Ascorbic acid oral treatment modifies lipolytic response and behavioural activity but not glucocorticoid metabolism in cafeteria diet‐fed rats

Aim: To analyse the effects of vitamin C (VC), a potent dietary antioxidant, oral supplementation on body weight gain, behavioural activity, lipolytic response and glucocorticoid metabolism in the early stages of diet‐induced overweight in rats. Methods: Food intake, locomotive activity and faecal corticosterone were assessed during the 14 day trial period. After 2 weeks, the animals were sacrificed and the body composition, biochemical markers and lipolytic response from isolated adipocytes from retroperitoneal white adipose tissue were examined. Results: The intake of a high‐fat diet by rats induced a significant increase in body weight, adiposity and insulin resistance markers as well as a decrease in faecal corticosterone levels compared with standard diet‐fed rats. Interestingly, the animals fed on the cafeteria diet showed a significant increase in the isoproterenol‐induced lipolytic response in isolated adipocytes. Furthermore, this cafeteria‐fed group showed a reduced locomotive behaviour than the control rats. On the other hand, oral VC supplementation in animals receiving the high‐fat diet restored the cafeteria diet effect in some of the analysed variables such as final body weight and plasma insulin to control group levels. Remarkably, increases in locomotive behaviour and a significant decrease in the lipolytic response induced by isoproterenol on isolated adipocytes from animals treated with VC were observed. Conclusion: This work demonstrates that an oral ascorbic acid supplementation has direct effects on behavioural activity and on adipocyte lipolysis in early obesity stages in rats, which could indicate a protective short‐term role of this vitamin against adiposity induced by chronic high‐fat diet consumption.     

GLP-1受体激动剂对肥胖小鼠脂肪组织的脂解作用及机制探讨

目的:探讨胰高血糖素样肽1(GLP-1)受体激动剂艾塞那肽(exendin-4)对肥胖小鼠脂肪组织的作用及机制。方法:8周龄C57BL/6J小鼠高脂喂养12周后随机分为艾塞那肽组和生理盐水对照组,另设正常饮食组。取附睾旁脂肪检测sirtuin 1(SIRT1)、脂肪甘油三酯脂酶(ATGL)、肿瘤坏死因子α(TNF-α)及脂联素mRNA的表达。Exendin-4或联合SIRT1激动剂/抑制剂处理3T3-L1脂肪细胞24 h;小鼠胚胎成纤维细胞(MEF)诱导成脂肪细胞后exendin-4干预24 h;检测SIRT1、ATGL和激素敏感性脂酶(HSL)的蛋白表达水平。结果:与生理盐水对照组相比,艾塞那肽组小鼠附睾旁脂肪量、空腹血糖及血甘油三酯水平降低(均P0.05),体重减轻,血TNF-α水平降低。艾塞那肽干预后,肥胖小鼠脂肪组织SIRT1、ATGL和脂联素mRNA表达明显上调,TNF-αmRNA表达明显下调(P0.05)。Exendin-4剂量依赖性促进3T3-L1脂肪细胞SIRT1、ATGL和HSL脂解相关蛋白的表达。联合SIRT1激动剂后,脂滴数量减少,上述脂解相关蛋白的表达上调。联合SIRT1抑制剂后上述作用减弱。敲除SIRT1后MEF脂肪细胞内脂滴增大,数量增多,exendin-4促进脂解的作用消失。结论:艾塞那肽通过激活SIRT1促进肥胖小鼠脂肪组织脂解作用。     

Repeated fasting/refeeding elevates plasma leptin without increasing fat mass in rats

It is known that repeated fasting and refeeding increase capacity of fat storage in adipose tissue as an adaptive response to fasting. However, the amount of weight gain in fasted/refed animals falls behind the control level in most rodent studies. Leptin, an adipocyte-derived hormone that impacts on energy homeostasis, may be up-regulated by repeated cycles of fasting and refeeding. In this study, we investigated the adaptive response of leptin to repeated cycles of 1-day fasting and 1-day refeeding for 42 days in rats. The repeated fasting and refeeding (RFR) rats gained less body weight than the controls. Daily food intake of the RFR rats was decreased after Day 16 and remained suppressed. Circulating leptin levels of the RFR rats were significantly elevated at Day 35 compared with the controls and at Day 44 compared with the controls and pair-fed (PF) rats. Leptin mRNA levels of these rats were also significantly increased in retroperitoneal white adipose tissue (RT-WAT) compared with the controls and PF rats. Moreover, hypothalamic proopiomelanocortic (POMC) gene expression was augmented in the RFR rats compared with the controls and PF rats. However, there was no statistical difference in percent visceral fat mass among the experimental groups. Lipoprotein lipase (LPL) mRNA levels of RFR rats were significantly increased in RT-WAT compared with the controls and PF rats. These data indicated that leptin was up-regulated in response to chronic repeated fasting and refeeding cycles without a concomitant increase in adiposity, and the augmented leptin levels were associated with an increase in POMC gene expression, reduced food intake, and diminished body weight gain.     

Molecular and morphometric description of adipose tissue during weight changes: a quantitative tool for assessment of tissue texture

The aim of this study was to evaluate the morphometric changes of adipose tissue of lean and obese rats as assessed by computerized image analysis (IA) system in experimental conditions, with different degrees of adiposity. Moreover, to validate measures obtained by image analysis by correlation with direct measures of adiposity (body weight, epididimal fat, mean fat cell size and serum leptin). Finally to correlate these changes to expression of genes involved in lipid deposition and mobilization in adipose tissue. Lean (Fa/?) and genetically obese (fa/fa) Zucker rats were studied. Obese rats were food-restricted or treated with retinoic acid (ATRA) in order to reduce body weight and fat content. Moreover, gene expression of two key enzymes involved in fat metabolism (HSL and DGAT) were assessed in adipose tissue by RT-PCR. Our results show that HSL expression in adipose tissue was lower in obese compared to lean rats (1.47+/-0.02 vs 0.35+/-0.03, p<0.005) and was upregulated during food restriction in obese rats. DGAT expression was similar in lean and obese rats and was reduced by treatment with ATRA in obese rats. Tissue texture assessed by IA was significantly higher in lean compared to obese rats (23.2+/-0.6 vs 11.6+/-2.4%; p=0.01). Tissue structure highly correlated with adiposity in obese rats with different amount of body fat (area fraction vs epididimal fat depot: p=0.001). Distribution of measures for each sample, an index of spread of adipose tissue texture, as expressed by the standard deviation, correlated with adiposity (standard deviation vs epididimal fat depot: p=0.002) thus suggesting that adipose tissue texture increases its heterogeneity when adiposity is lower. This observation is in agreement with the hypothesis that the process of lipid mobilization from adipose tissue is not uniform, but a subpopulation of slimming adipocytes undergoes a complete release of their fat content while the rest of the tissue is much less affected. Moreover, image analysis system seems a reliable quantitative tool for assessment of adipose tissue texture.     

Catecholamine-induced lipolysis in adipose tissue and skeletal muscle in obesity

Increased fat storage in adipose and non-adipose tissue (e.g. skeletal muscle) characterizes the obese insulin resistant state. Disturbances in pathways of lipolysis may play a role in the development and maintenance of these increased fat stores. A reduced catecholamine-induced lipolysis may contribute to the development and maintenance of increased adipose tissue stores. To data, a reduced hormone-sensitive lipase (HSL) expression is the best characterized defect contributing to this catecholamine resistance. The recently discovered adipose triglyceride lipase (ATGL) seems not to be involved in the catecholamine resistance of lipolysis observed in abdominal subcutaneous adipose tissue of obese subjects, which contrasts with findings in mice studies. So far, little is known on the regulation of skeletal muscle lipolysis. There is evidence of both HSL and ATGL activity and/or expression in skeletal muscle. A blunted fasting and/or catecholamine-induced lipolysis has been reported in skeletal muscle, but data require confirmation. It is tempting to speculate that an imbalance between ATGL and HSL expression results in incomplete lipolysis and increased accumulation of lipid intermediates in skeletal muscle of obese insulin resistant subjects. The latter may inhibit insulin signalling and play a role in the development of type 2 diabetes. This review summarizes the current knowledge on (intracellular) adipose tissue and skeletal muscle lipolysis in obesity, discusses the underlying mechanisms of these disturbances and will finally address the question whether disturbances in the lipolytic pathways may be primary factors in the etiology of obesity or adaptational responses to the obese insulin resistant state.     

Neu1 sialidase interacts with perilipin 1 on lipid droplets and inhibits lipolysis in 3T3‐L1 adipocytes

Fatty acids are stored within adipocytes in lipid droplets (LDs) as triacylglycerol (TG), which is converted to free fatty acid (FFA) and glycerol via lipolysis. Increased plasma FFA levels in obesity are associated with several clinical conditions. We previously found that Neu1 activity is aberrant in the epididymal fat and liver of obese and diabetic mice. Here, we examined involvement of Neu1 in lipolysis in 3T3‐L1 adipocytes. Small interfering RNA against Neu1 was introduced into adipocytes, and glycerol concentrations were measured in the culture medium. We then assessed the effects of Neu1 knockdown on lipolytic protein expression and phosphorylation, as well as interactions between perilipin 1 (Plin1) and hormone‐sensitive lipase (HSL) after isoproterenol (IS) stimulation. Interactions between Neu1 and Plin1 were analyzed by immunoprecipitation and immunofluorescent imaging using adipocytes transfected with pCMV6‐mNeu1‐myc‐DYKDDDDK (mNeu1DDK). Neu1 knockdown increased glycerol concentrations in culture media and Plin1 phosphorylation in whole lysates of IS‐stimulated cells. Neu1 knockdown increased interaction between Plin1 and HSL after IS stimulation whereas that between Neu1 and Plin1 on LD observed under basal conditions was lost. These results suggest that Neu1 inhibits lipolysis induced by β‐adrenergic stimulation in adipocytes via interactions with Plin1 on LD.     

Effects of prolonged food restriction on some aspects of lipid metabolism in Norwegian and Svalbard reindeer

The high-arctic Svalbard reindeer (SR) deposit great amounts of body fat in autumn for subsequent use during winter when food is often in short supply. Captive SR and, for comparative reasons, the sub-arctic Norwegian reindeer (NR) were offered 15% of their ad libitum food intake during a 21-day period in September/October and its effect on fat metabolism was investigated. Plasma free fatty acids (FFA), glycerol, glucose, insulin and urea as well as lipogenic and lipolytic activity of isolated adipocytes were determined. Levels of FFA and glycerol increased immediately when food intake was restricted, reaching the highest levels in SR. Plasma glucose was fairly constant in NR, but decreased in SR. Plasma insulin decreased in both species. Plasma urea increased steadily from day 5 in NR and from day 11 in SR, after a transient rise on day 1 in both. Lipogenic activity had vanished completely in both NR and SR adipocytes when tested after 13 days of food restriction, while lipolytic activity was initially increased, after which it decreased in adipocytes from both species. After 21 days of food restriction SR adipocytes exhibited another marked increase in lipolytic activity, while the fat deposits of NR at that time were too small to allow examination. Thus, reindeer do not differ from other species in their lipogenic responses, although they show some hitherto undescribed lipolytic responses to prolonged food restriction.     

Weight gain induced by an isocaloric pair-fed high fat diet: A nutriepigenetic study on FASN and NDUFB6 gene promoters

Experimental studies have demonstrated that dietary macronutrient distribution plays an important role in insulin regulation, a risk factor associated to obesity, diabetes and other metabolic disorders. To assess whether the macronutrient composition of the diet could be related to obesity onset by affecting the epigenetic regulation of gene expression, we investigated in rats the metabolic effects of two pair-fed isocaloric diets: control (rich in carbohydrates) and high fat diet (rich in fat; HFD). Compared to controls, HFD induced higher weight gain and adiposity and impaired glucose tolerance, which was accompanied by a slight increase in adiponectin levels and liver steatosis. Epididymal adipose tissue expression of the fatty acid synthase (FASN) gene and NADH dehydrogenase (ubiquinone) 1β-subcomplex 6 (NDUFB6) were significantly reduced in HFD group. These variations in mRNA levels were accompanied by changes in the methylation patterns of several CpG islands located in the promoter region of these genes. However, no correlations were found between gene expression and the methylation status. These results suggest that high fat intake produces overweighted rats independently of total energy intake. These diets could also induce some epigenetic changes in the promoters of key genes that could influence gene expression and may be behind metabolic alterations.     

黑质多巴胺参与调控大鼠腹膜后白色脂肪的合成与分解

目的:观察用6-羟基多巴胺(6-OHDA)诱导的帕金森病(PD)模型大鼠中腹膜后白色脂肪组织(rWAT)内脂肪酸合成酶(FAS)、激素敏感性脂肪酶(HSL)和磷酸化激素敏感性脂肪酶(p-HSL)的变化。方法:20只SD大鼠随机分为PD模型组(双侧黑质内注射6-OHDA)和对照组(注射等体积生理盐水)。每日测量进食量,手术后6周进行转棒实验,并测量体重和rWAT重量。免疫组织化学染色观察黑质中的酪氨酸羟化酶(TH)的表达变化。HE染色观察rWAT中脂肪细胞的形态改变,并用ImageJ软件测量脂肪细胞的直径。Western blot检测黑质中TH及rWAT中FAS、HSL和p-HSL的蛋白水平。结果:与对照组比较,PD模型大鼠黑质内阳性神经元显著减少,运动能力明显下降;每日进食量和体重均无明显变化,但rWAT重量/体重的比值明显降低(P<0.05)。rWAT中脂肪细胞直径明显减少。rWAT中FAS表达明显降低,p-HSL的水平明显升高,HSL变化的差异无统计学显著性。结论:黑质多巴胺参与大鼠rWAT的合成与分解调控。黑质破坏引起的rWAT代谢改变可能与PD病人体重下降具有一定的相关性。     

Lipolysis and the integrated physiology of lipid energy metabolism

Fat cell lipolysis, the cleavage of triglycerides and release of fatty acids and glycerol, evolved to enable survival during prolonged food deprivation but is paradoxically increased in obesity, in which a surfeit of all energy metabolites is found. Essential, previously-unsuspected components have been discovered in the lipolytic machinery, at the protective interface of the lipid droplet surface and in the signaling pathways that control lipolysis. At least two adipocyte lipases are important for controlling lipolysis, hormone-sensitive lipase (HSL) and adipocyte triglyceride lipase (ATGL). Perilipin (PLIN) and possibly other proteins of the lipid droplet surface are master regulators of lipolysis, protecting or exposing the triglyceride core of the droplet to lipases. The prototypes for hormonal lipolytic control are beta adrenergic stimulation and suppression by insulin, both of which affect cyclic AMP levels and hence the protein kinase A-mediated phosphorylation of HSL and PLIN. Newly-recognized mediators of lipolysis include atrial natriuretic peptide, cyclic GMP, the ketone body 3-hydroxybutyrate, AMP kinase and mitogen-activated kinases. Lipolysis must be interpreted in its physiological context since similar rates of basal or stimulated lipolysis occur under different conditions and by different mechanisms. Age, sex, anatomical site, genotype and species differences are each important variables. Manipulation of lipolysis has therapeutic potential in several inborn errors and in the metabolic syndrome that frequently complicates obesity.     

Effect of glucose on lipolysis and on release of lipolytic products in isolated adipocytes.

Isolated adipocytes were prepared from epididymal adipose tissues removed from rats which had been fed or starved for 48 h (fed adipocytes or fasted adipocytes). These cells were incubated at 37 degrees C for 90 min in media containing 0, 3, or 30 mM glucose, with or without norepinephrine (1.0 mug/ml). Then the concentrations of free fatty acids (FFA) and free glycerol (FG) in the total mixture (medium plus cells) and in the medium alone were measured. Addition of glucose to the medium increased the total PG, presumably by increasing the basal lipolysis, and it decreased the intracellular retention ratio of FG (the ratio of intracellular FG to total FG). Addition of glucose did not change the total FFA, but decreased the FFA/FG ratio, presumably by increasing reesterification. The increase in FG and decrease in the FFA/FG ratio on addition of glucose were greater in fed than in fasted adipocytes. The intracellular retention ratio of FFA also decreased on addition of glucose. Glucose enhanced norepinephrine-induced lipolysis (release of free glycerol), and this effect of glucose was greater in fasted adipocytes. However, the increase in FFA in fasted adipocytes induced by norepinephrine was not altered by addition of glucose. In fed adipocytes norepinephrine decreased the total FFA in the presence of glucose. Reesterification of FFA following norepinephrine was increased by addition of glucose. Norepinephrine decreased the intracellular retention ratios of FG and FFA in the presence of glucose. These results suggest that the passage of the lipolytic products, FFA and FG, through the cell membranes may not occur by simple diffusion, but may require energy.     

Calorigenic effects of noradrenaline and glucagon on white adipocytes in cold- and heat-acclimated rats

Calorigenic actions of noradrenaline and glucagon on isolated epididymal fat cells from warmacclimated controls, cold-acclimated and heatacclimated rats were measured by the use of a twin-type conduction microcalorimeter. Both noradrenaline and glucagon stimulated heat production in isolated adipocytes maximally in doses of 1 g/ml and 10g/ml, respectively. Maximal responsiveness of adipocytes per unit cell to noradrenaline was not influenced by cold acclimation, while it was reduced by heat acclimation. Maximal response in total epididymal fat cells to noradrenaline was increased in cold acclimation and not changed in heat acclimation at increased numbers of adipocytes in both cold-acclimated and heatacclimated animals. Maximal response per unit cell as well as per total epididymal fat cells to glucagon was increased in cold acclimation and reduced in heat acclimation.The present results indicate that the modified responses of target adipocytes to noradrenaline and glucagon are involved in the development of temperature acclimation.     

Adipocyte dysfunction in rats with streptozotocin–nicotinamide‐induced diabetes

Administration of streptozotocin (STZ) and nicotinamide (NA) to adult rats allows for the induction of mild diabetes. However, this experimental model has not been fully characterized. This study was undertaken to determine the metabolic and secretory activity of adipose tissue in rats with STZ‐NA‐induced diabetes. Experiments were performed using epididymal adipocytes isolated from control and mildly diabetic rats. Lipogenesis, glucose transport as well as glucose and alanine oxidation, lipolysis, anti‐lipolysis, cAMP levels and adipokine secretion were compared in cells isolated from the control and diabetic rats. Lipogenesis, glucose transport and oxidation were diminished in the adipocytes of diabetic rats compared with the fat cells of control animals. However, alanine oxidation appeared to be similar in the cells of non‐diabetic and diabetic animals. Lipolytic response to low epinephrine concentrations was slightly increased in the adipocytes of diabetic rats; however, at higher concentrations of the hormone, lipolysis was similar in both groups of cells. The epinephrine‐induced rise in cAMP levels was higher in the adipocytes of STZ‐NA‐induced diabetic rats, even in the presence of insulin. Lipolysis stimulated by dibutyryl‐cAMP did not significantly differ, whereas anti‐lipolytic effects of insulin were mildly decreased in the cells of diabetic rats. Secretion of adiponectin and leptin was substantially diminished in the adipocytes of diabetic rats compared with the cells of control animals. Our studies demonstrated that the balance between lipogenesis and lipolysis in the adipose tissue of rats with mild diabetes induced by STZ and NA is slightly shifted towards reduced lipid accumulation. Simultaneously, adiponectin and leptin secretion is significantly impaired.     

Serum leptin levels in Ache Amerindian females with normal adiposity are not significantly different from American anorexia nervosa patients.

Serum leptin, a polypeptide hormone secreted primarily by adipocytes, is a reflection of somatic fat availability in humans and other vertebrates. Among Ache Amerindians, leptin levels are very low in contrast to other populations, despite comparable adiposity with individuals exhibiting much higher leptin levels. In order to gain a greater understanding of these differences, leptin levels were compared between Ache Amerindian females (n = 12, mean age = 32.2 +/- 14.0 SD), American females diagnosed with anorexia nervosa (n = 22, mean age = 23.0 +/- 4.0), and non-anorectic American controls (n = 23, mean age = 23.0 +/- 4.0). Ache leptin (5.6 +/- 3.2 ng/ml) was not significantly different from anorectic patients (5.6 +/- 3.7 ng/ml; P > 0.98) despite greater adiposity (Ache 33.3% +/- 4.4% vs. anorectic 7.0% +/- 2.0%; P < 0.0001). Ache adiposity was also higher than American controls (leptin 19.1 +/- 8.1 ng/ml; fat 28% +/- 5.0%; P < 0.004) underscoring the uniqueness of Ache leptin profiles. This suggests a greater range of population variation in leptin physiology than previously suspected and intimates the potential role of chronic environmental conditions.     

Membrane responses to extracellular ATP in rat isolated white adipocytes

 We used whole-cell and perforated-patch voltage-clamp methods to study the membrane electrical properties of isolated rat epididymal and inguinal white adipocytes. We examined cells from both Sprague-Dawley and Zucker lean and Zucker obese (fa/fa) rats. A delayed-rectifier potassium current was present and similar in unstimulated white fat cells from all these sources. The potassium current activated rapidly with depolarization positive to about –30 mV and showed slow inactivation. Stimulation with extracellular ATP activated both hyperpolarizing and depolarizing conductances. ATP exposure also increased cell membrane capacitance by an average of 16%, suggesting that ATP activates exocytosis. Exposure to norepinephrine had little electrophysiological effect. We conclude that white adipocytes are very similar to brown adipocytes in their resting electrophysiological profile and in their responses to extracellular ATP. Received: 2 January 1997 / Received after revision: 14 April 1997 / Accepted: 15 April 1997     

Effects of a chronic corticotropin treatment on brown adipose tissue of cold acclimated rats

1. This study deales with the effects of chronic corticotropin treatment (daily injection of 2U · kg^?1 i.p. for 10 days) upon the lipolytic action of norepinephrine (NE) on abdominal and interscapular brown adipose tissues (BAT) and on epididymal white fat of rats acclimated to either 28°C or 5°C and compared to controls.
2. In incubated BAT pieces from both 28°C and 5°C control animals no stimulation of lipolysis (release of glycerol and fatty acids) was induced by NE (10^?4 mol). A similar absence of effect was observed in BAT from corticotropin-treated rats. In epididymal fat, the in vitro enhancement of lipolysis by NE (300–400%) in controls was not modified in ACTH-treated rats.
3. The in vivo lipolysis in interscapular BAT was estimated by determining the arteriovenous differences in free glycerol and fatty acids levels and measuring blood flow. In control animals the blood flow stimulation by NE and the in situ utilization of the hydrolyzed fatty acids were larger in 5°C rats than in 28°C ones. In corticotropin-treated animals compared to controls, blood flow stimulation and fatty acid utilization were enhanced in 28°C group, and in opposite they were reduced in 5°C group; no significant difference was observed between the two groups.
4. It is concluded that chronic corticotropin treatment induces a “pseudo cold-acclimation” of BAT in 28°C rats and, inversely, a “loss of acclimation” in cold-acclimated ones. These opposite effects may be related to both the corticotropic and lipolytic action of the hormone.

     

Hormone-sensitive lipase in skeletal muscle: regulatory mechanisms

AIM: The enzymatic regulation of intramuscular triacylglycerol (TG) breakdown has until recently not been well understood. Our aim was to elucidate the role of hormone-sensitive lipase (HSL), which controls TG breakdown in adipose tissue. METHODS: Isolated rat muscle as well as exercising humans were studied. RESULTS: The presence of HSL was demonstrated in all muscle fibre types by Western blotting of muscle fibres isolated by collagenase treatment or after freeze-drying. The content of HSL varies between fibre types, being higher in oxidative than in glycolytic fibres. Analysed under conditions optimal for HSL, neutral lipase activity in muscle can be stimulated by adrenaline as well as by contractions. These increases are abolished by presence of anti-HSL antibody during analysis. Moreover, immunoprecipitation with affinity-purified anti-HSL antibody causes similar reductions in muscle HSL protein concentration and in measured neutral lipase responses to contractions. The immunoreactive HSL in muscle is stimulated by adrenaline via beta-adrenergic activation of protein kinase A (PKA). From findings in adipocytes it is likely that PKA phosphorylates HSL at residues Ser563, Ser659 and Ser660. Contraction probably also enhances muscle-HSL activity by phosphorylation, because the contraction-induced increase in HSL activity is increased by the protein phosphatase inhibitor okadaic acid and reversed by alkaline phosphatase. A novel signalling pathway in muscle by which HSL activity may be stimulated by protein kinase C (PKC) via extracellular signal regulated kinase (ERK) has been demonstrated. In contrast to previous findings in adipocytes, in muscle activation of ERK is not necessary for stimulation of HSL by adrenaline. However, contraction-induced HSL activation is mediated by PKC, at least partly via the ERK pathway. In fat cells ERK is known to phosphorylate HSL at Ser600. So, phosphorylation of different sites may explain that in muscle the effects of contractions and adrenaline on HSL activity are partially additive. In line with the view that the two stimuli act by different mechanisms, training increases the contraction-mediated, but diminishes the adrenaline mediated HSL activation in muscle. CONCLUSION: The existence and regulation of HSL in skeletal muscle indicate a role of HSL in muscle TG metabolism.     

Growth and metabolic changes in weanling rats with lesions in the dorsomedial hypothalamic nuclei

Summary When compared with sham-operated ad libitum-fed controls, weanling rats with lesions primarily destroying the dorsomedial hypothalamic nuclei (DMNL rats) showed reduced ponderal and linear growth and food intake, normal carcass fat but increased carcass protein. Among the metabolic parameters measured, DMNL rats showed only decreased incorporation of palmitate into epididymal fat pad phospholipid and triglyceride. When sham-operated controls were pair-fed with DMNL rats, they showed growth changes almost identical with those observed in lesioned rats. However, their carcass protein was lower than both that of the lesioned rats and the ad libitum-fed controls. Metabolically, the sham-operated, pair-fed controls showed decreased incorporation of palmitate into triglyceride of epididymal fat pads and decreased oxidation of glucose and increased incorporation into total lipid of the diaphragm. When previous data on growth hormone, insulin, triglyceride and cholesterol are compared with the present findings it is suggested that dorsomedial lesions cause a subcaloric-type dwarfism that does not involve adenohypophyseal secretions and their target organs affecting growth.This investigation was supported by USPHS Grants HD 03331, AM 14418, NIH, a Veterans Administration Grant and a Veterans Administration Clinical Investigatorship (JKG).     

Structural and biochemical characteristics of various white adipose tissue depots

It is now widely accepted that white adipose tissue (WAT) is not merely a fuel storage organ, but also a key component of metabolic homoeostatic mechanisms. Apart from its major role in lipid and glucose metabolism, adipose tissue is also involved in a wide array of other biological processes. The hormones and adipokines, as well as other biologically active agents released from fat cells, affect many physiological and pathological processes. WAT is neither uniform nor inflexible because it undergoes constant remodelling, adapting the size and number of adipocytes to changes in nutrients' availability and hormonal milieu. Fat depots from different areas of the body display distinct structural and functional properties and have disparate roles in pathology. The two major types of WAT are visceral fat, localized within the abdominal cavity and mediastinum, and subcutaneous fat in the hypodermis. Visceral obesity correlates with increased risk of insulin resistance and cardiovascular diseases, while increase of subcutaneous fat is associated with favourable plasma lipid profiles. Visceral adipocytes show higher lipogenic and lipolytic activities and produce more pro-inflammatory cytokines, while subcutaneous adipocytes are the main source of leptin and adiponectin. Moreover, adipose tissue associated with skeletal muscles (intramyocellular and intermuscular fat) and with the epicardium is believed to provide fuels for skeletal and cardiac muscle contraction. However, increased mass of either epicardial or intermuscular adipose tissue correlates with cardiovascular risk, while the presence of the intramyocellular fat is a risk factor for the development of insulin resistance. This review summarizes results of mainly human studies related to the differential characteristics of various WAT depots.     

Hormone‐sensitive lipase in skeletal muscle: regulatory mechanisms

Aim: The enzymatic regulation of intramuscular triacylglycerol (TG) breakdown has until recently not been well understood. Our aim was to elucidate the role of hormone‐sensitive lipase (HSL), which controls TG breakdown in adipose tissue. Methods: Isolated rat muscle as well as exercising humans were studied. Results: The presence of HSL was demonstrated in all muscle fibre types by Western blotting of muscle fibres isolated by collagenase treatment or after freeze‐drying. The content of HSL varies between fibre types, being higher in oxidative than in glycolytic fibres. Analysed under conditions optimal for HSL, neutral lipase activity in muscle can be stimulated by adrenaline as well as by contractions. These increases are abolished by presence of anti‐HSL antibody during analysis. Moreover, immunoprecipitation with affinity‐purified anti‐HSL antibody causes similar reductions in muscle HSL protein concentration and in measured neutral lipase responses to contractions. The immunoreactive HSL in muscle is stimulated by adrenaline via beta‐adrenergic activation of protein kinase A (PKA). From findings in adipocytes it is likely that PKA phosphorylates HSL at residues Ser⁵⁶³, Ser⁶⁵⁹ and Ser⁶⁶⁰. Contraction probably also enhances muscle‐HSL activity by phosphorylation, because the contraction‐induced increase in HSL activity is increased by the protein phosphatase inhibitor okadaic acid and reversed by alkaline phosphatase. A novel signalling pathway in muscle by which HSL activity may be stimulated by protein kinase C (PKC) via extracellular signal regulated kinase (ERK) has been demonstrated. In contrast to previous findings in adipocytes, in muscle activation of ERK is not necessary for stimulation of HSL by adrenaline. However, contraction‐induced HSL activation is mediated by PKC, at least partly via the ERK pathway. In fat cells ERK is known to phosphorylate HSL at Ser⁶⁰⁰. So, phosphorylation of different sites may explain that in muscle the effects of contractions and adrenaline on HSL activity are partially additive. In line with the view that the two stimuli act by different mechanisms, training increases the contraction‐mediated, but diminishes the adrenaline mediated HSL activation in muscle. Conclusion: The existence and regulation of HSL in skeletal muscle indicate a role of HSL in muscle TG metabolism.