Effect of cold acclimation on brown adipose tissue fatty acid synthesis in rats adapted to a high-protein, carbohydrate-free diet

The effect of cold acclimation on brown adipose tissue (BAT) fatty acid synthesis was investigated in rats adapted to a high-protein, carbohydrate-free diet. At an ambient temperature (25 degrees C), rates of fatty acid synthesis in BAT from rats adapted to the high-protein diet were reduced to 27% of rats fed the balanced diet and increased markedly after cold acclimation (10 days at 4 degrees C), although the increase was smaller than in control rats. BAT weight increase induced by cold acclimation was smaller in rats fed the high-protein diet (30%) than in controls (100%). When expressed per whole tissue, maximal activities of BAT glucose-6-phosphate dehydrogenase, malic enzyme, adenosine triphosphate (ATP)-citrate lyase, and acetyl-coenzyme A carboxylase were markedly reduced in high-protein diet-adapted rats at 25 degrees C and increased after cold acclimation in BAT from the 2 groups. However, when expressed per milligram protein, only acetyl-coenzyme A carboxylase showed an increase in both controls and in rats fed the high-protein diet. G6P-dehydrogenase, malic enzyme, and ATP-citrate lyase increased (per milligram protein) only in rats adapted to the high-protein diet and actually decreased in BAT from cold-acclimated control rats. Initial (before activation) pyruvate dehydrogenase (PDH) complex activity was lower in BAT from rats fed the high-protein diet at 25 degrees C and increased in cold-acclimated rats from the 2 groups. Circulating levels of insulin decreased in the 2 groups after cold acclimation. The data suggest that the cold acclimation-induced increase in BAT lipogenesis in rats adapted to the high-protein diet was due to a restoration of sympathetic activity, which induced both BAT hyperplasia and activation of adipocyte free fatty acid (FFA) synthesis, with an important participation of acetyl-coenzyme A carboxylase and pyruvate dehydrogenase.     

Glucose uptake,glucose transporter GLUT4, and glycolytic enzymes in brown adipose tissue from rats adapted to a high-protein diet

In vivo rates of glucose uptake, insulin-responsive glucose transporter (GLUT4) content, and activities of glycolytic enzymes were determined in brown adipose tissue (BAT) from rats adapted to a high-protein, carbohydrate-free (HP) diet. Adaptation to the HP diet resulted in marked decreases in BAT glucose uptake and in GLUT4 content. Replacement of the HP diet by a balanced control diet for 24 hours restored BAT glucose uptake to levels above those in rats fed the control diet, with no changes in GLUT4 levels in 4 of 5 animals examined. BAT denervation of rats fed the control diet induced a 50% reduction in glucose uptake, but did not significantly affect the already markedly reduced BAT hexose uptake in HP diet-fed rats. It is suggested that the pronounced decrease in BAT glucose uptake in these animals is due to the combined effects of the HP diet-induced reductions in plasma insulin levels and in BAT sympathetic activity. Adaptation to the HP diet was accompanied by decreased activities of hexokinase, phosphofructo-1-kinase, and pyruvate kinase (PK). The activity of BAT PK in HP diet-fed rats was reduced to about 50% of controls, and approached normal levels 24 hours after diet reversion. BAT denervation induced a small (15%) decrease in BAT PK activity in control rats, but did not affect the activity of the enzyme in HP diet-adapted rats. Also, denervation did not interfere with the restoration of PK activity induced by diet substitution. Treatment with anti-insulin serum resulted in an almost 50% reduction in PK activity in both innervated and denervated BAT from rats fed the control diet, but caused a much smaller ( thick approximate 20%) decrease in BAT from HP diet-fed rats. Furthermore, anti-insulin serum administration completely suppressed the restoration of BAT PK activity induced by diet reversion. These data suggest that, differently from glucose uptake, BAT PK activity is predominantly controlled by hormonal/metabolic factors.     

Glucose uptake and glycolytic flux in adipose tissue from rats adapted to a high-protein, carbohydrate-free diet

Rates of glucose uptake by epididymal and retroperitoneal adipose tissue in vivo, as well as rates of hexose uptake and glycolytic flux in isolated adipocytes, were determined in rats adapted to a high-protein, carbohydrate-free (HP) diet and in control rats fed a balanced (N) diet. Adaptation to the HP diet induced a significant reduction in rates of glucose uptake, estimated with 2-deoxy-[1-(3)H]-glucose, both by adipose tissue (epididymal and retroperitoneal) in vivo and by isolated adipocytes. Twelve hours after replacement of the HP diet with the balanced diet, rates of adipose tissue uptake in vivo in HP-adapted rats returned to levels that did not differ significantly from those in N-fed rats. The rate of flux in the glycolytic pathway, estimated with (3)H[5]-glucose, was also significantly reduced in adipocytes from HP-fed rats. In agreement with the above findings, the activities of hexokinase (HK), phosphofructo-1-kinase (PFK-1), and pyruvate kinase (PK) were markedly reduced in adipose tissue from HP-adapted rats. The activity of pyruvate kinase was partially reverted by diet replacement for 12 hours. The low-plasma insulin and high-glucagon levels in HP-fed rats may have played an important role in the reduction of adipose tissue glucose utilization in these animals.     

Lipolysis and the antilipolytic effect of insulin in adipocytes from rats adapted to a high-protein diet

Free fatty acid (FFA) mobilization during fasting was investigated in rats fed a high-protein, carbohydrate-free (HP) diet (70% casein, 8% fat, wt/wt) or a balanced diet (66% carbohydrate, 17% casein, 8% fat) for 30 to 40 days. In vivo, rats on the HP diet showed reduced rates of plasma FFA increase during fasting. Their blood sugar remained unchanged and was higher than that of control rats 24 hours after removal of food. In the fed state, serum insulin levels were smaller in HP-fed rats but did not differ significantly in the two experimental groups during fasting. In vitro, the rates of glycerol and FFA release by epididymal fat pads obtained from fasted rats were similar in rats consuming the HP diet. Fat cells isolated from rats on the HP diet also had reduced rates of basal lipolysis. Furthermore, they showed a significant increase in responsiveness to the lipolytic action of noradrenaline and an increase in both sensitivity and responsiveness to the inhibitory effect of insulin on noradrenaline-stimulated lipolysis. Adipocytes from HP-fed and control rats had mean diameters of 51 and 60 mu, respectively, and estimated average volumes of 90 and 142 pL. On the basis of existing data on the correlation between size and lipolytic activity of fat cells, the smaller size of the adipocytes from HP-fed rats might account for the lower rate of basal lipolysis but not for the increased response to the hormones. The increased sensitivity of fat cells to the antilipolytic action of insulin may have been an important factor in the reduced lipomobilization during fasting in rats under the high-protein regimen.     

Increased glyceroneogenesis in adipose tissue from rats adapted to a high-protein, carbohydrate-free diet: role of dietary fatty acids

We have previously shown in in vivo experiments that adipose tissue glyceroneogenesis is increased in rats adapted to a high-protein, carbohydrate-free (HP) diet. The objectives of the present study were (1) to verify if the increased glyceroneogenic activity is also observed in isolated adipocytes and (2) to investigate the role of preformed fatty acids in the production of the increased adipose tissue glyceroneogenesis. Control rats received a balanced diet, with the same lipid content of the HP diet. Glyceroneogenic activity was found to be higher in adipocytes from HP rats than in controls, as evidenced by increased rates of conversion of pyruvate and lactate to triacylglycerol (TAG)-glycerol. Administration of Triton WR 1339, which blocks the removal of TAG incorporated into circulating lipoproteins, to HP diet-adapted rats caused a significant reduction in the incorporation of 14C-pyruvate into TAG-glycerol by adipose tissue, which was accompanied by a marked inhibition of phosphoenolpyruvate carboxykinase activity, the key enzyme of glyceroneogenesis. The inhibitory effect of Triton on TAG-glycerol synthesis by adipose tissue was also observed in vivo, after administration of 3H2O. Adaptation to the HP diet induced a marked increase in the activity of retroperitoneal and epididymal fat LPL, which was restored to control values 24 hours after replacement of the HP diet by the balanced diet. The data suggest that in rats adapted to a carbohydrate-free diet, adipose tissue glyceroneogenesis is activated by an increased use of diet-derived fatty acids.     

Reduced lipogenesis in rats fed a high-protein carbohydrate-free diet

Rates of fatty acid synthesis were assessed in carcass, liver, and adipose tissue from rats fed for 30 to 40 days a balanced diet (66% wt/wt carbohydrate, 17% casein, 8% fat) or a high-protein carbohydrate-free diet (70% casein, 8% fat). Despite similar body weight increases, carcass fatty acid content of rats on the high-protein (HP) diet was 13% less, and the weight of their epididymal fat pads was reduced by 29% in relation to the controls. In vivo incorporation of 3H2O into carcass fatty acids (FA) and into liver triacylglycerol (TAG) was significantly reduced in HP-fed rats. FA synthesis from 14C-acetate, glucose, or leucine and from 3H2O was also markedly decreased in liver slices from HP rats. The amount of 3H-TAG that accumulated in plasma of rats injected with triton WR 1339 to block peripheral utilization of lipoprotein corresponded in HP and control rats to only 4.1% and 5.0%, respectively, of 3H-FA recovered in carcasses from animals not treated with triton, indicating that almost all of the carcass 3H-TAG was synthesized in situ. However, on a long term basis, the reduced hepatic lipogenesis and the resulting decreased transport of TAG might affect lipid accumulation in HP rats. In vivo lipogenesis from 3H2O and in vitro FA synthesis from 3H2O and from 14C-precursors did not differ significantly in retroperitoneal and epididymal adipose tissue from HP and control rats. In both groups of animals, in vivo rates of lipogenesis were higher in retroperitoneal than in epididymal adipose tissue but still did not account for rates of FA synthesis by carcasses, suggesting the existence of other sites with higher lipogenic activity.     

Reduced lipolysis and increased lipogenesis in adipose tissue from pinealectomized rats adapted to training

The current study investigated the effects of chronic training and pinealectomy on the lipogenic and lipolytic activity of adipose tissue. Pinealectomized and sham-operated adult male Wistar rats were distributed in to four subgroups: pinealectomized untrained, pinealectomized trained, control untrained and control trained. At the end of the training period (8 wk) the rats were killed. Blood samples were collected for glucose, insulin and leptin determinations. Peri-epididymal adipocytes were isolated for measurement of in vitro rates of lipolysis and incorporation of substrates (D-[U-14C]-glucose, L-[U-14C]-lactate, [2-14C]-acetate and [1-14C]-palmitate) into lipids, and samples of epididymal adipose tissue were homogenized for evaluation of glucose-6-phosphate dehydrogenase maximal activity. Pinealectomy resulted in a significantly increased lipolytic capacity in response to isoproterenol and a decrease in circulating leptin levels without affecting the rates of incorporation of different substrates into lipids. However, only in the intact control group did training promote a higher basal and isoproterenol-stimulated lipolysis, increase the incorporation of palmitate (esterification), decrease the incorporation of acetate (lipogenesis) into lipids and diminish circulating leptin levels. These effects of exercise training were not seen in pinealectomized rats. However, pinealectomized trained animals showed a marked reduction in lipolysis and an increased rate of acetate incorporation. In conclusion, we demonstrated for the first time that the pineal gland plays an important role in the regulation of lipid metabolism in such a way that its absence caused a severe alteration in the balance between lipogenesis and lipolysis, which becomes evident with the adaptation to exercise training.     

Effect of sucrose overfeeding on brown adipose tissue lipogenesis and lipoprotein lipase activity in rats

During cold-induced nonshivering thermogenesis, interscapular brown adipose tissue (BAT) lipoprotein lipase (LPL) activity and lipogenesis are elevated. Because of the many similarities between cold- and diet-induced thermogenesis, we examined the effect of ad libitum access to a 32% sucrose solution on caloric intake, adiposity, and BAT enzyme activities in male rats. Daily caloric intakes of sucrose-fed animals were elevated by 20%-25%, and 8 wk of sucrose feeding doubled carcass fat content. This sucrose-feeding induced obesity was associated with increases in circulating triglyceride and insulin levels as well as increased retroperitoneal white adipose tissue LPL activity. However, the increased carcass lipid content accounted for less than half of the excess calories ingested by the sucrose-fed rats. Sucrose feeding stimulated in vivo lipogenesis in BAT and elevated BAT fatty acid synthetase and acetyl-CoA carboxylase activities but not LPL activity. These findings suggest that overeating enhances endogenous lipogenesis but not uptake of circulating triglyceride in BAT. Thus, both cold- and diet-induced thermogenesis increase BAT lipogenesis, while only cold-induced thermogenesis is associated with elevated LPL activity in BAT.     

Effects of sucrose feeding and denervation on lipogenesis in brown adipose tissue

Rats were fed standard laboratory chow or sucrose + chow for 2-3 weeks. Sucrose feeding significantly increased the size and norepinephrine content of interscapular brown adipose tissue (BAT). Sucrose feeding stimulated in vivo lipogenesis in BAT, white adipose tissue, and liver. Unilateral denervation reduced BAT norepinephrine content by 90%, and significantly reduced BAT protein and DNA content. Prior surgical denervation of BAT had no effect on lipogenesis in chow-fed rats, but reduced the sucrose-induced lipogenesis in BAT by 75% and increased lipogenesis in retroperitoneal white adipose tissue of sucrose-fed rats. These data demonstrate that the optimal stimulation of lipogenesis in BAT by sucrose feeding requires innervation of this tissue.     

Dual regulatory effects of orexins on sympathetic nerve activity innervating brown adipose tissue in rats

This study examined how orexin regulates the activity of the sympathetic nerves that innervate brown adipose tissue (BAT) in rats. Infusion of orexin A at a dose of 0.3 nmol into the third cerebral ventricle decreased BAT sympathetic nerve activity, compared with the effect of PBS (P < 0.05), whereas infusion of orexin B at the same dose caused a significant increase (P < 0.05). Pretreatment with a third cerebral ventricle injection of 2.24 micromol/kg alpha-fluoromethylhistidine, an irreversible inhibitor of the histamine-synthesizing enzyme histidine decarboxylase, attenuated the orexin B-induced response of BAT sympathetic nerve activity, but not that induced by orexin A. These results indicate that orexins may regulate both BAT energy expenditure and thermogenesis through their dual effects on sympathetic nerve activity. In particular, orexin B regulates BAT sympathetic nerve activity via neuronal histamine in the hypothalamus.     

Insulin sensitivity of adipose tissue and of diaphragm in rats adapted to periodic hyperphagia

Summary In the present paper the authors have investigated the sensitivity of adipose tissue and diaphragm to insulin in rats adapted to periodic hyperphagia. Insulin was administered by the intraperitoneal route in amounts that did not affect the blood sugar level. The incorporation of labelled glucose, administered simultaneously with insulin, into lipids of parametrial adipose tissue and into glycogen of the diaphragm was investigated, together with the glycogen content of the diaphragm. Moreover, the authors investigated the influence of insulin added to the medium on the glycogen content of the isolated diaphragm. It was found that adipose tissue of adapted rats responded to insulin at all levels used, more sensitively than adipose tissue of control rats fed ad libitum. On the other hand, no differences were found in the sensitivity of the diaphragm to insulin either in vivo or in vitro. The relationship between morphological and functional changes in the adipose tissue of adapted rats and the increased reactivity of this tissue to insulin is discussed.

---

Insulinempfindlichkeit des Fettgewebes und Zwerchfells von Ratten nach Adaptation an stoweise Nahrungszufuhr

Zusammenfassung Die Autoren überprüften die Empfindlichkeit des Fettgewebes und des Zwerchfells gegenüber Insulin bei Ratten, die sie an stoßweise Nahrungszufuhr gewöhnt hatten. Insulin wurde dabei in nicht blutzuckerwirksamen Mengen i. p. injiziert. Es wird über den Einbau von gleichzeitig verabreichter Radioglucose in die Lipide des parametranen Fettgewebes und das Zwerchfellglykogen und über den Glykogengehalt des Zwerchfells berichtet. Ferner untersuchten die Autoren die Wirkung von Insulinzusätzen auf den Glykogengehalt des isolierten Zwerchfells. Es zeigte sich, daß das Fettgewebe adaptierter Ratten auf alle verwandten Insulinkonzentrationen stärker ansprach als das Gewebe der Kontrolltiere mit freiem Zugang zum Futter. Andererseits wies die Insulin-Ansprechbarkeit des Zwerchfells weder in vivo noch in vitro Unterschiede auf. Die Beziehungen zwischen morphologischen und funktionellen Veränderungen des Fettgewebes adaptierter Ratten und seiner erhöhten Insulin-Empfindlichkeit werden besprochen.

---

Sensibilité à l'insuline du tissu adipeux et du diaphragme de rats adaptés à l'hyperphagie périodique

Résumé Dans cet article les auteurs ont étudié la sensibilité à l'insuline du tissu adipeux et du diaphragme de rats adaptés à l'hyperphagie périodique. L'insuline était administrée par voie intrapéritonéale en quantités ne modifiant pas le taux de la glycémie. L'incorporation du glucose marqué, administré en même temps que l'insuline, dans les lipides du tissu adipeux parametrial et dans le glycogène du diaphragme, a été étudiée en même temps que le contenu en glycogène du diaphragme. En outre, les auteurs ont étudié l'influence de l'addition d'insuline au milieu sur le contenu en glycogène du diaphragme isolé. On a constaté que le tissu adipeux des rats adaptés répondait à l'insuline à tous les taux utilisés, d'une manière plus sensible que le tissu adipeux des rats témoins nourris ad libitum. D'autre part on n'a trouvé aucune différence dans la sensibilité du diaphragme à l'insuline in vivo ou in vitro. La relation entre les variations morphologiques et fonctionnelles dans le tissu adipeux des rats adaptés et la réactivité augmentée de ce tissu à l'insuline est discutee.

     

Mechanism linking insulin resistance to defective thermogenesis in brown adipose tissue of obese diabetic SHR/N-cp rats.

We previously reported that the decreased sensitivity of brown adipose tissue (BAT) from obese Zucker rats to the calorigenic effects of norepinephrine is associated with a marked resistance to insulin, and we suggested that this defect may explain, at least in part, the increased energy gain efficiency of fa/fa rats. To test whether insulin resistance and/or diabetes leads to a reduced BAT thermogenesis in other genetic models of obesity, we compared BAT metabolic properties of obese Zucker rats with that of obese-nondiabetic LA/N-cp and obese-diabetic SHR/N-cp rats. It was found that the responsiveness and sensitivity of isolated brown adipocytes to the calorigenic effects of norepinephrine (10-100 mM) were markedly reduced in SHR/N-cp rats as compared to their lean controls (the Vmax was decreased by 3-4 times and the EC50 value was doubled). In the same cells, there was a similar decrease in the respiratory effects of dibutyryl cAMP (DBcAMP), revealing the presence of a major post-receptor defect. Remarkably, total cytochrome oxidase activity (an index of cell mitochondrial content) was also decreased by 3-4 times in SHR/N-cp rats, suggesting that a reduced BAT mitochondrial content is responsible for the defective thermogenesis. Similarly to Zucker rats, adipocytes isolated from SHR/N-cp rats were resistant to the metabolic effects of insulin (glucose transport and antithermogenesis). Cells from obese Zucker rats were also desensitized to the metabolic effects of norepinephrine and insulin but their thermogenic capacity was not reduced. In contrast, all the above parameters were normal in obese-nondiabetic LA/N-cp rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Impaired metabolic response to nerve stimulation in brown adipose tissue of hypothyroid rats

In brown adipose tissue of the rat, chemically or surgically induced hypothyroidism caused the following effects. A large decrease of the magnitude of the metabolic response to electrical nerve stimulation. The deactivation half-time of the response was reduced to 70% of the control value, with no change in catechol O-methyltransferase activity. Pre-incubation of tissues with norepinephrine, 10(-5) M, increased the response to subsequent nerve stimulation almost to that of the controls. The catecholamine analogue dose-response curves were shifted to the right. The shift was very pronounced for isoproterenol (K50 426 nM versus 2 nM), somewhat less marked for norepinephrine (7373 nM versus 194) and very slight for phenylephrine (2803 nM versus 1649); there was almost no change in Emax values. An increase of octanoate oxidative capacity. A decrease of the capacity of the stereoselective binding of (-)-[3H]dihydroalprenolol of the high-affinity (Kd 2.0 nM) sites to a fourth of the control value and an increase by a factor of 2.9 of the Kd of the low-affinity binding sites. This decrease of binding to the beta-receptors was not sufficient quantitatively to explain the decrease in the metabolic response, suggesting the existence of an additional defective reaction which could occur between the binding to the beta-receptors and the activation of the triglyceride lipase. These results show that the sharp decrease of the metabolic response of brown adipose tissue to nerve stimulation has multiple causes. The findings are discussed in the context of the drastic decrease of cold resistance in hypothyroid rats.     

The sympathetic nervous system regulates the three glycerol-3P generation pathways in white adipose tissue of fasted, diabetic and high-protein diet-fed rats

The aim of the present study was to investigate the participation of the sympathetic nervous system (SNS) in the control of glycerol-3-P (G3P) generating pathways in white adipose tissue (WAT) of rats in three situations in which the plasma insulin levels are low. WAT from 48h fasted animals, 3day-streptozotocin diabetic animals and high-protein, carbohydrate-free (HP) diet-fed rats was surgical denervated and the G3P generation pathways were evaluated. Food deprivation, diabetes and the HP diet provoke a marked decrease in the rate of glucose uptake and glycerokinase (GyK) activity, but a significant increase in the glyceroneogenesis, estimated by the phosphoenolpyruvate carboxykinase (PEPCK) activity and the incorporation of 1-[(14)C]-pyruvate into glycerol-TAG. The denervation provokes a reduction (~70%) in the NE content of WAT in fasted, diabetic and HP diet-fed rats. The denervation induced an increase in WAT glucose uptake of fed, fasted, diabetic and HP diet-fed rats (40%, 60%, 3.2 fold and 35%, respectively). TAG-glycerol synthesis from pyruvate was reduced by denervation in adipocytes of fed (58%) and fasted (36%), saline-treated (58%) and diabetic (23%), and HP diet-fed rats (11%). In these same groups the denervation reduced the PEPCK mRNA expression (75%-95%) and the PEPCK activity (35%-60%). The denervation caused a ~35% decrease in GyK activity of control rats and a further ~35% reduction in the already low enzyme activity of fasted, diabetic and HP diet-fed rats. These data suggest that the SNS plays an important role in modulating G3P generating pathways in WAT, in situations where insulin levels are low.     

Centrally administered resistin enhances sympathetic nerve activity to the hindlimb but attenuates the activity to brown adipose tissue

Resistin, an adipokine, is believed to act in the brain to influence energy homeostasis. Plasma resistin levels are elevated in obesity and are associated with metabolic and cardiovascular disease. Increased muscle sympathetic nerve activity (SNA) is a characteristic of obesity, a risk factor for diabetes and cardiovascular disease. We hypothesized that resistin affects SNA, which contributes to metabolic and cardiovascular dysfunction. Here we investigated the effects of centrally administered resistin on SNA to muscle (lumbar) and brown adipose tissue (BAT), outputs that influence cardiovascular and energy homeostasis. Overnight-fasted rats were anesthetized, and resistin (7 μg) was administered into the lateral cerebral ventricle (intracerebroventricular). The lumbar sympathetic nerve trunk or sympathetic nerves supplying BAT were dissected free, and nerve activity was recorded. Arterial blood pressure, heart rate, body core temperature, and BAT temperature were also recorded. Responses to resistin or vehicle were monitored for 4 h after intracerebroventricular administration. Acutely administered resistin increased lumbar SNA but decreased BAT SNA. Mean arterial pressure and heart rate, however, were not significantly affected by resistin. BAT temperature was significantly reduced by resistin, and there was a concomitant fall in body temperature. The findings indicate that resistin has differential effects on SNA to tissues involved in metabolic and cardiovascular regulation. The decreased BAT SNA and the increased lumbar SNA elicited by resistin suggest that it may contribute to the increased muscle SNA and reduced energy expenditure observed in obesity and diabetes.     

Enzymes related to lipogenesis in the adipose tissue of obese subjects.

In a group of ten adult obese subjects, maintained for 15 days on a normal caloric intake and balanced diet, the activity of hexokinase (EC 2.7.1.1),6-phosphofructokinase (EC 2.7.1.11), and ATP citratelyase (EC 4.1.3.8) in the adipose tissue was significantly increased, both on a protein and on a fat cell number basis, compared to matched normal subjects. The activity of glucose-6-phosphate dehydrogenase (EC 1.1.1.49), malate dehydrogenase (EC 1.1.1.37), and malate dehydrogenase (decarboxylating) (NADP) (EC 1.1.1.40), on the other hand, was unchanged. Since both hexokinase and 6-phosphofructokinase are rate-limiting in glycolysis, their enhanced activity would indicate the occurrence of an increased capacity to metabolize glucose and therefore to generate alpha-glycerophosphate. The elevation of ATP citrate-lyase would suggest increased lipogenesis, owing to the regulatory role that this enzyme plays in fatty acid synthesis. The normal activity of glucose-6-phosphate dehydrogenase and malate dehydrogenase (decarboxylating) (NADP), which supply NADPH for the reduction of acetyl-CoA to fatty acids, would suggest that the change in lipogenesis is of moderate degree, thereb) affecting only the most rate-limiting enzyme, ATP citrate-lyase. These data, on the whole, are consistent with the occurrence of enhanced triglyceride formation. Whether the enzyme changes observed are adaptive or genetic in nature remains to be clarified.     

Dietary-induced permanent changes in brown and white adipose tissue composition in rats

We have previously observed that feeding rats a cafeteria diet causes excess weight gain and changes in tissue composition. The object of this study was to assess whether these alterations were sustained after withdrawal of the palatable diet in the rat. The results showed that the obesity was not reversed by feeding a standard diet ad libitum for five months after withdrawal of the cafeteria diet. Body weight was 26 per cent greater than in control rats and tissue composition showed permanent alterations. The excess weight of lumbar white adipose tissue was due mainly to lipid content (86 per cent) and this was also true, but to a less extent, for interscapular brown fat (59 per cent). Increased brown fat mass was a result of hyperplasia and hypertrophy, whereas increased lumbar white fat was mainly a result of hyperplasia alone. In conclusion, changes in tissue composition, particularly in fat depots, were permanent and could be ascribed to the obesity per se, and not to the diet composition.     

Thermogenesis and brown adipose tissue activity in high fat fed Osborne-Mendel rats

The consequences of hyperlipidic feeding on interscapular brown adipose tissue (IBAT) activity were examined in the cold-acclimated rat. Male Osborne-Mendel rats (7 weeks old) were exposed for 10 weeks at either 28 or 5 degrees C. The rats were fed a semipurified diet (normal fat (NL): 5% lard; high fat (HL): 54% lard) for the last 9 weeks. IBAT weight was greater in HL than in NL rats. This increase was entirely due to accumulation of neutral lipids. Among different IBAT components (proteins, DNA, phospholipids) no variations were observed in HL 28 degrees C rats. HL diet did not modify lipoprotein lipase and cytochrome oxidase activities, but an increase in purine nucleotide binding (taken as an index of thermogenic activity) was observed in HL 28 degrees C rats. Cold acclimation led to comparable stimulation in NL and HL rats. The calorigenic effect of norepinephrine in vivo was not modified by HL diet. Study of arteriovenous differences showed that IBAT fatty acid and glycerol release by norepinephrine was slightly increased in HL 28 degrees C rats. No effect of HL diet was observed at 5 degrees C. These results indicate that HL feeding leads to a moderate increase in the IBAT thermogenic capacity of Osborne-Mendel rats. HL diet does not modify the normal development of nonshivering thermogenesis when rats are kept in a cold environment.     

Modulation of fatty acid synthesis in vivo in brown adipose tissue, liver and white adipose tissue of cold-acclimated rats

The present experiment was an appraisal of the relative importance of fatty acid synthesis in brown adipose tissue (BAT) in young 28 or 5 degrees C adapted rats (9 weeks old). With a low-fat diet in vivo incorporation of 3H2O into BAT fatty acids was 8 times lower during the day than during the night and was not modified by a 6-hour fast during the day (28 degrees C). Cold acclimation doubled (night) or increased 8 times (day) BAT lipogenesis. Fasting led to a halving of the diurnal rate. A high-fat diet led to large decrease in synthesis rate during the night but had a weak effect on diurnal synthesis. The specific activity of fatty acids was 3 times lower in phospholipids than in neutral lipids. A comparison between 9- and 15-week-old rats indicated that in older warm-adapted rats BAT lipogenesis decreased by half but that cold stimulation was unaltered. These results were compared with hepatic and epididymal white adipose tissue lipogenesis. In conclusion, we showed that BAT of 5 degrees C rats is an important but not the major site for the conversion of carbohydrate to fat and that the proportional involvement of each tissue is age-dependent.