The obese rats had hepatomegaly due mainly to high levels of triglycerides. The plasma of the obese animals contained high levels of triglycerides, phospholipids, cholesterol, and nonesterified fatty acids. Obese rats had slightly higher levels of ketone bodies than nonobese rats. In the liver of obese rats, we observed a decreased level of citrate/g and an elevated malate level. Levels of adenine nucleotides were similar in all rats.     

Cellularity of adipose depots in the genetically obese Zucker rat

Cell size and number of three adipose depots, epididymal, retroperitoneal, and subcutaneous, were determined during growth of the obese Zucker rat ("fatty") and nonobese Zucker control. Cellularity of these depots in the adult "fatty" was compared with that in nonobese controls and in nonobese Zucker rats made obese by ventromedial hypothalamic lesions. Epididymal and retroperitoneal depots in the nonobese rat grew by cell enlargement and increase in cell number until the 14th wk, when number became fixed; further increase in depot size occurred by cell enlargement. The subcutaneous depot added cells until the 26th wk. In the Zucker "fatty," cell number increased until the 26th wk in all depots, accompanied by extreme cell enlargement. The enlarged adipose depots of the adult Zucker "fatty," when compared with the nonobese control, are the result of both hypertrophy and hyperplasia. Depot enlargement in the lesioned animal is the result of hypertrophy. "Fatties" have more cells in adipose depots than do lesioned rats. Genetic obesity in the Zucker rat is clearly different from the obesity produced by hypothalamic lesioning.     

Decreased responsiveness of gluconeogenesis to the modulation by sulfonylureas in hepatocytes isolated from obese (fa/fa) Zucker rats

The influence of the hypoglycemic agent glipizide (0-100 microM) on the rate of gluconeogenesis from lactate, as well as on the levels of fructose 2,6-bisphosphate, has been investigated in hepatocytes isolated from genetically obese (fa/fa) Zucker rats and from their corresponding lean (Fa/-) littermates. As compared to lean rat hepatocytes, liver cells isolated from obese animals showed a lower rate of basal gluconeogenesis (0.9 +/- 0.2 vs 5.4 +/- 0.5 micromol of lactate converted to glucose/g cell x 30 min, n=4) and higher levels of fructose 2,6-bisphosphate (11.5 +/- 1.0 vs 5.9 +/- 0.4 nmol/g cell, n=8-9). In lean rat hepatocytes, the presence of glipizide in the incubation medium caused a dose-dependent inhibition of the rate of lactate conversion to glucose (maximal inhibition=46%; EC50 value=26 microM), and simultaneously raised the cellular content of fructose-2,6-bisphosphate (maximal increment=40%; EC50 value=10 microM). In contrast, in hepatocytes isolated from obese rats, the inhibition of gluconeogenesis and the increment in fructose-2,6-bisphosphate levels elicited by glipizide were significantly reduced (maximal effects of 22 and 13%, respectively). Similarly, the activation of glycogen phosphorylase and the increase in hexose 6-phosphate levels in response to glipizide were less marked in obese rat hepatocytes than in liver cells isolated from lean animals. These results demonstrate that the efficacy of sulfonylureas as inhibitors of hepatic gluconeogenesis is reduced in the genetically obese (fa/fa) Zucker rat.     

No altered blood pressure and serum markers of oxidative stress after a long time dietary fish oil in the genetically 9 month-old type-2 diabetes Zucker rat

In this study, we investigated the effect of a high n-3 fatty acid diet (eicosapentaenoic and docosahexaenoic acids) in Zucker obese and lean rats on blood pressure in association with physiological parameters, serum biochemistry and oxidative stress analysis.After 150 days of treatment, dietary fish oil supplementation in Zucker obese rats (9 months of age) reduces bodyweight gain and serum triglyceridemia and nitrite levels, increases serum glucose and angiotensin converting enzyme activity, but does not alter blood pressure, cholesterol levels and serum markers of oxidative stress (malondialdehyde, glutathione), compared to the Zucker rats fed control diet.According to these results, we can consider that after 150 days of treatment, fish oil is not enough to regulate parameters involved in the metabolic syndrome, such as cholesterolemia and blood pressure, in a 9 month-old genetically type-2 diabetes rat.     

Skeletal muscle contraction stimulates capillary recruitment and glucose uptake in insulin-resistant obese Zucker rats

Exercise and insulin increase muscle glucose uptake by different mechanisms and also increase capillary recruitment, which is proposed to facilitate access for hormones and nutrients. The genetically obese Zucker rat shows impaired insulin- but not contraction-mediated glucose uptake in muscle. Recently, we have shown the genetically obese Zucker rats to have impaired insulin-mediated capillary recruitment and proposed that this contributes to the insulin resistance of muscle in vivo. Because this might imply a general loss of recruitable capillaries, we now assess responses to contraction in muscles of 18 +/- 3-wk-old lean and obese Zucker rats in vivo. Field stimulation (2 Hz, 0.1 ms) was conducted for 1 h on one leg of anesthetized instrumented rats, and measurements were made of femoral blood flow (FBF), heart rate (HR), blood pressure (BP), hindleg metabolism of 1-methylxanthine (a measure of capillary recruitment), hindleg glucose uptake (HGU), and lower leg muscle glucose uptake by 2-deoxyglucose (R'g). Lean animals (311 +/- 9 g) developed tension at 219 +/- 27 g/g muscle with no change in BP but with significant increases in HR, FBF, HGU, 1-MX metabolism, and R'g (P < 0.05), compared with nonstimulated control leans. Obese animals (469 +/- 7 g) developed tension at 265 +/- 31 g/g muscle with no change in HR or BP but with significant increases in FBF, HGU, 1-MX metabolism, and R'g (P < 0.05) compared with nonstimulated control obese rats. Muscle contraction of lean animals led to a greater increase in lower leg R'g, similar responses in HGU and 1-MX, and a smaller increase in FBF than in obese animals. A tight correlation between FBF and capillary recruitment was noted for all data (P < 0.001). It is concluded that contraction-mediated muscle capillary recruitment and glucose uptake are essentially normal in the obese Zucker rat and that control of FBF and capillary recruitment in exercise is closely linked.     

Reduced sensitivity of the heart ventricle of obese Zucker rats to isoprenaline

In heart of genetically obese (fa/fa) 12-week-old Zucker rats, ventricle contractility (4 mM KCl medium), amplitude and duration of slow action potentials (14 mM KCl medium) were less increased by isoprenaline than in heart of non obese (Fa/fa) rats. This difference could be related to a lower reactivity of beta-adrenoceptors in obese rat heart.     

Comparison of the lipolytic activity of adipocytes isolated from white adipose tissue in Zucker and Wistar rats. In vitro effects of 3,5,3'-triiodo-L-thyronine

The in vitro effects of 3,5,3'-triiodo-L-thyronine (L-T3) on (-) epinephrine stimulated lipolytic activity were studied in adipocytes isolated from white adipose tissue of Zucker and Wistar male rats. Basic lipolytic activity was small and nearly insensitive to epinephrine in Zucker genetically obese rats. On the other hand, lipolytic activity was stimulated by epinephrine in Zucker lean rats and Wistar rats in the same way. There was no synergistic interaction between epinephrine and L-T3 on lipolytic activity, since the released glycerol levels were nearly the same. These data corroborate the hypothyroid status of Zucker genetically obese rats.     

Effects of a Fat Substitute,Sucrose Polyester,on Food Intake,Body Composition,and Serum Factors in Lean and Obese Zucker Rats

Sucrose polyester, a fat substitute, has shown promise in reducing blood cholesterol and body weight of obese individuals. Effects of this compound in the Zucker rat, a genetic model of obesity, are unknown. Thus, we examined food intake, body weight, body composition, and several metabolic parameters in sera of lean and obese female Zucker rats. Eight-week-old lean and obese animals were given a choice between a control diet (15% corn oil) and fat substitute diet (5% corn oil and 10% sucrose polyester) for 2 days. Next, one-half of the lean and obese groups received control diet; the remaining lean and obese rats received fat substitute diet for 18 days. Cumulative food intake was depressed in fat substitute groups relative to control-fed animals; however, this effect was more predominant in obese animals. Obese rats consuming fat substitute diet (O-FS) gained less weight as compared to obese control-fed animals (O-C). Lean rats given fat substitute (L-FS) did not have significantly different body weights as compared to the L-C group. Fat substitute groups, combined, had lower body fat and higher body water as compared to controls. The O-FS group had lower serum glucose and insulin and higher fatty acid levels compared to the O-C group. There were no differences in serum cholesterol, HDL, or triglyceride levels due to fat substitute diet. These data suggest that the obese Zucker rat is unable to defend its body weight when dietary fat is replaced with sucrose polyester.     

Serum leptin, lipids, free fatty acids, and fat pads in long-term dehydroepiandrosterone-treated Zucker rats

The obese Zucker rat has a genetically flawed leptin system and is a model of hyperphagia, obesity, hyperlipidemia, and markedly elevated leptin levels. Dehydroepiandrosterone (DHEA) administration reduces hyperphagia, hyperlipidemia, and obesity in Zucker rats. Since serum leptin levels are associated with body fat, we wondered what the effects of fat pad weight reduction from DHEA administration would have on leptin levels. This experiment investigated the effects of DHEA on intra-abdominal fat pads, serum lipids, and peripheral leptin in male lean and obese Zucker rats that were administered DHEA in their food from 4 weeks of age to 20 weeks. Lean and obese rats received plain chow or chow containing DHEA. Additional chow-fed groups of lean and obese weight-matched controls and obese pair-fed rats helped to control for the reduced body weight, food intake, and fat pad weights seen with DHEA administration. DHEA administration to lean Zucker rats reduced body weight and fat pad weights, but leptin levels showed a lower trend. Among obese rats, both DHEA treatment and pair-feeding reduced body weight and fat pad weights, but only DHEA lowered leptin levels. The weight-matched controls had reductions in fat pad weights similar to the DHEA-treated group, but with increased leptin levels. Thus, DHEA may exert a small, independent effect on leptin levels in this animal model, but the reduction is less than what would be expected.     

Adipogenic activity in the plasma of genetically obese Zucker rats

The potential of plasma to stimulate differentiation and lipid filling of adipose precursors in primary culture was investigated in the groups of genetically obese Zucker rats (fafa) and their lean littermates (FaFa). The effect of age, feeding status and possible role of growth hormone in the process of adipogenesis was also studied. Differences in lipid-filling activity of the tested plasma samples were much more dependent on age than the genotype of plasma donors were. The plasma taken from the oldest (20-week-old) rats stimulated the accumulation of triglycerides in the cells to significantly higher levels than the plasma from other rats. The influence of the feeding status on the lipid-filling activity of plasma was not significant. The differentiation potential of plasma in terms of the stimulation of glycerophosphate dehydrogenase activity measured in adipocyte precursors was 30-50% higher when the culture medium contained plasma from obese rats. Furthermore, glycerophosphate dehydrogenase activity in the growing cells declined with age and tended to be higher in the presence of plasma from fed rats. It was the growth hormone that was in a considerable degree responsible for the differentiation potential of Zucker rat plasma. This effect of growth hormone seemed to be less dependent on fafa genotype. It is, therefore, suggested that in addition to growth hormone, other factors in the plasma of genetically obese Zucker rats might be important in the development of obesity in this rat strain.     

Thyroid status in the obese syndrome of rats

The thyroid function was explored by comparing serum total and free iodothyronine levels in young male genetically obese Zucker rats and in their lean littermates, aged from 6 to 8 weeks old. Total and free thyroxine (T4) and 3,5,3'triiodothyronine (T3) levels were significantly decreased in obese rat serum while total 3,3',5'-triiodothyronine (rT3) remained constant. Radioactive T4 half life is slower in the plasma of obese rats. Peripheral synthesis of T3 from deiodination of T4 is also decreased in obese rat liver homogenate. These modifications produce changes in liver mitochondria oxidative phosphorylation and in marker enzyme activity, which are usually associated with hypothyroidism and hypothalamic disturbances. Genetic obesity probably involves activation of peripheral deiodination of T4 to rT3 which induces biochemical and metabolic changes.     

Subchronic treatment with metformin produces anorectic effect and reduces hyperinsulinemia in genetically obese Zucker rats.

The effect of subchronic metformin treatment on food intake, weight gain and plasma and tissue hormone levels was investigated in genetically obese male Zucker rats and in their lean controls. Metformin hydrochloride (320 mg/kg/day for 14 days in the drinking water) significantly reduced 24 hour food intake both after one and two weeks treatment in obese rats. In contrast, metformin had only a transient effect on food intake in lean animals. The reduced food intake was associated with body weight decrease, particularly in obese rats. Metformin markedly reduced also the hyperinsulinemia of the obese animals without altering their plasma glucose or pancreatic insulin content which may reflect an improved insulin sensitivity after metformin treatment. Metformin did not change plasma corticosterone levels or insulin and somatostatin concentrations in the pancreas. Metformin reduced pyloric region somatostatin content in lean rats. It is concluded that metformin has an anorectic effect and reduces body weight and hyperinsulinemia in genetically obese Zucker rat.     

Studies on the mechanism of hypertriglyceridemia in the genetically obese Zucker rat

The possibility that impaired removal of lipoprotein triglyceride from the circulation may be a participating factor in the hypertriglyceridemia of the obese Zucker rat was examined. We found no significant differences in the heparin-released lipoprotein lipase (LPL) activities of the adipose tissue, skeletal muscle, and heart (expressed per gram of tissue) from the lean and obese Zucker rats. Furthermore, the kinetic properties of adipose tissue and heart LPL from the lean and obese rats were similar, indicating that the catalytic efficiency of the enzyme was unaltered in the obese animals. The postheparin plasma LPL activities of lean and obese rats were also similar. However, the postheparin plasma hepatic triglyceride lipase (H-TGL) activity in the obese rats was elevated. The higher activity of H-TGL could not alleviate the hypertriglyceridemia in these animals. Since hypertriglyceridemia in the obese rats could also be due to the hepatic production of triglyceride-rich lipoproteins which are resistant to lipolysis, we therefore isolated very low density lipoproteins (VLDL) from lean and obese rat liver perfusates and examined their degradation by highly purified human milk LPL. Although certain differences were observed in hepatic VLDL triglyceride fatty acid composition, the kinetic patterns of LPL-catalyzed triglyceride disappearance from lean and obese rat liver perfusate VLDL were similar. The isolated liver perfusate VLDL contained sufficient apolipoprotein C-II for maximum lipolysis. These results indicate that impaired lipolysis is not a contributing factor in the genesis of hypertriglyceridemia in the genetically obese Zucker rat. The hyperlipemic state may be attributed to hypersecretion of hepatic VLDL and consequent saturation of the lipolytic removal of triglyceride-rich lipoproteins from the circulation.     

Insulin binding in the hypothalamus of lean and genetically obese Zucker rats

Recent reports have suggested that the obesity and hyperphagia of the genetically obese Zucker rat may be related to defective insulin action or binding in the hypothalamus. We used quantitative autoradiography to determine if insulin binding is altered in specific hypothalamic nuclei associated with food intake. Insulin binding was measured in the arcuate (ARC), dorsomedial (DMN), and ventromedial (VMN) hypothalamic nuclei of 3–4-month-old lean (Fa/Fa) and genetically obese (fa/fa) Zucker rats. A consistently reproducible 15% increase in the total specific binding of 0.1 nM [¹²⁵I]-insulin was found in the ARC of the obese genotype. A slight increase in insulin binding in the DMN was also found. No difference in specific insulin binding was found between genotypes in the VMN. Nonlinear least squares analysis of competitive binding studies showed that the K_d of the ARC insulin binding site was 33% higher in the lean rats than in the obese rats, indicating an increased affinity for insulin. No difference in site number (B_max) was found in the ARC, DMN or VMN, and no evidence was found for reduced insulin binding in the hypothalamus of the obese (fa/fa) genotype. The results suggest that hyperphagia and obesity of the obese (fa/fa) Zucker rat genotype may be associated with increased insulin binding in the arcuate nucleus.     

Dynamic of the GRF-induced GH response in genetically obese Zucker rats: influence of central and peripheral factors

To determine the time onset of the growth hormone (GH) alteration in the genetically obese rat, we studied the in vivo and in vitro rat growth hormone releasing factor (rGRF(1-29)NH2)-induced GH secretion in 6- and 8-week-old lean and obese male Zucker rats. Under sodium pentobarbital anesthesia, rGRF(1-29)NH2 (GRF) was injected intravenously at two doses: 0.8 and 4.0 micrograms/kg b.w. Basal serum GH concentrations were similar in lean and obese age-matched animals. The GH response to both GRF doses tested was unchanged in 6-week-old obese rats as compared to their lean litter mates. In contrast, a significant decrease of the GH secretion in response to 4.0 micrograms/kg b.w. GRF was observed in the 8-week-old obese rats. The effect of GRF (1.56, 6.25 and 12.5 pM) was further studied in vitro, in a perifusion system of freshly dispersed anterior pituitary cells of lean and obese Zucker rats. Basal GH release was similar in the 6-week-old animal group. In contrast, it was significantly decreased in 8-week-old obese rats as compared to their lean litter mates. Stimulated GH response to 1.56 and 6.25 pM GRF was significantly greater in the 6-week-old obese group than in the age-matched control group. In contrast, the GH response to all GRF concentrations tested was significantly decreased in the 8-week-old obese rats as compared to their respective lean siblings. In 8-week-old obese rats, a decrease of GH pituitary content and an increase of hypothalamic somatostatin (SRIF) concentration were observed. Insulin and free fatty acid serum were significantly increased in 8-week-old obese rats. In contrast, lower insulin-like growth factor I serum levels were observed in the obese animals as compared to their lean litter mates. Finally, to further clarify the role of the periphery in the inhibition of GH secretion observed in the 8-week-old fatty rats, we exposed cultured pituitary cells of 8-week-old lean animals to 17% serum of their obese litter mates. A significant decrease of GRF-stimulated GH secretion of lean rat pituitary cells exposed to the obese serum was noted (P less than 0.05). This study demonstrates that, in the obese Zucker rat, an alteration of the GH response to GRF is evident by the 8th week of life. This defective GH secretion could be related to peripheral and central abnormalities.     

Tissue and serum somatostatin-like immunoreactivity in lean and obese Zucker rats

Tissue and serum somatostatin levels were measured in genetically lean and obese Zucker rats. Immunoreactive somatostatin content was decreased in three central nervous system regions (hypothalamus, septum and preoptic area and thalamus) of obese rats but was increased in cerebral cortex. No differences were observed in antral or colonic somatostatin content but obese animals had significantly elevated pancreatic levels. Portal vein somatostatin-like immunoreactivity in contrast was significantly lower in obese rats. The widespread alterations in tissue and serum somatostatin-like immunoreactivity suggest either a diffuse abnormality of somatostatin physiology or a response to a generalised feature of the obese hyperinsulinaemic state.     

Expression of hepatic microsomal cholesterol 7 alpha-hydroxylase activity in lean and obese Zucker rats

The activity of hepatic microsomal cholesterol 7 alpha-hydroxylase was studied in genetically obese and lean Zucker rats. The liver microsomal cholesterol 7 alpha-hydroxylase activity in fatty Zucker rats (fa/fa) is about 50% to 70% lower than that of the lean (Fa/-) rats of the same sex, when animals were sacrificed at the middle of the dark cycle. When rats were sacrificed at the middle of the light cycle, cholesterol 7 alpha-hydroxylase activity was the same as in the dark cycle in obese rats of both sexes, but was 65% lower in lean rats. However, cholesterol 7 alpha-hydroxylase activity was stimulated by the treatment with cholestyramine in both obese and lean rats. Our results suggested that the diurnal regulation of cholesterol 7 alpha-hydroxylase activity is lost in obese rats but was present under cholestyramine treatment in the genetically obese strain of rats.     

The Zucker-fatty rat: a review.

The Zucker (fatty) rat is one of a group of animals that inherit obesity as an autosomal Mendelian recessive trait. These rats are obese, hyperphagic, and hyperinsulinemic, but blood glucose remains at normal levels. Although these rats eat more than normal rats, their response to the addition of adulterants to the food or after exposure to the cold is more like that of normal rats than rats with hypothalamic obesity. The hypertriglyceridemia which characterized these animals is due to the increased hepatic production of a very low density lipoproteins. Adipocytes are increased in size and in number with the subcutaneous fat depot showing the largest increase in the number of fat cells. Lipogenesis from glucose is brisk in the young animals but declines with age. Enzymatic patterns of glycolysis and gluconeogenesis appear to reflect the altered internal milieu rather than specific defects. Endocrine changes in the fatty rat include hyperinsulinemia, reduced levels of glucagon, hypothyroidis, and impaired reproductive function. A model is presented in which the features of the genetically obese (Zucker) fatty rat are compared with those of animals with hypothalamic obesity.     

Altered Kidney CYP2C and Cyclooxygenase‐2 Levels Are Associated with Obesity‐Related Albuminuria

Objective: To determine cytochrome P450 (CYP450) and cyclooxygenase (COX) expression and metabolite regulation and renal damage in the early stages of obesity‐related hypertension and diabetes. Research Methods and Procedures: Obese and lean Zucker rats at 10 to 12 weeks of age were studied. Blood pressure was measured in the conscious state using radiotelemetry. Blood glucose levels and body weight were measured periodically. Protein expression of CYP450 and COX enzymes in the kidney cortex, renal microvessels, and glomeruli was studied. The levels of CYP450 and COX metabolites in urine were measured, and urinary albumin excretion, an indicator of kidney damage, was measured. Results: Body weight and blood glucose averaged 432 ± 20 grams and 105 ± 5 mg/dl, respectively, in obese Zucker rats as compared with 320 ± 8 grams and 91 ± 5 mg/dl, respectively, in age‐matched 10‐ to 12‐week‐old lean Zucker rats. Renal microvascular CYP4A and COX‐2 protein levels were increased 2.3‐ and 17.0‐fold, respectively, in obese Zucker rats. The protein expression of CYP2C11 and CYP2C23 was decreased 2.0‐fold in renal microvessels isolated from obese Zucker rats when compared with lean Zucker rats. The urinary excretion rate of thromboxane B₂ was increased significantly in obese Zucker as compared with lean Zucker rats (22.0 ± 1.8 vs. 13.4 ± 1.0 ng/d). Urinary albumin excretion, an index of kidney damage, was increased in the obese Zucker rat at this early age. Discussion: These results suggest that increased CYP4A and COX‐2 protein levels and decreased CYP2C11 and CYP2C23 protein levels occur in association with microalbuminuria during the onset of obesity‐related hypertension and type 2 diabetes.     

Starvation increases gastrointestinal somatostatin in normal and obese Zucker rats: a possible regulatory mechanism

Normal, male Sprague-Dawley (S-D) rats and female, lean and obese Zucker rats were studied in the fed state and after 48 hours of food deprivation. Somatostatin-like immunoreactivity (SLI) was measured from acetic acid extracts of oesophagus-cardia, stomach, small and large intestine, pancreas, hypothalamus, pituitary and cerebellum. Within the CNS, the highest levels of SLI were found in the hypothalamus, while in the gut, these levels were highest in the stomach and pancreas. All Zucker rats displayed higher hypothalamic levels of SLI than did S-D rats. Obese Zucker rats in the fed state differed from their lean littermates in that SLI levels were lower in oesophagus-cardia, stomach and hypothalamus, while being higher in pancreas and pituitary. The response to starvation in both obese and lean Zucker rats was qualitatively similar, and included significant increases in stomach and oesophagus-cardia SLI, but with a significant fall hypothalamic SLI. We have concluded that the increase in gastrointestinal SLI with starvation in Zucker as well as in S-D rats may represent a significant regulatory mechanism in nutrient homeostasis. We postulate that gastric SLI may decrease the availability of intestinal insulin secretagogues in the fasting state. This adaptive mechanism appears to be intact in the obese Zucker rat.