CCK-resistance in Zucker obese versus lean rats

Obese Zucker rats are less sensitive to the satiety effect of CCK than lean litter mates. The present studies further characterised this CCK resistance. Subcutaneous injection of the CCK agonist caerulein dose-dependently decreased food intake in Zucker obese and lean rats whereas the CCK-B agonist gastrin-17 did not. Caerulein at 4 μg/kg, which resulted in CCK plasma bioactivity slightly above postprandial levels, decreased food intake in lean rats but not in obese rats. The decrease in food intake was also more marked at higher caerulein doses (20–100 μg/kg) in lean versus obese rats. In lean animals the satiety effects of the “near physiological” 4 μg/kg caerulein dose was abolished after blockade of vagal afferents with capsaicin, whereas the effects of higher caerulein doses were not. CCK-stimulated amylase secretion from pancreatic acini and binding capacity of ¹²⁵I- labelled CCK-8 were decreased in obese versus lean rats. The CCK-A antagonist loxiglumide at 20 mg/kg, a dose which abolished the action of all caerulein doses on food intake, failed to alter the food intake either in obese or in lean rats when given without an agonist. The results suggest that the satiety effects of “near physiological” doses of caerulein in lean rats are mediated by vagal afferents whereas pharmacological doses act via non-vagal mechanisms. The differences in CCK's satiety effect between lean and obese rats may be due to differences in CCK-receptor binding and action at peripheral vagal sites. However, the failure of the CCK-A antagonist to increase food intake questions whether any of the effects of exogenous CCK are of physiological relevance.     

Individual amino acid balances in young lean and obese Zucker rats fed a cafeteria diet

The amino acid composition of the diet ingested by reference and cafeteria diet-fed lean and obese Zucker rats has been analyzed from day 30 to 60 after birth. Their body protein amino acid composition was measured, as well as the urinary and faecal losses incurred during the period studied. The protein actually selected by the rats fed the cafeteria diet had essentially the same amino acid composition as the reference diet. The mean protein amino acid composition of the rat showed only small changes with breed, age or diet.Cafeteria-fed rats had a higher dietary protein digestion/absorption efficiency than reference diet-fed rats. Obese rats wasted a high proportion of dietary amino acids when given the reference diet, but not on the cafeteria diet. In all cases, the amino acids lost as such in the urine were a minimal portion of available amino acids.In addition to breed, the rates of protein accretion are deeply influenced by diet, but even more by the age — or size — of the animals: cafeteria-fed rats grew faster, to higher body protein settings, but later protein accrual decreased considerably; this is probably due to a limitation in the blueprint for growth which restricts net protein deposition when a certain body size is attained. Obese rats, however, kept accuring protein with high rates throughout.Diet composition — and not protein availability or quality-induced deep changes in amino acid metabolism. Since the differences in the absolute levels of dietary protein or carbohydrate energy ingested by rats fed the reference or cafeteria diets were small, it can be assumed that high (lipid) energy elicits the changes observed in amino acid metabolism by the cafeteria diet. The effects induced in the fate of the nitrogen ingested were more related to the fractional protein energy proportion than to its absolute values. Cafeteria-fed rats tended to absorb more amino acids and preserve them more efficiently; these effects were shown even under conditions of genetic obesity.There were deep differences in handling of dietary amino acids by dietary or genetically obese rats. The former manage to extract and accrue larger proportions of their dietary amino acids than the latter. The effects of both models of amino acid management were largely additive, suggesting that the mechanisms underlying the development of obesity did not run in parallel to those affecting the control of amino acid utilization. Obesity may be developed in both cases despite a completely different strategy of amino acid assimilation, accrual and utilization. (Mol Cell Biochem121: 45–58, 1993)     

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.     

Kinetic parameters for plasma beta-endorphin in lean and obese Zucker rats

To determine plasma clearance kinetics for beta-endorphin (BE) by empirical compartmental analysis, a bolus of radioactive labeled 125I-BE was rapidly injected into a carotid artery catheter of unanesthetized lean (L) and obese (O) Zucker rats. The plasma disappearance of 125I was followed over a 3-h period. A 3-component exponential equation provided the best fit for plasma data. Plasma transit times were very short (10 s); however, plasma fractional catabolic rate was much slower. Plasma mean residence time was similar for both groups (50 min) as was recycle time (1.3 min). These data suggest that BE plasma disappearance kinetics are similar in L and O rats.     

Effects of 2G exposure on lean and genetically obese Zucker rats

Changes in the ambient force environment alter the regulation of adiposity, food intake and energy expenditure (i.e., energy balance). Lean (Fa/Fa) and obese (fa/fa) male Zucker rats were exposed to 2G (twice Earth's normal gravity) for eight weeks via centrifugation to test the hypothesis that the Fa/Fa rats recover to a greater degree from the effects of an increased ambient force environment on body mass and food intake, than do the fa/fa rats which have a dysfunctional leptin regulatory system. The rats (lean and obese exposed to either 1G or 2G) were individually housed in standard vivarium cages with food and water provided ad libitum. The acute response to 2G included a transient hypophagia accompanied by decreased body mass, followed by recovery of feeding to new steady-states. In the lean rats, body mass-independent food intake had returned to 1G control levels six weeks after the onset of centrifugation, and body mass increased towards that of the 1G rats. In contrast, food intake and body mass of the 2G obese rats plateaued at a level lower than that of the 1G controls. Although percent carcass fat was reduced more in the 2G leans vs. 2G obese rats, the latter lost significantly more grams of fat than did the leans. Our data suggest that with respect to food intake and body mass, the lean rats recover from the initial effects of 2G exposure to a greater degree than do the fatty rats, a difference that likely reflects the functionality of the leptin regulatory system in the leans.     

Effects of CCK on gastrointestinal function in lean and obese Zucker rats

Cholecystokinin (CCK), a hormone affecting several gastrointestinal functions, has also been shown to elicit satiety and affect daily meal patterns. Since Zucker obese rats are less sensitive to the satiety effects of CCK, two experiments were designed to determine if they are also less sensitive to the gastric emptying and intestinal transit rate effects of CCK. In the first experiment phenol red was administered to 5.5 hr fasted rats 15 minutes after intraperitoneal injection of CCK-8 or saline. Rats were sacrificed after 30 minutes, the stomach and small intestine were removed, and phenol red content was measured. More phenol red was in the stomach of obese but not lean rats treated with CCK-8. The rate of transit of the contents of the small intestine was increased by CCK-8 and the percent of phenol red in the fourth quarter of the small intestine was greater in obese than lean rats (91 vs 37%, p<0.05). In the second experiment gastrointestinal transit of ferric oxide was measured during the light and dark phases of the diurnal cycle, and when obese rats were ad lib or yoke-fed to lean pair-mates. Total gastrointestinal transit time of the ferric oxide was decreased 15% when CCK-8 was administered to yoke-fed obese rats in either the light or dark portions of the diurnal cycle but was not affected in ad lib-fed obese rats or lean rats. Thus, while Zucker obese rats are less sensitive to satiety effects of CCK, they appear to be more sensitive to the gastrointestinal effects of CCK, and therefore it is not clear what role these gastrointestinal responses have on the feeding behavior responses.     

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.     

Hepatic conversion of thyroxine to triiodothyronine in obese and lean Zucker rats

The in vitro conversion of thyroxine (T4) to triiodothyronine (T3) was studied in liver homogenates from fed and fasted lean and obese Zucker rats. T3 generation was decreased in fed young (2 month) obese rats as compared to values in fed lean controls. This was not corrected by the addition of dithiothreitol (DTT), suggesting a deficiency in 5'-deiodinase activity in young obese rats. Both lean and obese 2 month old rats responded to a 2 day fast by decreasing hepatic T3 generation as is always observed in other strains of rats. The hepatic conversion rate was not decreased in older (5 month) fed obese rats when compared to age-matched lean controls. Hepatic conversion of T4 to T3 was markedly decreased in 5 month old lean Zucker rats fasted for 4 days. In contrast, a 4 day fast had no effect on the hepatic conversion rate in the 5 month old obese rats. The hepatic conversion rate was assessed in 5 month old obese rats fasted for up to 28 days and hepatic conversion still did not decrease. This paradoxical response of the 5 month old obese rat may provide a new model to further evaluate the control of hepatic T3 generation from T4.     

Behavioral thermoregulation in obese and lean Zucker rats in a thermal gradient

Genetically obese Zucker (Z) rats have been reported to display a body core temperature (Tb) that is consistently below that of their lean littermates. We asked the question whether the lower Tb was a result of deficits in thermoregulation or a downward resetting of the set point for Tb. For a period of 45 consecutive hours, lean and obese Z rats were free to move within a thermal gradient with an ambient temperature (T(a)) range of 15-35 degrees C, while subjected to a 12:12-h light-dark cycle. Tb was measured using a miniature radio transmitter implanted within the peritoneal cavity. Oxygen consumption (VO2) was measured using an open flow technique. Movements and most frequently occupied position in the gradient (preferred T(a)) were recorded using a series of infrared phototransmitters. Obese Z rats were compared with lean Z rats matched for either age (A) or body mass (M). Our results show that obese Z rats have a lower Tb [37.1 +/- 0.1 degrees C (SD) vs. 37.3 +/- 0.1 degrees C, P < 0.001] and a lower VO2 (25.3 +/- 1.9 ml x kg(-1) x h(-1)) than lean controls [33.1 +/- 3.7 (A) and 33.9 +/- 3.9 (M) ml x kg(-1) x h(-1), P < 0.001]. Also, the obese Z rats consistently chose to occupy a cooler T(a) [20.9 +/- 0.6 degrees C vs. 22.7 +/- 0.6 degrees C (A) and 22.5 +/- 0.7 degrees C (M), P < 0.001] in the thermal gradient. This suggests a lower set point for Tb in the obese Z rat, as they refused the option to select a warmer T(a) that might allow them to counteract any thermoregulatory deficiency that could lead to a low Tb. Although all rats followed a definite circadian rhythm for both Tb and VO2, there was no discernible circadian pattern for preferred T(a) in either obese or lean rats. Obese Z rats tended to show a far less definite light-dark activity cycle compared with lean rats.     

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.     

Lipoprotein lipase and cholesterol transfer activities of lean and obese Zucker rats.

Adult female lean and obese Zucker rats maintained under standard conditions were used for the estimation of plasma, liver and white adipose tissue (WAT) activity of lipoprotein lipase, plasma and liver hepatic lipase and plasma lecithin-cholesterol acyltransferase. No differences in plasma or tissue levels of lipoprotein lipase between lean and obese rats were detected, but the larger WAT size of the obese rats resulted in higher lipase activity per unit of rat weight. Hepatic lipase levels in plasma were higher in the obese, but in liver, the higher activity was found in lean rats. No significant differences were found for lecithin-cholesterol acyltransferase activity, except when the levels in the HDL fraction were expressed per unit of protein weight, showing lower activity in the obese rats. In conclusion, the essentially maintained enzyme activities in obese rat tissues suggest that they cannot explain the deficient lipoproteins processing of obese rats, and, consequently their dislipidaemia.     

Regulation of lypolysis in white adipose tissues of lean and obese Zucker rats

Obese Zucker rat is often used as a model of genetic obesity to understand the mechanism of the development of obesity. In the present work, in order to better understand the regulation of lipolysis in the Zucker rat, the lipolytic activities of adipocytes isolated from different adipose depots of lean and obese Zucker rats, in the basal state or after catecholamine stimulation have been measured. The obese Zucker rat presents hyperinsulinemia without hyperglycemia and with elevated plasma free fatty acids, suggesting a dyslipidemia. Morphological studies of three adipose deposits show a marked hypertrophic and hyperplastic type of obesity, much pronounced in the subcutaneous depot. In the current study we show that the basal lipolytic rate is higher in adipocytes from each deposit of obese rats (when results are corrected for cell surface area). This finding, associated with the increase of all deposits, could contribute to the elevated plasma FFA observed. Investigation of the responsiveness of dibutyril cAMP (DBcAMP) points out that the defect in the NE responsiveness is essentially located at post-receptor level. Nevertheless, a receptor defect could not be excluded as suggested by a decrease of the beta-ARs observed in all deposits. Our study points out that the lipolytic resistance to catecholamines in adipose tissue of obese Zucker rats appears to counteract the increase in the lipolytic rate, in order to moderate the increase in plasma FFA levels that may contribute to the hyperinsulinemia observed, characteristic of an insulino-resistant state.     

Feeding response to ghrelin agonist and antagonist in lean and obese Zucker rats

Ghrelin is a new orexigenic and adipogenic peptide primarily produced by the stomach and the hypothalamus. In the present experiment, we determined the circulating ghrelin levels in 60-week old fa/fa Zucker rats with a well-established obesity (n = 12) and in their lean (FA/FA) counterparts (n = 12). We also tested the feeding response of both groups to intra-peritoneal (I.P.) injection of ghrelin agonist and antagonist. Obese rats ate significantly more than the lean rats (21.7 +/- 1.1 vs. 18.3 +/- 0.3 g/day; p < 0.01). Their plasma ghrelin concentration was 35% higher than that in the lean homozygous rats (p < 0.025). GHRP-6 (1 mg/kg I.P, a GHS-R agonist) stimulated food intake in lean but not in obese rats (p < 0.01), whereas [D-Lys)]-GHRP-6 (12 mg/kg I.P., a GHS-R antagonist) decreased food intake in both groups (p < 0.0001). These results indicate that the obese Zucker rat is characterized by an increase in plasma ghrelin concentrations and by an attenuated response to a GHS-R agonist. They support a role for ghrelin in the development of obesity in the absence of leptin signaling.     

Effect of exercise training on calpain systems in lean and obese Zucker rats

Exercise training plays a major role in the improving physiology of diabetes. Herein we aimed to investigate the influence of exercise upon the calcium-dependent calpain-isoform expressions of lean or obese Zucker rats, a model of obesity and type II diabetes (NIDDM). Five-month-old rats were divided: (1) obese sedentary (OS, n=7); (2) obese exercise (OE, n=7); (3) lean sedentary (LS, n=7); (4) lean exercise (LE, n=7). After 2-month exercise (treadmill running), the body weight (BW) and expression of calpain 10, μ-calpain, and m-calpain in skeletal muscles were determined by RT-PCR, using β-actin as internal standard. We found exercise is useful for BW lossing, especially in the obese rats. The BW difference between OS and OE rats (69 g vs. 18.2 g) was more significantly than that between LS and LE rats (41.8 g vs. 28.7g). The calpain 10 expression of LS rats (0.965) was lower than that of LE rats (1.006), whereas those of OS and OE were comparable. The μ- or m-calpain expressions of sedentary groups (OS, LS) was significantly higher than those of exercise groups (OE, LE). The μ-calpain expression (1.13/0.92) and m-calpain expression (1.01/0.99) of OS/LS rats was significantly higher than those of OE/LE rats [1.07/0.9 (μ-calpain); 0.97/0.95 (m-calpain)]. We concluded that the μ- or m-calpains in skeletal muscle are regulated by exercise in both lean and obese Zucker rats. Exercise and BW controlling might improve the physiopathology of obesity and diabetes. Both μ- or m-calpains might become useful markers for prognoses of diabetes.     

Ammonium uptake and urea production in hepatocytes from lean and obese Zucker rats

The metabolic differences in vitro between genetic and dietary obese rats in the uptake of ammonium and amino acids by the liver and their use for ureogenesis have been assayed using hepatocytes isolated from Lean, Obese Zucker (Genetic obese) rats and Dietary obese rats. The hepatocytes of genetic obese animals took up more ammonium and produced higher amounts of urea from ammonium and alanine than those of lean and dietary obese groups (2 and 5 times more respectively). In the lean and dietary obese groups urea synthesis accounted for almost all the nitrogen taken up as ammonium. Thus, dietary and genetic obesity show a widely different handling of nitrogen, and the genetic obese rats need to break down protein to maintain their hepatocyte function.     

Different susceptibility of liver grafts from lean and obese Zucker rats to preservation injury

We compared the susceptibility of liver grafts from lean and obese Zucker rats to preservation injury, using two organ-preservation techniques: conventional static preservation (SP) and machine perfusion (MP) preservation. SP: livers preserved by UW solution at 4, 8 or 20 °C for 6-h. MP: livers perfused for 6-h with an improved oxygenated Krebs–Henseleit solution (KH) at 4, 8 or 20 °C. Reperfusion with KH (2-h) was performed either with the SP or MP preserved livers. Fatty livers tolerate SP poorly at 4, 8 and 20 °C as compared with MP at the same temperatures. SP induced a decrease in the ATP/ADP ratio both at 8 and 20 °C in obese rats while an increase in energy status was found with MP at 8 and 20 °C. Nitrate/nitrite (NOx) concentration was higher and bile flow lower in livers preserved with SP than MP. In lean rats, no differences were observed between MP and SP as regards enzyme release, bile production and NOx levels except for SP at 20 °C in which high enzyme release and low bile flow were observed. In lean rats ATP/ADP was higher and NOx was lower with MP at 20 °C than with SP at 20 °C. To optimize steatotic liver preservation SP should be avoided because it is particularly detrimental as compared with MP.     

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.     

Serum hormone levels and tissue metabolism in pair-fed lean and obese Zucker rats.

Obese Zucker rats were either pair-fed to their lean litter-mates or fed ad lib, to determine the effect of hyperphagia on serum hormone levels and tissue metabolism as indicated by enzyme activities and in vitro metabolite flux. Hyperphagia was shown to be non-essential for the elevation in serum insulin and suppression in serum growth hormone and prolactin in the genetically obese rat. It was also shown that the increased liver cell lipogenic rate was not dependent on hyperphagia in the obese rat and that adipose cell lipogenesis was not significantly altered in the pair-fed obese rat. The utilization of alanine for glucose synthesis in vitro was similar for both lean and obese rats, but its utilization for fatty acid synthesis was higher in the obese rat. Data is presented which suggest that the inhibitory effect of glucagon on liver lipogenesis is blunted in the obese rat.