Roland Favier

Time course of the development of non-alcoholic hepatic steatosis in response to high-fat diet-induced obesity in rats.

The aim of the study was to characterize the time course of the development of high-fat diet-induced hepatic steatosis and its relation to body fat accretion and changes in plasma lipid profile. Female Sprague-Dawley rats were high-fat fed (HF; 42 %, kJ) for 1, 2, 4, 6, 12 and 16 weeks and compared to standard fed rats (SD). Data obtained from HF rats were further analysed by classifying the animals into obesity-prone and obesity-resistant. In HF rats, liver lipid content increased rapidly by approximately 200 % during the first 2 weeks, decreased almost to baseline levels between weeks 2 and 6, and re-increased by 17 % between weeks 6 and 16 (P<0.05). Body weight, body fat accretion, plasma leptin, NEFA and glycerol concentrations were higher in HF than in SD rats (P<0.05). These higher values were established in 2 weeks and the differences between the groups did not further enlarge from weeks 2 to 16. Obesity-prone rats depicted higher body weight and body fat accretion than obesity-resistant and SD rats. Surprisingly, however, liver lipid content was the same in obesity-prone as in obesity-resistant rats as they were both higher than in SD rats (weeks 2 and 16; P<0.05). Our data support the hypothesis that the liver acts as a systemic buffer, largely increasing its lipid content in the early stage of high-fat feeding. Our results also suggest that the development of non-alcoholic hepatic steatosis is more linked to dietary fat ingestion than to body weight gain.

Aerobic capacity and skeletal muscle properties of normoxic and hypoxic rats in response to training

The aim of this study was to determine, in the rat, the effects of chronic exposure (7–9 weeks) to normobaric hypoxia (FIO2 = 0.13, equivalent to 3700 m altitude) on cardiac and skeletal muscle properties, on maximal oxygen uptake (VO2max), and endurance time to exhaustion (ETE). In addition, we evaluated the impact of endurance training (90 min of treadmill running per day, 5 days per week, for 9 weeks) on these parameters. The results were compared to normoxic rats fed ad libitum (NAL) and to normoxic pairweight (NPW) animals in order to take into account the influence of hypoxia on growth rate. It was found that, in sedentary rats, hypoxia results in stunted growth, adrenal atrophy, a significant reduction of cross-sectional area of fast-twitch (type II) fibres, a reduced capillary to-fibre ratio (C/F), and a reduced oxidative capacity (decreases in citrate synthase and 3-hydroxy-Acyl CoA dehydrogenase activities) of the plantaris muscle. These effects are mainly related to the anorexic effects of prolonged exposure to hypoxia. Nevertheless, hypoxic (H) rats displayed higherVO2max and ETE values when compared either to NAL or to NPW animals Endurance training resulted, in all groups (H, NAL, NPW), in a significant change of the fibre type distribution of the plantaris which displayed an increased number of type IIA fibres and a decreased proportion of type IIB fibres. In addition, the C/F ratio and cross-sectional area of fast-twitch fibres were normalized by superimposition of training on hypoxia. BothVO2max and ETE were significantly higher in trained H rats than in NAL, but these improvements were mainly related to the reduced body weight induced by hypoxia. These data suggest that the greater aerobic capacity and tolerance for prolonged exercise induced by chronic exposure to hypoxia can be mainly accounted for by the anorexic effects of hypoxia, although other factors (e.g. increase in oxygen carrying capacity induced by hypoxia acclimatization) may play a significant role in some circumstances (e.g. in sedentary rats)

Molecular system bioenergetics: regulation of substrate supply in response to heart energy demands

This review re‐evaluates regulatory aspects of substrate supply in heart. In aerobic heart, the preferred substrates are always free fatty acids, and workload‐induced increase in their oxidation is observed at unchanged global levels of ATP, phosphocreatine and AMP. Here, we evaluate the mechanisms of regulation of substrate supply for mitochondrial respiration in muscle cells, and show that a system approach is useful also for revealing mechanisms of feedback signalling within the network of substrate oxidation and particularly for explaining the role of malonyl‐CoA in regulation of fatty acid oxidation in cardiac muscle. This approach shows that a key regulator of fatty acid oxidation is the energy demand. Alterations in malonyl‐CoA would not be the reason for, but rather the consequence of, the increased fatty acid oxidation at elevated workloads, when the level of acetyl‐CoA decreases due to shifts in the kinetics of the Krebs cycle. This would make malonyl‐CoA a feedback regulator that allows acyl‐CoA entry into mitochondrial matrix space only when it is needed. Regulation of malonyl‐CoA levels by AMPK does not seem to work as a master on–off switch, but rather as a modulator of fatty acid import.

Skeletal muscle HSP72 and norepinephrine response to static magnetic field in rat

Summary.The present work was undertaken in order to investigate the noradrenergic system and skeletal muscle heat shock protein 72 (HSP72) response to static magnetic field (MF) in male rats. At thermoneutrality (25°C), the exposition of rats 1u2009hour/day for 5 consecutive days to MF of 128u2009mT (m tesla) induced an increase in norepinephrine content in gastrocnemius muscle (+25%, p < 0.05) but had no effect at 67u2009mT (+1%, p > 0.05), indicating a stimulatory effect of sub-acute MF exposure on the noradrenergic system activity. Moreover, exposed rats to MF displayed a non-significant increase of HSP72 levels in gastrocnemius muscles (+29%, p > 0.05). The results indicate that noradrenergic systems in rat’s gastrocnemius muscles are affected by MF exposure. Interestingly, sub-acute exposure insufficiency increased HSP72 levels in gastrocnemius muscles.

Inhibition of xanthine oxidase reduces hyperglycemia-induced oxidative stress and improves mitochondrial alterations in skeletal muscle of diabetic mice.

Reactive oxygen species (ROS) have been widely implicated in the pathogenesis of diabetes and more recently in mitochondrial alterations in skeletal muscle of diabetic mice. However, so far the exact sources of ROS in skeletal muscle have remained elusive. Aiming at better understanding the causes of mitochondrial alterations in diabetic muscle, we designed this study to characterize the sites of ROS production in skeletal muscle of streptozotocin (STZ)-induced diabetic mice. Hyperglycemic STZ mice showed increased markers of systemic and muscular oxidative stress, as evidenced by increased circulating H(2)O(2) and muscle carbonylated protein levels. Interestingly, insulin treatment reduced hyperglycemia and improved systemic and muscular oxidative stress in STZ mice. We demonstrated that increased oxidative stress in muscle of STZ mice is associated with an increase of xanthine oxidase (XO) expression and activity and is mediated by an induction of H(2)O(2) production by both mitochondria and XO. Finally, treatment of STZ mice, as well as high-fat and high-sucrose diet-fed mice, with oxypurinol reduced markers of systemic and muscular oxidative stress and prevented structural and functional mitochondrial alterations, confirming the in vivo relevance of XO in ROS production in diabetic mice. These data indicate that mitochondria and XO are the major sources of hyperglycemia-induced ROS production in skeletal muscle and that the inhibition of XO reduces oxidative stress and improves mitochondrial alterations in diabetic muscle.

Influence of gender and endogenous sex steroids on catecholaminergic structures involved in physiological adaptation to hypoxia

Abstractu2002Mechanisms underlying sex-related differences in adaptation to high altitude were investigated by assessing the turnover of dopamine and noradrenaline in structures of the chemoafferent pathway, i.e. carotid body and brainstem noradrenergic cell groups (A1, A5, A6, A2 to which chemosensory fibres project). The influence of gender was assessed in male and female rats reared at an altitude of 3600 m, whereas the influence of endogenous sex hormones was evaluated by castration. Haematocrit, red blood cell count and plasma erythropoietin levels were lower in females than in males (–5%, –15%, –53%, respectively). Dopamine and noradrenaline turnover were higher in female structures (carotid body: +51%; A2: +140%; A1: +54%; A5: +27%). Dopamine and noradrenaline turnover in carotid body and brainstem cell groups were differently affected by castration, i.e. enhanced by orchidectomy (carotid body: +134%; A2: +120%; A1: +69%; A5: +67%) but inhibited by ovariectomy (carotid body: –33%; A2: –92%). Orchidectomy elicited a reduction in haematocrit (–10%), haemoglobin concentration (–8%) and red blood cell count (–24%), whereas haematological status remained unaltered after ovariectomy. Therefore, both gender and endogenous sex steroids may control catecholamine activity differently in structures involved in the chemoafferent pathway, thus providing a neurochemical basis for sex-related differences in adaptation to hypoxia.

Skeletal muscle HSP72 level during endurance training: influence of peripheral arterial insufficiency.

Abstract. Heat shock protein 72 (HSP72), the inducible isoform of the HSP70 family, is synthesized in exercised rat muscles and in the ischaemic heart. To determine the isolated and combined effects of chronic ischaemia and repeated exercise on skeletal muscle HSP72 expression, male Wistar rats were subjected to unilateral occlusion of the iliac artery. Beginning 1xa0week after ischaemia, half the rats were exercised on a motor-driven treadmill once a day, 5xa0days/week, the other half were restricted to cage activity. Rats were sacrificed after 2, 4, or 8xa0weeks of endurance training, together with the age-matched sedentary rats. Tissue samples were obtained from the plantaris and the red portion of the quadriceps of both hind-limbs. Endurance-trained rats displayed significantly increased HSP72 levels in skeletal muscles. Occlusion of iliac artery did not affect the HSP72 level in muscle from sedentary rats but enhanced that in the trained rats. Mitochondrial oxidative capacity, as assessed from cytochrome oxidase and citrate synthase activities, decreased during growth in sedentary animals, but was significantly improved by endurance training. Nevertheless, increased oxidative capacity induced by endurance training was partially prevented by arterial occlusion. It is concluded that both HSP72 levels and mitochondrial oxidative capacity are affected by ischaemia and training but these changes are not necessarily related. Whereas superimposition of chronic exercise on peripheral arterial insufficiency increased HSP72 levels, our results demonstrate that endurance training even for extended period of time is not effective for improving oxidative capacity of ischaemic muscle.

High Expression of Thyroid Hormone Receptors and Mitochondrial Glycerol-3-phosphate Dehydrogenase in the Liver Is Linked to Enhanced Fatty Acid Oxidation in Lou/C, a Rat Strain Resistant to Obesity

Besides its well recognized role in lipid and carbohydrate metabolisms, glycerol is involved in the regulation of cellular energy homeostasis via glycerol-3-phosphate, a key metabolite in the translocation of reducing power across the mitochondrial inner membrane with mitochondrial glycerol-3-phosphate dehydrogenase. Here, we report a high rate of gluconeogenesis from glycerol and fatty acid oxidation in hepatocytes from Lou/C, a peculiar rat strain derived from Wistar, which is resistant to age- and diet-related obesity. This feature, associated with elevated cellular respiration and cytosolic ATP/ADP and NAD+/NADH ratios, was linked to a high expression and activity of mitochondrial glycerol-3-phosphate dehydrogenase. Interestingly, this strain exhibited high expression and protein content of thyroid hormone receptor, whereas circulating thyroid hormone levels were slightly decreased and hepatic thyroid hormone carrier MCT-8 mRNA levels were not modified. We propose that an enhanced liver thyroid hormone receptor in Lou/C may explain its unique resistance to obesity by increasing fatty acid oxidation and lowering liver oxidative phosphorylation stoichiometry at the translocation of reducing power into mitochondria.

Differential effects of ventilatory stimulation by sex hormones and almitrine on hypoxic erythrocytosis

Abstractu2002In the absence of pulmonary disease, hypoventilation is considered to be the primary cause of Chronic Mountain Sickness, and there is some reason to believe that chronic administration of respiratory analeptics could be useful for treatment of this disease. The present study was intended to define comparatively the influence of two potent ventilatory stimulants, namely a combination of progesterone and estrogen and the pharmacological agent almitrine, on catecholaminergic structures implicated in the chemoreflex pathway and on hypoxia-induced polycythemia. Three groups of young male rats born and living at high altitude (3 600 m) were examined: untreated animals (n = 25), rats given ovarian steroids (progesterone plus 17β-estradiol, n = 25) or almitrine (n = 25) for 6 weeks until sacrifice. Ovarian steroids or almitrine had pronounced neurochemical effects on the afferent chemoreflex circuitry. Both treatments inhibited norepinephrine (NE) and dopamine (DA) turnover in the carotid body, but central processing of chemosensory inputs differed between the two respiratory drugs. Ovarian steroids inhibited noradrenergic activity in the projection area of the chemosensory nerve fibers within the caudal portion (A2C) of the nucleus tractus solitarius (NTS). In contrast, almitrine stimulated neurochemical activity of other brainstem noradrenergic cell groups involved in cardiorespiratory control, i.e., the rostral portion (A2R) of the NTS, the nucleus reticularis lateralis (A1), the nucleus olivaris superior (A5) and the locus ceruleus (A6). Although both treatments increased chemoreflex drive and ventilation, only sex hormones decreased erythropoietin (EPO) levels and the degree of polycythemia. These results suggest that stimulation of ventilation through activation of peripheral arterial chemoreceptors activation alone is not sufficient for reducing EPO levels and polycythemia. The better efficiency of female sex hormone treatment as compared to almitrine could be related either to the central effects of progesterone and estrogen and/or to the impact of these hormones on erythropoiesis at the kidney/bone marrow level.

Lack of starvation-induced activation of AMP-activated protein kinase in the hypothalamus of the Lou/C rats resistant to obesity

Objective:The AMP-activated protein kinase (AMPK) is involved in the control of food intake by the hypothalamus. The aim of this work was to investigate if modification of hypothalamic AMPK regulation could be related to the spontaneous food restriction of Lou/C rats, a strain resistant to obesity exhibiting a 40% reduction in caloric intake compared with their lean Wistar counterparts.Design:Three-month-old male Lou/C rats were compared with age-matched male Wistar rats in both fed ad libitum and 24-h food deprivation state.Measurements and results:We first confirmed that starvation activated both isoforms of AMPK catalytic α subunits and enhanced the phosphorylation state of its downstream targets acetyl-CoA carboxylase and elongation factor 2 in the hypothalamus of Wistar rats. These changes were not observed in the hypothalamus of Lou/C rats. Interestingly, the starvation-induced changes in hypothalamic mRNA levels of the main orexigenic and anorexigenic neuropeptides were also blunted in the Lou/C rats. Analysis of the concentrations of circulating substrates and hormones known to regulate hypothalamic AMPK indicated that the starvation-induced changes in ghrelin, adiponectin and leptin were not observed in Lou/C rats. Furthermore, an increased phosphorylation state of signal transducer and activator of transcription 3 (STAT3), which admittedly mediates leptin signaling, was evidenced in the hypothalamus of the starved Lou/C rats, as well as modifications of expression of the leptin-sensitive genes suppressor of cytokine signaling-3 and stearoyl-coenzyme A desaturase 1. In addition, despite reduced leptin level in fed Lou/C rats, the phosphorylation state of hypothalamic STAT3 remained similar to that found in fed Wistar rats, an adaptation that could be explained by the concomitant increase in ObRb leptin receptor mRNA expression.Conclusion:Activation of hypothalamic AMPK by starvation, which stimulates food intake through changes in (an)orexigenic neuropeptides in the normal rats, was not observed in the spontaneously hypophagic Lou/C rats.