Effect of meal-feeding on the daily variations of insulin, glucose, and NADP-linked dehydrogenases in rats.

Normal and adrenalectomized rats were either ad libitum fed or meal-fed a 66.5% glucose diet for 1 to 3 weeks. Ad libitum-fed rats exhibited synchronized daily rhythms in the levels of immunoreactive insulin, glucose, hepatic glucose-6-phosphate dehydrogenase, and malic enzyme activities. These rhythms were shifted late-morning 2-hour feeding period. The rhythms of adrenalectomized ad libitum-fed rats were different from those for nonadrenalectomized rats. These results indicate that differences between ad libitum- and meal-fed animals may be attributable to the timing of food intake with respect to the timing of observation rather than to meal-feeding alone. It is further concluded that the adrenals play a role in the maintenance of these rhythms.     

Paradoxical effect of ethanol on liver lipogenesis in the genetically-obese Zucker rat

Sixteen obese (fa/fa) Zucker rats, sixteen lean (Fa/-) Zucker rats and sixteen Wistar rats, all male rats aged 7-8 weeks, were given either a control (C) diet containing no ethanol or an ethanol (E) diet in which 36% of the energy was supplied by ethanol, for a period of 4 weeks. The activities of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), glucose-6-phosphatase (EC 3.1.3.9) and glycerol kinase (EC 2.7.1.30) and the glycogen content in the livers of obese (fa/fa) rats were lower in animals given diet E than in those given diet C. As a result, hepatic lipogenesis and fatty degeneration of the liver were reduced in obese (fa/fa) rats given diet E.     

Response of adult lean and obese female Zucker rats to food restriction and refeeding

Lean and obese female Zucker rats were either fed ad libitum (ad libitum-fed lean and ad libitum-fed obese), food-restricted (restricted lean and restricted obese) at 50% of ad libitum intake for 3 weeks from 19–20 weeks of age, or food-restricted and then refed ad libitum for 1 week (restricted-refed lean and restricted-refed obese). Following food restriction, body weights of restricted rats were significantly lower than ad libitum-fed rats within genotype. Body weights of restricted-refed rats were not different from either ad libitum-fed or restricted rats within genotype. Restricted-refed lean rats returned to their previous ad libitum food intake, whereas restricted-refed obese rats ate significantly more food. Both restricted and restricted-refed lean rats had lowered serum insulin levels compared to ad libitum-fed lean rats, whereas there was no effect on serum insulin levels in obese rats. Liver weights and hepatocyte conversion of glucose to fatty acids, glyceride-glycerol and CO₂ were not affected by food restriction or refeeding in lean rats. Among obese rats, restricted obese rats had the smallest liver weights, and restricted-refed obese rats had the highest. Restricted-refed obese rats had greater rates of hepatic glucose metabolism compared to all other groups. Ad libitum-fed lean and restricted-refed lean rats had similar fat pad weights that were significantly greater than those of restricted lean rats. Dietary intervention had no effect on fat pad weight in obese rats. There was no effect of food restriction or refeeding on adipocyte glucose metabolism except for higher glucose conversion to CO₂ for restricted lean rats in comparison to values for all other groups. These results demonstrate that body weight changes of adult female obese rats in response to food restriction and refeeding are similar to those of lean rats; but the effects of liver and adipose tissue weights are different.     

Effect of pancreatectomy or adrenalectomy on the responses of rats to meal-feeding.

The effects of partial pancreatectomy or adrenalectomy and insulin or corticosterone replacement on the responses of rats to meal-feeding were studied. Partial pancreatectomy lowered glucose-6-phosphate dehydrogenase (G6PD) and malic enzyme (ME) activities and resulted in higher blood glucose levels. Partial pancreatectomy did not affect the ability of the animals to adapt to meal-feeding. Insulin supplementation of the pancreatectomized rats restored G6PD and ME activities to those observed in the intact animals and normalized the blood glucose levels in the ad libitum-fed rats. Adrenalectomy decresed the survival of rats subjected to meal-feeding. Eighty percent of the rats died when meal-fed a high glucose diet. Survival was improved when either a 66.5% starch diet or a 40.5% fat diet was substituted for the 66.5% glucose diet. Adrenalectomized meal-fed animals fed 66.5% glucose had higher G6PD and ME activities and higher liver lipid levels than both the adrenalectomized ad libitum-fed and the sham-operated meal-fed rats. Glucocorticoid supplementation lowered G6PD activity in the adrenalectomized meal-fed rats but had no effect on ME activity or liver lipid. Meal-fed adrenalectomized rats had lower liver and serum cholesterol levels than meal-fed intact rats and ad libitum-fed adrenalectomized rats. These cholesterol levels were increased with glucocorticoid supplementation. It was concluded that adaptation to meal-feeding involves an adrenal response to the periodic absence of dietary energy intake, and that the degree of involvement of this response is determined by the composition of the diet.     

Metabolic effects of coconut, safflower, or menhaden oil feeding in lean and obese Zucker rats.

The aim of the present investigation was to study the effects of fish oil feeding in obese Zucker rats to establish its suitability as an animal model of hyperlipidaemia, and to understand the possible mechanism of fish oil-induced perturbations in cell metabolism. Lean and obese Zucker rats were fed on diets containing 180 g coconut, safflower, or menhaden oil/kg for 10 weeks. Body-weights and food intakes of lean coconut (LC), safflower (LS), and menhaden (LM) groups were similar. Obese menhaden (OM) rats had lower food intakes and body-weights compared with obese coconut (OC) and obese safflower (OS) groups, but values for all obese rats were higher than those for lean rats. Liver weights were higher in obese compared with lean rats, but on a percentage body-weight basis menhaden oil rats had higher values within genotype. Serum cholesterol and triacylglycerol levels were lower in the OM group compared with the OC and OS groups, and in the LM group compared with the LC group. Glucose and insulin levels were highest in OS rats followed by OC and OM rats and then the lean rats. Serum triiodothyronine and thyroxine were lower in OM rats compared with OC and OS rats. Liver mitochondrial state 3 rates with glutamate-malate and succinate were lower; mitochondrial beta-oxidation was unaffected and peroxisomal beta-oxidation was higher in menhaden oil rats compared with both coconut and safflower oil rats. In general, consumption of menhaden oil lowered hepatic malic enzyme (EC 1.1.1.38, 1.1.1.40), glucose-6-phosphate dehydrogenase (EC 1.1.1.49) and glutathione peroxidase (EC 1.11.1.9) activities and elevated long-chain fatty acyl-CoA hydrolase (EC 3.1.2.2) activity when compared with the two other diets. It is concluded that obese Zucker rats do respond like human subjects to fish oil feeding but not to vegetable oils. The hypolipidaemic effect of fish oil appears to be mediated through a lowering of lipogenic enzymes, glucose-6-phosphate dehydrogenase and malic enzyme.     

Cholinesterase activity in the serum and liver of Zucker fat rats and controls

Serum and liver cholinesterase activity was observed to be significantly higher in chow fed Zucker fat rats than in chow fed lean controls. Similarly, serum triglycerides and insulin showed significantly higher levels in the fat rats compared to the controls. In contrast, no significant difference was observed for adipose tissue cholinesterase, serum glucose and cholesterol.Correlation analyses revealed a positive correlation between serum and liver cholinesterase activity when data were combined from both obese and lean rats indicating liver as the possible source of cholinesterase. Serum cholinesterase also correlated positively with triglycerides, body weight and food intake. Food intake also showed a strong positive correlation with serum triglycerides and body weight.     

Effects of short-term dehydroepiandrosterone treatment on serum and pancreatic insulin in Zucker rats

Lean and obese female Zucker rats were treated with 0.6% dehydroepiandrosterone (DHEA) from 5 until 10 wk of age and comparisons made to both ad libitum--fed and weight-matched groups. Within genotype, body weights and cumulative food intakes of DHEA and weight-matched groups were not different. Fat pad weights of DHEA rats were significantly lower than those of non-treated groups. Pancreatic insulin content, whether expressed per pancreas or per gram pancreas, was not lowered by DHEA treatment. Fasting serum insulin levels were not altered in lean rats but were significantly lower in DHEA obese rats than in either nontreated obese group. Glucose metabolism (conversion to CO2, fatty acids or glyceride-glycerol) in isolated adipocytes was similar in all groups except that obese rats had greater glyceride-glycerol production than lean rats. Glucose conversion to CO2 in soleus muscle was lower in obese rats than in lean rats. Basal and insulin-stimulated glycogen production was lower in DHEA and weight-matched obese rats than in any other group. Ad libitum-fed lean rats had higher insulin-stimulated glycogen production than DHEA lean and all groups of obese rats. Short-term DHEA treatment of obese Zucker rats lowers serum insulin levels without improvement of insulin resistance in peripheral tissues and without lowering pancreatic insulin content.     

Effects of fructose feeding on lipid parameters in obese and lean, diabetic and nondiabetic Zucker rats

Effects of fructose feeding in moderate amounts on lipid metabolism of obese versus lean, and diabetic versus nondiabetic Zucker rats, were studied. Forty pairs of male lean and obese animals were assigned to two dietary groups, fructose and glucose. For each diet, one-half of lean and obese animals were injected with streptozotocin intraperitoneally (i.p.) to induce diabetes, and the other half were injected with buffer i.p. as a nondiabetic control group. After 9 wk of feeding, animals were fasted overnight, decapitated and exsanguinated. Organs were removed and weighed. Blood glucose, insulin, lactic acid, triglycerides, cholesterol, total liver lipids and urinary glucose were determined. Hyperphagia was observed in obese, non-diabetic and lean-diabetic animals. Streptozotocin injection drastically reduced insulin levels, and produced an impairment of growth, hyperglycemia, glucosuria, polydipsia and polyuria. Fructose feeding increased organ weights in kidney, liver and retroperitoneal adipose tissue, regardless of diabetic state. However, lactic acid levels were lower in fructose-fed groups than glucose-fed groups. In obese rats serum triglyceride levels were also lower in fructose-fed groups than in glucose-fed groups. Serum cholesterol was not affected by fructose feeding. The results indicated that fructose feeding did not produce hyperlipemia and lactic acidosis in the blood circulation in Zucker rats. However, fructose feeding did not improve glucose intolerance in diabetic animals, rather fructose feeding produced hyperinsulinemia in nondiabetic, obese animals.     

Effects of long-term moderate food restriction on growth, serum factors, lipogenic enzymes and adipocyte glucose metabolism in lean and obese Zucker rats

The effects of long-term moderate food restriction were assessed in lean and obese male Zucker rats. A 30% reduction in food intake from 5 to 68 wk of age resulted in parallel lowering of body weight in both lean and obese rats compared to their respective ad libitum-fed control groups. In lean rats, epididymal and retroperitoneal fat pad weights and cell size were lowered by food restriction. In obese rats there was an effect of food restriction on growth of the epididymal pad but not the retroperitoneal pad. Hyperinsulinemia, hyperlipidemia and elevated serum albumin levels, as well as higher activity of lipogenic enzymes, were also not affected by food restriction in the obese rat. In a second experiment, long-term food restriction resulted in greater glucose conversion to CO2 in response to insulin in adipocytes from lean rats but not obese rats compared to their respective control groups. These results indicate that food restriction throughout the first year of life in the obese Zucker rat does not alter the development of hyperplastic obesity and insulin resistance.     

Response of adult lean and obese female Zucker rats to intermittent food restriction/refeeding

Previously, it was found that lean and obese Zucker rats (9-15 wk of age) responded differently to the first of four cycles of food restriction/refeeding. In later cycles, they responded similarly. The present study was undertaken to determine if this finding was due to age, adaptation to the intervention or the obesity. Adult (35-wk-old) lean and obese rats were classified into four groups, ad libitum-fed lean and obese and food-restricted lean and obese. Food-restricted rats underwent four 3-wk periods when they were fed 50% of their ad libitum intake, each followed by a 3-wk period of ad libitum refeeding. Food-restricted rats lost and regained sufficient weight in each cycle to weigh a similar amount as their ad libitum-fed groups by the end of each refeeding period. In lean rats, there were no permanent effects of this intervention except for a 25% reduction in carbohydrate intake. Similar results were found in obese rats, although they did have significantly lower retroperitoneal fat pad weight and serum triacylglycerol levels than ad libitum-fed obese rats at the end of the experiment. These results indicate that lean and obese adult rats respond to each food restriction/refeeding cycle in a similar manner. Results in the earlier experiment would appear to be due both to age and genotype.     

Influence of thyroid status on body weight gain, food intake and serum lipid levels in genetically obese Zucker rats

Body weight gain, food intake and serum concentrations of total lipids, cholesterol, triglycerides, phospholipids and free fatty acids were measured in genetically obese and lean female Zucker rats after surgical thyroidectomy and treatment with 3,5,3'-triiodothyronine (T3) (3 micrograms/100 g body wt per day) for 4 wk. There were large differences in the major serum lipids between the control obese and lean rats. Obese rats were characterized by high levels of the various categories of lipids, especially of triglycerides. Thyroidectomized rats had higher serum concentrations of total lipids and cholesterol than controls in both obese and lean rats. The serum concentration of triglycerides was higher in the thyroidectomized lean rats than in lean controls but lower in their thyroidectomized obese littermates than in obese controls. The T3 treatment resulted in a marked reduction in serum total lipid, cholesterol, triglyceride and phospholipid levels in both thyroidectomized and control obese rats. The same treatment given to thyroidectomized and control lean rats also produced a lower concentration in serum total lipid, cholesterol and triglyceride levels but had no significant effect on serum phospholipid levels. The serum concentration of free fatty acid was not significantly affected by T3 administration in either obese or lean rats. Thyroidectomy induced a lower body weight gain in both obese and lean rats. The T3 treatment restored the body weight gain in the thyroidectomized lean rats but not in the thyroidectomized obese rats, although the food intake was increased by T3.(ABSTRACT TRUNCATED AT 250 WORDS)     

Consequences of restricted feeding/refeeding cycles in lean and obese female Zucker rats

Lean and obese female Zucker rats underwent four, 6-wk cycles of restricted feeding/refeeding. Restricted-fed, lean (RL) rats lost and then regained sufficient weight in each cycle to weigh the same amount as ad libitum-fed, lean (AL) rats at the end of each refeeding period. After the final period of restricted intake, RL rats had significantly lower parametrial and retroperitoneal pad weights and fat cell sizes than AL rats, but their organ growth and lipogenic enzymes were not affected. After the final refeeding period, there were no differences between RL and AL rats except in cumulative food intake. During the first period of restricted intake restricted-fed, obese (RO) rats did not lose weight; however, they were unable to attain the body weight of ad libitum-fed, obese (AO) rats during the subsequent refeeding period. In later cycles, RO rats lost and regained weight, but always weighed significantly less than AO rats. Following the final restricted feeding and refeeding periods, fat pad weights and cell numbers were significantly lower in RO than AO rats, but fat cell size was not affected. Liver weight and lipogenic enzymes were lower in RO than in AO rats after the final period of restricted intake, but were similar to AO rats after the final refeeding period. Permanent effects on heart and kidney growth were found in RO rats. Obese rats appeared to respond differently than lean rats to this form of dietary intervention.     

Obesity-related promotion of aberrant crypt foci in DMH-treated obese Zucker rats correlates with dyslipidemia rather than hyperinsulinemia

BACKGROUND: Obesity and energy restriction modulate the development of precancerous aberrant crypt foci (ACF) in animal models of colon cancer. AIM: Investigation of the major obesity-associated determinants for ACF-development and underlying mechanisms leading to ACF-modulation, such as changes in DNA damage or colonocytes hyperproliferation. METHODS: Lean and obese Zucker rats fed ad libitum (a.l.) or obese pair fed (p.f.) were induced with 1,2-dimethylhydrazine (DMH) for colon cancer. Multiple regression analyses were performed to identify major metabolic factors correlated with ACF number and size (aberrant crypts/ACF). DNA damage is analyzed by the comet-assay, epithelial proliferation by immunohistochemistry. RESULTS: Aberrant crypt foci number was significantly elevated in Zucker obese a.l. (205.7+/-65.4 vs. lean 9.5+/-6.3, P<0.05) and is reduced by pair feeding in Zucker obese rats (81.4+/-28.5 vs. obese a.l., P<0.05). Compared to lean the ACF size was higher in Zucker obese a.l. (2.1+/-0.3 vs. lean 1.3+/-0.2., P<0.05) but is not reduced by pair feeding (1.7+/-0.2; P>0.05). While ACF number and size were modulated by genotype and/or pair feeding the DMH-induced DNA damage and hyperproliferation in colonocytes did not differ significantly between groups. Regression analysis showed that plasma parameters associated with lipid-metabolism (triglycerides, cholesterol, malondialdehyde) significantly correlated with the ACF number and size while parameters linked to carbohydrate-metabolism (glucose, insulin) were weaker determinants. CONCLUSION: Obesity or pair feeding-associated modulation of ACF correlate with parameters related to lipid-metabolism but is not accompanied by changes in DNA damage and proliferation.     

Effects of simmondsin on food intake, growth, and metabolic variables in lean (+/?) and obese (fa/fa) Zucker rats.

Incorporation of 2.5 g/kg of the anorexigen, simmondsin, in the diet resulted in food intake reduction in both lean and obese Zucker rats; however, the obese rats were much more sensitive to the food intake-reducing activity of simmondsin. In both obese and lean simmondsin-treated Zucker rats, growth was slower than in control rats, but was the same as that in pair-fed animals. The 24 h heat production pattern showed a smaller diurnal variation and a lower mean in obese rats than in lean rats. Food intake reduction, as a result of either simmondsin treatment or pair feeding, caused a decrease in mean heat production. Simmondsin treatment, but not pair feeding, caused a decrease in the diurnal variation of heat production. Plasma total cholesterol levels were increased in both simmondsin-treated and pair-fed obese and lean Zucker rats compared with control animals; this increase was mainly due to an increase in HDL-cholesterol levels. Blood leptin levels in both obese and lean rats decreased with decreased food intake and decreased fat deposition, but in obese rats, simmondsin treatment resulted in an additional decrease in leptin levels. It is concluded that the food intake-reducing effect of simmondsin is more pronounced in obese Zucker rats than in their lean littermates, and except for the simmondsin-specific effects on leptin and total cholesterol values in obese littermates, the effects of simmondsin are related to food intake restriction in obese and lean Zucker rats.     

Regional fat pad growth and cellularity in obese zucker rats: modulation by caloric restriction

OBJECTIVE: To investigate, in young obese male Zucker rats, the effects of chronic food restriction and subsequent refeeding on: 1). parameters of nonadipose and adipose growth, 2). regional adipose depot cellularity [fat cell volume (FCV) and number], and 3). circulating leptin levels. RESEARCH METHODS AND PROCEDURES: Obese (fa/fa) and lean (Fa/?) male Zucker rats were studied from age 5 to 19 weeks. After baseline food intake monitoring, 10 obese rats were subjected to 58 days of marked caloric restriction from ad libitum levels [obese-restricted (OR)], followed by a return to ad libitum feeding for 22 days. Ten lean control rats and 10 obese control rats were fed ad libitum for the entire experiment. All rats were fed using a computer-driven automated feeding system designed to mimic natural eating patterns. RESULTS: After food restriction, OR rats weighed significantly less than did lean and obese rats and showed a significant diminution in body and adipose growth as compared with obese rats. Relative adiposity was not different between obese and OR rats and was significantly higher than that of lean rats. The limitation in growth of the adipose tissue mass in OR rats was due mostly to suppression of fat cell proliferation because the mean FCV in each of the four depots was not affected. Serum leptin levels of OR and obese rats were not different from each other but were significantly higher than those of lean rats. DISCUSSION: Marked caloric restriction affects obese male Zucker rats in a manner different from that of nongenetic rodent models (i.e., Wistar rats). In comparison with the response to caloric deprivation of Wistar rats, these calorically restricted obese male Zucker rats appeared to defend their relative adiposity and mean FCV at the expense of fat cell number. These findings indicate that genetic and/or tissue-specific controls override the general consequences of food restriction in this genetic model of obesity.     

Circadian Rhythms in the Zucker Obese Rat: Assessment and Intervention

Body temperature (T_b) and activity were recorded by telemetry in obese and lean Zucker rats in light–dark (LD), constant dark (DD) and constant light (LL). In LD, obese rats, by comparison with lean rats, exhibited a 2–4-h phase advance and attenuated amplitude of T_band activity rhythms. These differences persisted on the first day of DD, and thus were not due to differential sensitivity to masking effects of light. In LL, obese and lean rats exhibited similar free-running periods, thus the phase advance in LD was also not due to a short intrinsic period. In LD, obese rats exhibited more diurnal food intake and a reduced LD intake ratio. To assess the role of diurnal feeding in weight gain, one group of obese rats was fedad libitum, and another fed only at night. Food intake did not differ significantly between groups, butad libitumfed rats gained 23% more weight (60 g) over 60 days, suggesting that excessive diurnal feeding may contribute adversely to body weight regulation in this animal model of obesity.     

Indices of skeletal muscle growth in lean and obese Zucker rats

Skeletal muscle growth was studied in gastrocnemius muscle of lean and obese ad libitum or pair-fed Zucker rats. Skeletal muscle cellularity was determined by DNA, RNA and protein analysis. The length and weight of the tibia and femur were determined. All rats were killed at 15 weeks of age. Lean rats had heavier gastrocnemius, total DNA and total protein than either the ad libitum (ALO) or pair-fed (PFO) obese rats. Lean rats had longer and heavier tibias. Femur length was greater in lean animals while femur weight did not differ between lean and ALO groups. Both had heavier femur than the PFO rats. Comparison of the obese rats revealed that the ALO rats had greater gastrocnemius weight, total DNA and total protein than the PFO group. Total RNA was not different between the lean and ALO group. Comparison of tibias and femurs showed the ALO rats to have longer and heavier bones than the PFO rats. In summary, there are marked differences in the bones of the hindlimb and the cellularity of the gastrocnemius muscle between lean and obese Zucker rats. The differences were accentuated by pair feeding the obese rat.     

Dietary fatty acids on the control of glucose-6-phosphate dehydrogenase and malic enzyme in the starved-refed rat.

The role of dietary unsaturated fat in the control of hepatic glucose-6-phosphate dehydrogenase (G6PD) (EC 1.1.1.49) and malic enzyme (ME) (EC 1.1.1.40) was studied in rats subjected to one or two cycles of starvation-refeeding. Rats starved and refed a control (5% corn oil) diet showed a threefold increase in G6PD activity and a twofold increase in ME activity compared to ad libitum-fed rats. After a second cycle of starvation-refeeding G6PD and ME activities showed fourfold and threefold increases, respectively, as compared to ad libitum-fed rats. Feeding rats diets containing 8% linoleic acid (as triglycerides) prevented the increase in G6PD and ME activities upon starvation-refeeding, diets with oleic, palmitic, and stearic acis when fed did not prevent this increase. Feeding rats various combinations of linoleic, linolenic and oleic acids following starvation prevented the additional increase in G6PD and ME activities after a second starvation-refeeding cycle; however, linoleic acid fed alone during the first refeeding prevented the additional increase in ME activity but not in G6PD activity. It is suggested that the dietary control of these enzymes involves one or more specific polyunsaturated fatty acids.     

Effects of carbohydrate restriction on renal injury in the obese Zucker rat

The obese Zucker rat model of nonimmune-mediated, spontaneous focal glomerulosclerosis is ideally suited to study the influence of diet on the initiation and progression of glomerular injury. Young (6 wk) and old (33 wk) lean and obese female Zucker rats were fed a carbohydrate-restricted diet intermittently for 27 wk. Carbohydrate restriction resulted in lower body weight (460 +/- 16 versus 310 +/- 7 g, p less than 0.025), kidney weight (1.26 +/- 0.04 versus 1.07 +/- 0.05 g, p less than 0.025), and glomerular area (6930 +/- 290 versus 5780 +/- 230 micron2, p less than 0.025) in young obese Zucker rats compared to ad libitum-fed rats. Although urine-albumin excretion was substantially reduced by carbohydrate restriction in young obese Zucker rats (41.1 +/- 12.3 versus 6.9 +/- 2.9 mg/24 h, p less than 0.01), glomerular injury was not significantly altered. In old obese rats, carbohydrate restriction did not significantly reduce albuminuria or prevent the progression of glomerular injury. Thus, intermittent carbohydrate restriction failed to alter significantly either the initiation of glomerular injury in young, or the progression of nephron damage in old, obese Zucker rats.