Rat tail suspension causes a decline in insulin receptors

Physical inactivity, insulin resistance, and the oxidative-inflammatory loop

Epidemiological data indicate that physical inactivity, a main factor of global energetic imbalance, is involved in the worldwide epidemic of obesity and metabolic disorders such as insulin resistance. Although the complex pathogenesis of insulin resistance is not fully understood, literature data accumulated during the past decades clearly indicate that the activation of the oxidative-inflammatory loop plays a major role. By activating the oxidative-inflammatory loop in insulin-sensitive tissues, fat gain and adipose tissue dysfunction likely contribute to induce insulin resistance during chronic and prolonged physical inactivity. However, in the past years, evidence has emerged showing that early insulin resistance also occurs after very short-term exposure to physical inactivity (1-7 days) without any fat gain or energetic imbalance. The possible role of liver disturbances or endothelial dysfunction is suggested, but further studies are necessary to really conclude. Inactive skeletal muscle probably constitutes the primary triggering tissue for the development of early insulin resistance. In the present review, we discuss on the current knowledge about the effect of physical inactivity on whole-body and peripheral insulin sensitivity, and how local inflammation and oxidative stress arising with physical inactivity could potentially induce insulin resistance. We assume that early muscle insulin resistance allows the excess nutrients to shift in the storage tissues to withstand starvation through energy storage. We also consider when chronic and prolonged, physical inactivity over an extended period of time is an underestimated contributor to pathological insulin resistance and hence indirectly to numerous chronic diseases.

Animal model of simulated microgravity: a comparative study of hindlimb unloading via tail versus pelvic suspension

The aim of this study was to compare physiological effects of hindlimb suspension (HLS) in tail- and pelvic-HLS rat models to determine if severe stretch in the tail-HLS rats lumbosacral skeleton may contribute to the changes traditionally attributed to simulated microgravity and musculoskeletal disuse in the tail-HLS model. Adult male Sprague-Dawley rats divided into suspended and control-nonsuspended groups were subjected to two separate methods of suspension and maintained with regular food and water for 2 weeks. Body weights, food and water consumption, soleus muscle weight, tibial bone mineral density, random plasma insulin, and hindlimb pain on pressure threshold (PPT) were measured. X-ray analysis demonstrated severe lordosis in tail- but not pelvic-HLS animals. However, growth retardation, food consumption, and soleus muscle weight and tibial bone density (decreased relative to control) did not differ between two HLS models. Furthermore, HLS rats developed similar levels of insulinopenia and mechanical hyperalgesia (decreased PPT) in both tail- and pelvic-HLS groups. In the rat-to-rat comparisons, the growth retardation and the decreased PPT observed in HLS-rats was most associated with insulinopenia. In conclusion, these data suggest that HLS results in mild prediabetic state with some signs of pressure hyperalgesia, but lumbosacral skeleton stretch plays little role, if any, in these pathological changes.

Effect of dietary saturated fatty acids on HNF-4α DNA binding activity and ApoCIII mRNA in sedentary rat liver

Hind limb-suspended rats represent a sedentary-hyperinsulinemic model with a liver dyslipidemia mainly related to changes in sterol regulatory element-binding protein 1 (SREBP-1) and peroxisome proliferator-activated receptor-α (PPARα) expression and activity. To assess the effects of dietary fatty acids on hepatic lipid homeostasis, the hepatic expression and activity of PPARα, SREBP-1, and hepatocyte nuclear factor-4α (HNF-4α) were investigated in this animal model. In control and sedentary rats, diets enriched with saturated, monounsaturated, and polyunsaturated fatty acids (PUFA) enhanced the expression of the PPARα target genes carnitine palmitoyltransferase 1 and acyl-CoA oxidase, the highest effect being exerted by ω-3. The same diets reduced SREBP-1 mRNA and target lipogenic gene expression, as indicated by the reduction in fatty acid synthase and acetyl-CoA carboxylase mRNA content. Effects were greater in sedentary rat liver than in controls on the same diet. Only the ω-3 enriched diet decreased liver triglyceride content as well as plasma cholesterol and triglyceride levels in sedentary rats. This effect may be mainly related to the enhanced mitochondrial and peroxisomal β-oxidation genes expression. On the other hand, saturated fatty acid-enriched diet induced an increase in liver triglyceride content and enhanced plasma cholesterol and triglyceride levels, both in control and immobilized rats. This detrimental effect may be ascribed to the induced HNF-4α binding activity on ApoCIII promoter and to the enhanced ApoCIII mRNA levels both in control and in sedentary rat livers. In conclusion, we can speculate that dietary saturated fats, acting at apolipoprotein transcriptional level, may impact on the close relationship existing among high ApoCIII plasma level, dyslipidemia, and atherosclerosis.

Feeding a corn oil/sucrose-enriched diet enhances steatohepatitis in sedentary rats

The current study investigated the combined effects of feeding a high-fat/high-sucrose (HF/HS) diet to rodents rendered sedentary via hindlimb unloading (HU). For 3 wk before HU, male Wistar rats were fed chow or a diet in which 32% of calories were derived from corn oil fat and 48% of calories from sucrose. Feeding continued during an additional 3-wk period of HU. Subsequently, blood samples were collected for determination of circulating leukocyte counts, insulin levels, and portal vein endotoxin. Inflammation, necrosis, and steatosis were assessed in formalin-fixed liver sections. No biochemical or histological evidence of injury was observed in control rats fed chow or HF/HS. HU increased circulating neutrophils and resulted in hyperinsulinemia. Mild hepatic fat accumulation and minimal focal necroinflammation were observed in this group. Feeding HF/HS during HU exacerbated hyperinsulinemia, hepatic steatosis, Kupffer cell content, and cytokine expression. Significant portal endotoxemia was noted in HU rats but was not influenced by HF/HS diet. On the other hand, feeding HF/HS significantly enhanced lipid peroxidation end products in liver of HU rats by approximately threefold compared with chow-fed rats. In summary, these findings demonstrate that feeding a high-calorie diet potentiates steatosis and injury in sedentary HU rats. Mechanisms underlying enhanced injury most likely involved lipid peroxidation. Importantly, these findings suggest that dietary manipulation combined with physical inactivity can be used to model steatohepatitis.

Phosphorylation of insulin receptor substrate-1 serine 307 correlates with JNK activity in atrophic skeletal muscle

c-Jun NH(2)-terminal kinase (JNK) has been shown to negatively regulate insulin signaling through serine phosphorylation of residue 307 within the insulin receptor substrate-1 (IRS-1) in adipose and liver tissue. Using a rat hindlimb suspension model for muscle disuse atrophy, we found that JNK activity was significantly elevated in atrophic soleus muscle and that IRS-1 was phosphorylated on Ser(307) prior to the degradation of the IRS-1 protein. Moreover, we observed a corresponding reduction in Akt activity, providing biochemical evidence for the development of insulin resistance in atrophic skeletal muscle.

Enhanced expression of hepatic lipogenic enzymes in an animal model of sedentariness

The hindlimb-suspended rat was used as animal model to investigate the effects induced by immobilization of the skeletal muscle in the expression of the genes encoding hepatic lipogenic enzymes. Following a 14-day period of immobilization, rats were injected intraperitoneally with radioactive acetate, and the labeling of hepatic lipids and cholesterol was evaluated 15 min after the isotope injection. The incorporation of labeled acetate in lipids and cholesterol was almost three times higher in the liver of immobilized rats than in control animals as a consequence of the enhanced transcription of the genes encoding acetyl-CoA synthase, acetyl-CoA carboxylase, fatty acid synthase, and 3-hydroxy-3-methylglutaryl-CoA reductase. The high expression of the key enzymes for fatty acid and cholesterol synthesis induced by immobilization was not paralleled by an increase of the hepatic sterol-regulatory element binding protein (SREBP)-1 and SREBP-2 mRNA content. However, the expression of the mature form of SREBP-1 and SREBP-2 was higher in the nuclear fraction of immobilized rat liver than in controls due to a significant increase of the cleavage of the native proteins. Immobilization also affected the expression of proteins involved in lipid degradation. In fact, the hepatic content of peroxisome proliferator-activated receptor-alpha (PPARalpha) mRNA and of PPARalpha target genes encoding carnitine palmitoyl transferase-1 and acyl-CoA oxidase were significantly increased upon immobilization.

Effects of space food bar feeding on bone mass and metabolism in normal and unloaded rats

During spaceflights in the shuttle, rats are provided specific food bars. To determine whether this diet allows normal body and skeletal growth, we used four groups of rats fed either standard pellet food or space food bars during a 2-wk unloading experiment. We recorded food intake, body weight, tibial bone mass, and mineral content by ash analyses, cancellous bone volume, and cell activities by histomorphometry. We found that food intake was not different when comparing the two types of food, but that suspended animals had a lower food intake than normal loaded animals. Body weight and bone mass were found lower in suspended animals than in normal loaded animals. Finally, longitudinal growth rate, cancellous bone volume, and bone formation rate were lower in suspended animals, irrespective of the type of food. These results show that space food bar feeding did not affect normal body and skeletal growth, and that body and bone changes due to unloading were not significantly different in animals fed space food bars and standard food.