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.     

Insulin- and exercise-stimulated skeletal muscle blood flow and glucose uptake in obese men

OBJECTIVE: Insulin resistance in obese subjects results in the impaired use of glucose by insulin-sensitive tissues, e.g., skeletal muscle. In the present study, we determined whether insulin resistance in obesity is associated with an impaired ability of exercise to stimulate muscle blood flow, oxygen delivery, or glucose uptake. RESEARCH METHODS AND PROCEDURES: Nine obese (body mass index = 36 +/- 2 kg/m(2)) and 11 age-matched nonobese men (body mass index = 22 +/- 1 kg/m(2)) performed one-legged isometric exercise during hyperinsulinemia. Rates of femoral muscle blood flow, oxygen consumption, and glucose uptake were measured simultaneously in both legs using [(15)O]H(2)O, [(15)O]O(2), [(18)F]fluoro-deoxy-glucose, and positron emission tomography. RESULTS: The obese subjects exhibited resistance to insulin stimulation of glucose uptake in resting muscle, regardless of whether glucose uptake was expressed per kilogram of femoral muscle mass (p = 0.001) or per the total mass of quadriceps femoris muscle. At similar workloads, oxygen consumption, blood flow, and glucose uptake were lower in the obese than the nonobese subjects when expressed per kilogram of muscle, but similar when expressed per quadriceps femoris muscle mass. DISCUSSION: We conclude that obesity is characterized by insulin resistance of glucose uptake in resting skeletal muscle regardless of how glucose uptake is expressed. When compared with nonobese individuals at similar absolute workloads and under identical hyperinsulinemic conditions, the ability of exercise to increase muscle oxygen uptake, blood flow, and glucose uptake per muscle mass is blunted in obese insulin-resistant subjects. However, these defects are compensated for by an increase in muscle mass.     

Food intake and choice in lean and obese Zucker rats after intragastric carbohydrate preloads

Feeding behavior in response to a carbohydrate preload was determined in lean and obese Zucker rats. Rats were adapted to a schedule of feeding during a 12-h daily dark period. Carbohydrate preloads (cornstarch in water) were given by intragastric intubation 30 min before rats had access to a choice of two diets that differed in their protein and carbohydrate contents. Behavioral responses of lean rats to carbohydrate preloads were prompt and selective. In the first hour of feeding, reduction in intake, mainly of the high carbohydrate-low protein diet, more than compensated for the energy content of the preload. In contrast, obese rats failed to achieve a similar degree of compensation to the energy surplus from the carbohydrate preload during the first hour of feeding. Also, their decreases in food intake were nonselective. In general, it took obese rats two or more hours to adjust their feeding behavior in response to a carbohydrate preload. Increasing the size of the preload did not improve the overall response of obese rats, but an extra 30-min lapse between preload and food access led to selective suppression in first-hour intake from the high carbohydrate-low protein diet. These findings suggest that altered food intake and selection behavior of the obese Zucker rat may be, in part, due to a delay in response to physiologic and metabolic changes arising from carbohydrate ingestion.     

Levels of hypothalamic neurotransmitters in lean and obese Zucker rats

Hypothalamic neurotransmitter levels were compared between groups of Zucker rats. Animals were grouped by gender, phenotype and preference for dietary fat. Before sacrifice all animals consumed a standard rat chow diet and were fasted overnight. Five rats from each of eight groups were assayed. Hypothalamic regions (lateral, LH; ventromedial, VMH; paraventricular, PVN) and the raphe were isolated and analyzed for dopamine, norepinephrine, epinephrine, serotonin and 5-hydroxyindoleacetic acid. A factorial analysis of variance was used to compare the concentrations of these biogenic amines in the four regions across phenotypic, gender and fat preference profiles. No differences were demonstrated between groups based upon fat food preference. Epinephrine and 5-HIAA content varied between lean and obese animals but there were no differences in the content of serotonin, norepinephrine or dopamine. The results are consistent with the hypothesis that the obese animal eats more because it releases less of the satiety-inducing neurotransmitter serotonin in the hypothalamus.     

Carnitine metabolism in lean and obese Zucker rats during starvation

Carnitine metabolism during starvation was studied in adult lean and obese female Zucker rats. Comparisons were made between rats starved for 0, 3, 6 or 9 d. Total plasma carnitine was not affected by obesity or starvation, but free plasma carnitine decreased with starvation. Plasma acid-soluble acylcarnitine was lower in obese than in lean rats, and increased with starvation in both lean and obese rats. Plasma acid-insoluble acylcarnitine was not affected by obesity but increased with starvation. Liver free and acid-soluble acylcarnitine were lower in obese rats than lean rats, and starvation increased liver free carnitine and acid-insoluble acylcarnitine. Free carnitine was lower in muscle from obese rats than from lean rats. In kidney, free carnitine decreased during starvation. Heart carnitine was not affected by obesity or starvation. Urinary free carnitine and acid-soluble acylcarnitine clearance decreased during starvation. These studies indicate that: 1) lean and obese Zucker rats conserve carnitine during starvation; and 2) the decreases in liver carnitine concentration reflect the loss of cellular constituents rather than increases in total hepatic carnitine.     

Effect of treadmill training on muscle oxidative capacity and accretion in young male obese and nonobese Zucker rats

This study was designed to determine if treadmill training of the male obese Zucker rat could reverse its deficit in muscle accretion, expose a possible latent defect in its muscle oxidative capacity or significantly alter its food intake and lipid deposition. At 12 wk of age muscle mass and myofibrillar protein concentration were significantly lower and body lipid and food intake were significantly higher in the sedentary obese than in the nonobese rat. Exercise, by both inducing hypophagia and increasing energy output, led to a lower body weight, body lipid, and muscle mass in the exercised than in the nonexercised rats. This response to exercise did not differ between both phenotypes, except for body lipid. In that case the reduction of body lipid was greater for the obese rats. Muscle mitochondrial enzyme activities and rates of mitochondrial respiration in the obese rats were not different or greater than those of their sedentary or pair-exercised nonobese counterparts. Taken together these data indicate that oxidative capacity per unit of muscle is not significantly lower in the obese rats than in nonobese rats in both sedentary and exercised states, but that total muscle oxidative capacity is lower on a whole-animal basis since total muscle mass is lower. Further, exercise reduces, but does not prevent the enhanced weight gain and lipid accretion that characterizes the obese rat.     

Effect of dehydroepiandrosterone on growth in lean and obese Zucker rats

Several studies were undertaken to determine the effect of dehydroepiandrosterone (DHEA) on growth in Zucker rats. In experiment 1, 3 weeks of DHEA treatment in lean rats resulted in decreased body weight gain in comparison to control rats. In experiment 2, both lean and obese rats were treated with DHEA from 6 to 21 weeks of age. Significant decreases in body weight were found for both lean and obese DHEA-treated rats. The food efficiency ratio (FER) was significantly decreased in both DHEA-treated groups. Significant decreases in parametrial and retroperitoneal fat pads were found in both lean and obese DHEA-treated rats. This was primarily attributed to a decrease in fat cell number in lean rats and to decreases in both number and size of fat cells in obese rats. In experiment 3 obese female rats were treated with DHEA from 6 to 21 weeks of age followed by 15 weeks with DHEA removed from the diet. Significantly more weight was gained by the rats previously treated than by the control rats, but body weight remained significantly lower than in the control groups. These data indicate DHEA has an effect on altering body weight and body fat in lean and obese Zucker rats.     

Comparison of dehydroepiandrosterone and clofibric acid treatments in obese Zucker rats

The effects of either dehydroepiandrosterone (DHEA) or clofibric acid (CFA) treatment on obese female Zucker rats were compared. After 5 wk of treatment, food intake, body weight gain and food efficiency ratio of DHEA-treated rats were 85, 64 and 75%, respectively, of those of CFA-treated and control rats. Liver weights of DHEA- and CFA-treated rats were higher than those of control rats. Non-fasting serum glucose levels were similar in all groups, but serum insulin level of DHEA-treated rats was 59% of that in CFA-treated and control rats. Results of glucose tolerance tests were not different among the groups. Mitochondrial state 3 and 4 rates expressed per g liver or per liver with either glutamate-malate or succinate as substrate were higher in DHEA-treated rats than in CFA-treated or control rats. Mitochondrial long-chain fatty acyl-CoA hydrolase activity was 8.5 and 4 times higher in DHEA- and CFA-treated rats, respectively, than in control rats. These results suggest that although DHEA and CFA affect some hepatic biochemical parameters similarly, they are distinct in their effects on body weight, serum insulin and mitochondrial respiration in obese Zucker rats.     

Hypothalamic galanin and early state of hyperphagia in obese Zucker rats

Galanin (GAL) stimulates food intake in normal rats when it is injected in different hypothalamic areas involved in feeding such as the paraventricular and ventromedial nuclei and the lateral hypothalamus. At adulthood, the hyperphagic obese Zucker rat is characterized by a general dysregulation of some important neuropeptides involved in the regulation of food intake including GAL. The aim of this study was to measure GAL in different microdissected brain areas in 2- and 4-week-old lean (FA/-) and obese (fa/fa) male Zucker rats in order to know if GAL actively participates in triggering abnormal feeding behavior in obese rats. There was a significant increase (40%-220%) in GAL concentration with age in the arcuate and dorsomedial nuclei and in the above areas except for the lateral hypothalamus. Genotype differences were observed in the arcuate and paraventricular nuclei only. GAL levels were globally lower in obese than in lean rats (-15% to -25%) and the difference was significant at 2 weeks of age in the paraventricular nucleus and at 4 weeks of age in the arcuate nucleus. In agreement with human observations, these data suggest that GAL is not an early player in the development of overeating.     

Protein turnover in insulin-treated, alloxan-diabetic lean and obese Zucker rats

Obese and lean Zucker rats were made diabetic by intracardiac injections of alloxan (65-72 mg/kg body weight) and then given daily injections of protamine zinc insulin [1.25 U/(100 g/d)] for 6, 9 and 12 d. Body weight, food intake, plasma glucose and immunoreactive insulin concentrations were not different for lean and obese diabetic rats of similar ages. Rates of increase in carcass protein, mixed muscle protein and myofibrillar protein were less in obese than in lean rats. However, rates of increase for the sarcoplasmic fraction were not different. Fractional rates of synthesis of total muscle protein and myofibrillar protein, as determined by continuous intravenous infusion of [14C]tyrosine, were comparable in the two genotypes. Fractional rate of myofibrillar protein degradation, as determined by urinary 3-methylhistidine excretion, was higher in obese than in lean rats. Differences in calculated absolute rates between genotypes did not parallel differences in the fractional rates, due mainly to a smaller protein mass in obese rats. As a consequence, absolute synthetic rates were lower in obese rats, while absolute degradation rates were similar in the two genotypes. In contrast, rates of liver protein synthesis were similar in obese and lean rats, whether expressed as fractional or absolute rates. These results indicate that decreased protein deposition in the obese animal is a consequence of both an absolute decrease in protein synthesis in muscle as well as a disproportionately elevated protein degradation in muscle. Hyperinsulinemia normally seen in obese rats may be an adaptive response to minimize the impaired balance between protein synthesis and degradation.     

Dietary single cell protein reduces fatty liver in obese Zucker rats

There is growing evidence that dietary proteins may interfere with lipid metabolism. We therefore examined the effects of feeding obese Zucker rats a single cell protein (SCP) with low ratios of methionine:glycine and lysine:arginine for 6 weeks. SCP feeding reduced the hepatic steatosis and lowered the plasma transaminase levels when compared with casein-fed rats (controls). The fatty acid oxidation was increased in liver mitochondria and peroxisomes, whereas the activities of enzymes involved in lipogenesis and TAG biosynthesis were unaffected. SCP feeding affected the fatty acid composition of liver lipids and plasma, and reduced the mRNA levels of the fatty acid desaturases. The decreased gene expression of stearoyl-CoA desaturase suggested that the fatty acids were directed towards oxidation rather than esterification as TAG. The decreased mRNA levels of VLDL-receptor and lipoprotein lipase in the liver after SCP feeding suggested that the uptake of TAG-rich lipoprotein to the liver was decreased. To conclude, the reduced fatty liver by SCP feeding may be caused by the increased capacity for fatty acid beta-oxidation in the liver, combined with changed fatty acid composition and possibly a reduced hepatic clearance of circulating VLDL. An increased awareness of the effect of dietary proteins on lipid metabolism could be of relevance in future dietary treatment of non-alcoholic fatty liver disease.     

Effect of fasting on vascular contractility in lean and obese Zucker rats

We compared the effects of fasting (36 h) on blood pressure and aortic contractile responsiveness in lean and obese Zucker rats. Fasting of lean animals resulted in a significant loss in body weight (-9.1 +/- 0.1%) and reduction in systolic blood pressure (-11.4 +/- 1.9 mmHg). Fasting plasma triacylglycerols (-76%) and beta-hydroxybutryic acid (beta-HBA) (+ 218%) were significantly decreased and increased, respectively. The fasting plasma concentrations of insulin (-72%) were significantly decreased, whereas glucose and epinephrine (Epi) were not changed in lean rats. The fasting of obese rats also resulted in weight loss (-5.6 +/- 1.3%) but did not cause a significant reduction of blood pressure. The plasma total cholesterol (+18%) was increased, triacylglycerols (-42%) were decreased and beta-HBA levels were unchanged in fasted obese rats. Similar to lean animals, the insulin levels of fasted obese rats were significantly decreased (-85%), whereas glucose and Epi concentrations were not significantly changed. Fasting of lean animals had no effect on the maximal contractile response of aortae to high K(+) and phorbol 12, 13 dibutyrate (PDBu) but significantly reduced the response to norepinephrine (NE) (% reference: fed, 61.1 +/- 11.0; fasted, 45.6 +/- 4.5). In addition, the concentration for half-maximal response (ED(50)) to NE was increased in fasted lean rats (fed, 1.8+/-0.2 x 10(-8)M; fasted, 3.0+/-0.3 x 10(-8)M). By comparison, fasting of obese rats had no significant effect on the contractile response to K(+), NE, or PDBu. The results show that short-term food withdrawal induces significant changes in vascular contractile properties of lean but not obese rats. Because fasting-induced changes were variable depending on the agonist, the results further suggest that the mechanism did not involve a general loss or enhancement in functional status.     

Opioidergic modulation of ventilatory response to sustained hypoxia in obese Zucker rats.

OBJECTIVE: To determine whether altered central and/or peripheral opioidergic mechanisms contribute to the altered ventilatory response to sustained hypoxia in obese Zucker rats. RESEARCH METHODS AND PROCEDURES: Eight lean (176 +/- 8 [SEM] g) and eight obese (225 +/- 12 g) Zucker rats were studied at 6 weeks of age. Pulmonary ventilation ((E)), tidal volume (V(T)), and breathing frequency (f) at rest and in response to sustained (30 minutes) hypoxic (10% O(2)) challenges were measured on three separate occasions by the barometric method after the randomized, blinded administration of equal volumes of saline (control), naloxone methiodide (N(M); 5 mg/kg, peripheral opioid antagonist), or naloxone hydrochloride (N(HCl); 5 mg/kg, peripheral and central opioid antagonist). RESULTS: Administration of N(M) and N(HCl) in lean animals had no effect on (E) either at rest or during 30 minutes of sustained exposure to hypoxia. Similarly, N(M) failed to alter (E) in obese rats. In contrast, N(HCl) significantly (p < 0.05) increased (E) and V(T) both at rest and during 2 to 10 minutes of hypoxic exposure in obese rats. After 20 to 30 minutes of hypoxic exposure, V(T) remained elevated with N(HCl), but the earlier elevation of (E) seemed to be attenuated due to a decrease in f at 20 minutes of exposure to hypoxia. DISCUSSION: Thus, endogenous opioids modulate both resting (E) and the ventilatory response to sustained hypoxia in obese, but not in lean, Zucker rats by acting specifically on opioid receptors located within the central nervous system.     

Antioxidant effects of a grapeseed procyanidin extract and oleoyl-estrone in obese Zucker rats

ObjectiveTo substantiate the relation between obesity and oxidative stress and to assess the potential beneficial properties of a grapeseed proanthocyanidin extract (GSPE), the amelioration of obesity with oleoyl-estrone (OE), and the possible combined effect of GSPE and OE on the hepatic and renal antioxidant enzyme system in obesity-induced oxidative stress.MethodsMale obese Zucker rats were divided into four groups: GSPE, OE, GSPE + OE, and OC (control). For 30 d they were gavaged with GSPE, OE, GSPE + OE, or sunflower oil as the control vehicle (OC). Lean Zucker rats gavaged with the vehicle comprised the lean control group. Hepatic and renal antioxidant enzymes and oxidative biomarkers were determined at the end of the experimental period.ResultsHepatic antioxidant activities were higher in obese rats than in lean ones. All these activities decreased when obese rats were treated with GSPE, whereas only some of these activities decreased with OE and GSPE + OE treatments. In the kidney, few antioxidant enzymes had higher activities in obese than in lean rats, and OE and GSPE + OE were the treatments that inhibited most enzymes studied. Glutathione S-transferase activity was always lower with the exception of the kidney of obese rats and all treatments used increased the low glutathione levels found in obesity.ConclusionGSPE and OE improve oxidative stress in obese Zucker rats. The effect of GSPE + OE is comparable to GSPE for the liver and to OE for the kidney. Thus the effects of GSPE and OE are not additive and are organ dependent.     

Effect of bran and cellulose on lipid metabolism in obese female Zucker rats

The effects on lipid metabolism of bran and cellulose added to a low-cholesterol purified diet were studied in female obese Zucker rats. The obese Zucker rat was chosen for the experiment because of its sensitivity to dietary changes with regard to lipid metabolism. Cellulos, as well as bran, had an effect on lipid metabolism, which is expressed as an increase in the excretion of fecal bile acid and cholesterol and, in the case of bran feeding, as a decrease in liver cholesterol level and an increase of fecal fat excretion. The increased excretion rate of cholesterol and bile acids did not result in a reduction of the plasma cholesterol levels, which suggests a compensatory mechanism in the form of either an increase of hepatic cholesterol synthesis and/or changes within the body cholesterol pools. The increased excretion of fecal fat that occurred when bran was added to the diet, is not likely to influence cholesterol metabolism, since fecal fat excretion only appeared to form a small percentage of the total fat intake. Bran as well as cellulose caused a significant increase in fecal wet weight and a decrease in transit time. Though both cellulose and bran are capable of binding bile acid, cholesterol and/or fat, the intestinal transit time and the fecal bulk might also be a cause of increased steroid excretion. The fact that our results are inconsistent with those of some investigations described in the literature but in agreement with the findings of other workers, again stresses the primary need for a better chemical characterization of the various fiber sources.     

Maintenance requirement and energetic efficiency of lean and obese Zucker rats.

The body balance technique was used to determine the maintenance requirement and the energetic efficiency of 6-week-old male lean and obese Zucker rats. Fifteen lean and 15 obese rats were assigned to three groups of five each and fed three levels of food intake: ad libitum, 75% of ad libitum, and 50% of ad libitum, for a 4-week period. The obese rats consumed more food and gained significantly more weight than the lean rats. Obese rats utilized dietary energy more efficiently than lean rats but the opposite was true for dietary protein. The efficiency with which obese rats utilized metabolizable energy for gain was 36% compared with efficiency of utilization of metabolizable energy above maintenance for obese and lean were 51.4% and 21.4%, respectively. The maintenance requirement on the basis of metabolic body size for the obese and lean was calculated to be 122 and 130.5 kcal ME per day per kg.75 for the 4-week period. These results indicate that the energetic efficiency is higher in obese than in lean Zucker rats and suggest that the higher energetic efficiency is not the result of a significantly lower maintenance requirement.     

Dietary conjugated linoleic acid reduces adiposity in lean but not obese Zucker rats

Recent studies have demonstrated a reduction in body fat in growing animals fed conjugated linoleic acid (CLA). Two experiments were conducted to extend these observations to obese rats so that the mechanism of the actions of CLA might be more easily elucidated. In experiment 1, male lean and obese Zucker rats were fed diets containing either 0 or 0.5% CLA for 5 wk. There was no effect of diet on growth rate or food intake. Dietary CLA reduced retroperitoneal and inguinal fat pad weights in the lean rats but increased fat pad weights in the obese genotype (diet x genotype interaction; P < 0.05). Determination of fat pad cellularity indicated that these changes in fat pad weight were due to a reduction or increase in average fat cell size for the lean and obese Zucker rats, respectively. In experiment 2, we sought to reproduce these effects on fat pad size, as well as to determine the effect of dietary CLA on the catabolic response to bacterial endotoxin injection in obese Zucker rats. Growing female lean and obese Zucker rats were fed diets containing 0 or 0.5% CLA for 8 wk. On d 28, each rat was injected intraperitoneally with lipopolysaccharide from Escherichia coli serotype 055:B5 (1 mg/kg body weight) and body weight was determined over the next 96 h. There was a diet x genotype interaction (P < 0.05) for the body weight response to lipopolysaccharide 24 h postinjection. Lean rats fed CLA lost less weight than did lean controls, but obese rats fed CLA lost more weight than did obese controls. As in the first experiment, there was a diet x genotype (P < 0.05) for the effect of treatment on retroperitoneal fat pad weights determined at the end of the experiment. Lean rats fed CLA had smaller RP fat pads than did lean controls, but obese rats fed CLA once again had heavier RP fat pads than did obese controls. These results indicate that CLA reduces body fat and catabolic response to endotoxin injection in lean Zucker rats but not in the obese genotype. The observed interaction between diet and genotype warrants additional investigation into the specific mechanism(s) of the biological activities of CLA.     

Pancreatic enzyme activity in obese and lean Zucker rats: a developmental study

Exocrine pancreatic enzyme activities were measured in female, obese and lean Zucker rats at 1, 3, 6 and 12 months of age. When expressed per 100 g body weight, food intake, pancreatic weight, and activities of trypsin, chymotrypsin and lipase were highest at 1 month and showed a similar decline with age in both obese and lean rats. In contrast, amylase activity declined only in the obese and remained elevated in the lean. Expressed per milligram pancreas, amylase activity was lower in the obese after 3 months, trypsin was elevated in the obese at 6 and 12 months, lipase was lower at 1 month and elevated at 6 months, whereas chymotrypsin did not differ. Hyperinsulinemia was present in obese rats at all ages, and yet amylase levels were reduced. The inability of insulin to elevate amylase activity in obese animals may be a reflection of the insulin resistance of these animals.     

Increased A-ring reduction of glucocorticoids in obese Zucker rats: effects of insulin sensitization

OBJECTIVE: Obesity is associated with altered glucocorticoid metabolism, which may impact on hypothalamic-pituitary-adrenal axis activity. Here we characterize hepatic 5alpha- and 5beta-reductase in obese rats and their responses to insulin sensitization. RESEARCH METHODS AND PROCEDURES: Hepatic A-ring reductase protein and mRNA were assessed in lean and obese Zucker rats after insulin sensitization with metformin or rosiglitazone (n = 7 to 8/group). RESULTS: Hepatic 5alpha-reductase 1 and 5beta-reductase mRNA and protein (p < 0.01) were increased in obese rats. Insulin sensitization ameliorated increased 5alpha-reductase 1 mRNA in obese rats (p < 0.01) and partially reversed increased 5beta-reductase activity. DISCUSSION: Hepatic clearance of glucocorticoids by 5alpha- and 5beta-reductase is increased in obese Zucker rats, and this increase in clearance is attenuated by insulin sensitization. This increased hepatic clearance may underpin compensatory activation of the hypothalamic-pituitary-adrenal axis in obesity.