Brown Adipose Tissue Depots Research Articles

Differential expression of genes for uncoupling proteins 1, 2 and 3 in brown and white adipose tissue depots during rat development.

The different expression patterns of genes for uncoupling proteins (UCPs) 1, 2 and 3 (ucp1, ucp2 and ucp3) were studied in interscapular brown adipose tissue (BAT) and in four white adipose tissue (WAT) depots (epididymal, inguinal, mesenteric and retroperitoneal) in male rats of different ages (18 days-12 months). UCP mRNA expression levels were determined by Northern blotting. In BAT, there were high levels of expression of UCP1 and UCP3 mRNA, but no detectable levels of UCP2 mRNA. Both ucp1 and ucp3 followed a similar expression pattern with age, with high levels in suckling rats which decreased to 50% or less in rats just under 2 months old, declining thereafter until 5 months and then recovering with age. However, an additional peak of expression was observed for ucp3 at the age of 3 months. In WAT, ucp1 expression was rare: occasional expression was found for UCP1 mRNA in the retroperitoneal depot in suckling rats and in the epididymal and inguinal depots in suckling and mature adult rats. ucp2 and ucp3 had different developmental expression patterns, but these were similar for each gene in the different depots studied. UCP3 mRNA was highly expressed in rats soon after birth, it decreased until 3 months, and increased thereafter, except for the mesenteric WAT where ucp3 expression decreased until 7 months before recovering. The fact that changes with age of both ucp1 and ucp3 expression have a similar profile in BAT, which is also similar to the ucp3 and also ucp1 profiles in some WAT depots, might reflect a common regulatory pattern for the expression of these genes, and also a common function. In contrast to ucp1 and ucp3, ucp2 had a peak of expression at about 2 months, and lower expression at 3 months, suggesting different regulation and probably a different role for this UCP.

Can non-shivering thermogenesis in brown adipose tissue following NA injection be quantified by changes in overlying surface temperatures using infrared thermography?

We aimed to investigate whether infra red thermography (IRT) can be used to measure and quantify non-shivering thermogenesis (NST) in the short-tailed field vole Microtus agrestis, by directly comparing it with a standard method, i.e. metabolic response following Noradrenaline injection (NA). Mean skin surface temperature overlying Brown adipose tissue (BAT) depot was 0.82°C higher than mean surface temperature that did not overly BAT. The difference in temperature increased by 1.26°C after NA was administered. Mean skin surface temperature overlying BAT increased by 0.32°C after NA was administered; however, surface temperature decreased by 1.32°C after saline was administered. Mean skin surface temperature overlying BAT did not change significantly between warm and cold acclimated voles; in contrast metabolic peak following NA injection significantly increased in cold acclimated voles. There was no significant correlation between change in surface temperature after NA injection and metabolic peak following NA injection. The results of this study suggest that IRT is not a sensitive enough method to measure changes in NST capacity in BAT following NA injection, or to detect changes in NST capacity induced by cold acclimation. However, IRT can distinguish between skin surfaces overlying BAT and skin surfaces that do not.

Peroxisome Proliferator-Activated Receptors and Mediate in Vivo Regulation of Uncoupling Protein (UCP-1, UCP-2, UCP-3) Gene Expression

A role for peroxisome proliferator-activated receptors, PPARγ and PPARα, as regulators of energy homeostasis and lipid metabolism, has been suggested. Recently, three distinct uncoupling protein isoforms, UCP-1, UCP-2, and UCP-3, have also been identified and implicated as mediators of thermogenesis. Here, we examined whether in vivo PPARγ or PPARα activation regulates the expression of all three UCP isoforms. Rats or lean and db/db mice were treated with PPARγ [thiazolidinedione (TZD)] or PPARα (WY-14643) agonists, followed by measurement of messenger RNAs (mRNAs) for UCP-1, UCP-2, and UCP-3 in selected tissues where they are expressed. TZD treatment (AD 5075 at 5 mg/kg·day) of rats (14 days) increased brown adipose tissue (BAT) depot size and induced the expression of each UCP mRNA (3× control levels for UCP-1 and UCP-2, 2.5× control for UCP-3). In contrast, UCP-2 and UCP-3 mRNA levels were not affected in white adipose tissue or skeletal muscle. Chronic (30 days) low-dose (0.3 mg/kg·day) TZD treatment induced UCP-1 mRNA and protein in BAT (2.5× control). In contrast, chronic TZD treatment (30 mg/kg·day) suppressed UCP-1 mRNA (>80%) and protein (50%) expression in BAT. This was associated with further induction of UCP-2 expression (>10-fold) and an increase in the size of lipid vacuoles, a decrease in the number of lipid vacuoles in each adipocyte, and an increase in the size of the adipocytes. TZD treatment of db/db mice (BRL 49653 at 10 mg/kg·day for 10 days) also induced UCP-1 and UCP-3 (but not UCP-2) expression in BAT. PPARα is present in BAT, as well as liver. Treatment of rats or db/db mice with WY-14643 did not affect expression of UCP-1, -2, or -3 in BAT. Hepatic UCP-2 mRNA was increased (4× control level) in db/db and lean mice, although this effect was not observed in rats. Thus, in vivo PPARγ activation can induce expression of UCP-1, -2, and -3 in BAT; whereas chronic-intense PPARγ activation may cause BAT to assume white adipose tissue-like phenotype with increased UCP-2 levels. PPARα activation in mice is sufficient to induce liver UCP-2 expression.

Peroxisome Proliferator-Activated Receptors γ and α Mediate in Vivo Regulation of Uncoupling Protein (UCP-1, UCP-2, UCP-3) Gene Expression

A role for peroxisome proliferator-activated receptors, PPAR gamma and PPAR alpha, as regulators of energy homeostasis and lipid metabolism, has been suggested. Recently, three distinct uncoupling protein isoforms, UCP-1, UCP-2, and UCP-3, have also been identified and implicated as mediators of thermogenesis. Here, we examined whether in vivo PPAR gamma or PPAR alpha activation regulates the expression of all three UCP isoforms. Rats or lean and db/db mice were treated with PPAR gamma [thiazolidinedione (TZD)] or PPAR alpha (WY-14643) agonists, followed by measurement of messenger RNAs (mRNAs) for UCP-1, UCP-2, and UCP-3 in selected tissues where they are expressed. TZD treatment (AD 5075 at 5 mg/kg x day) of rats (14 days) increased brown adipose tissue (BAT) depot size and induced the expression of each UCP mRNA (3x control levels for UCP-1 and UCP-2, 2.5x control for UCP-3). In contrast, UCP-2 and UCP-3 mRNA levels were not affected in white adipose tissue or skeletal muscle. Chronic (30 days) low-dose (0.3 mg/kg x day) TZD treatment induced UCP-1 mRNA and protein in BAT (2.5x control). In contrast, chronic TZD treatment (30 mg/kg x day) suppressed UCP-1 mRNA (>80%) and protein (50%) expression in BAT. This was associated with further induction of UCP-2 expression (>10-fold) and an increase in the size of lipid vacuoles, a decrease in the number of lipid vacuoles in each adipocyte, and an increase in the size of the adipocytes. TZD treatment of db/db mice (BRL 49653 at 10 mg/kg x day for 10 days) also induced UCP-1 and UCP-3 (but not UCP-2) expression in BAT. PPAR alpha is present in BAT, as well as liver. Treatment of rats or db/db mice with WY-14643 did not affect expression of UCP-1, -2, or -3 in BAT. Hepatic UCP-2 mRNA was increased (4x control level) in db/db and lean mice, although this effect was not observed in rats. Thus, in vivo PPAR gamma activation can induce expression of UCP-1, -2, and -3 in BAT; whereas chronic-intense PPAR gamma activation may cause BAT to assume white adipose tissue-like phenotype with increased UCP-2 levels. PPAR alpha activation in mice is sufficient to induce liver UCP-2 expression.

Contrasting adrenergic effects on lipoprotein lipase gene expression in the brown adipose tissue of intact mice and in cultured brown adipocytes from mice

To examine the regulation of lipoprotein lipase (LPL) gene expression, LPL mRNA levels in the brown adipose tissue of intact mice and in mouse brown adipocyte cultures were examined. In intact mice, exposure to cold resulted in a rapid, transient, 5-fold increase in LPL mRNA level. Norepinephrine (NE) injection could fully mimic the effect of acute exposure to cold, and LPL mRNA and enzymatic activity were increased in parallel after NE injection. These results indicated positive adrenergic control of LPL gene expression in the brown adipose tissue of intact mice. In cultured mouse brown adipocytes, the level of spontaneously expressed LPL mRNA decreased in parallel with the progression of brown adipocyte differentiation. NE treatment of undifferentiated cells led to a decrease in LPL mRNA levels. In brown adipocytes that had reached a mature state, NE had a small negative or no effect on LPL mRNA levels, irrespective of whether the experiment was performed in the presence or absence of insulin or of newborn-calf serum. It was concluded that LPL gene expression in brown adipose tissue in intact mice is under adrenergic control but that this gene is not under positive adrenergic control in cultured brown adipocytes from mice, although these cells are otherwise adrenergically sensitive. The presence of additional factors may be necessary to confer adrenergic sensitivity to the LPL gene in the cultured brown adipocytes; alternatively, cells other than the mature brown adipocytes may confer the positive adrenergic sensitivity to the brown adipose tissue depots in situ. © 1997 Elsevier Science B.V. All rights reserved.

Seasonal variation in brown adipose tissue mass and lipid droplet size of Sorex minutus, the pygmy shrew; The relationship between morphology and metabolic rate

1. 1. Metabolic rate, brown adipose tissue (BAT) mass and BAT lipid droplet size, were measured in winter-, summer- and spring-acclimatized Pygmy shrews, Sorex minutus. 2. 2. BAT mass changed significantly with season both in relative and absolute terms. Winter-acclimatized shrews had the smallest (135 ± 10 mg) and spring-acclimatized shrews had the heaviest BAT depots (310 ± 28 mg). Relative BAT mass varied between 2–7% depending on season. Median BAT lipid droplet size (μm 2) was smallest in winter-acclimatized (106 μm 2) and largest in spring-acclimatized shrews (326 μm 2). 3. 3. Resting metabolic rate and thermoregulatory thermogenesis were significantly positively related to BAT mass. There was a significant inverse relationship between mass independent metabolic rate and lipid droplet size in all seasonally acclimatized shrews. 4. 4. Winter-acclimatization of BAT in S. minutus is characterised by a decrease in BAT mass signifying a marked utilisation of lipid stores and by a reduction in BAT lipid droplet size, signifying an increase in thermogenic activity.

A hypercaloric load induces thermogenesis but inhibits stress responses in the SNS and HPA system.

Caloric overingestion generates a sympathetic nervous system (SNS)-mediated increase in brown adipose tissue (BAT) thermogenesis; its effect on the hypothalamo-pituitary-adrenal (HPA) axis is unknown. To determine whether metabolic activation affects the HPA axis, male rats were provided palatable sucrose ad libitum. After 5 or 10 days of sucrose ingestion, BAT and basal and restraint-induced HPA variables were measured. Some rats were instrumented with temperature probes. BAT temperature and HPA axis responses to restraint were measured. Although caloric intake increased > or = 18%, body weight gain did not change after sucrose ingestion; DNA, protein, and uncoupling protein increased in BAT depots, and white adipose tissues were heavier after both 5 and 10 days. During days 5-10, the BAT-core temperature difference was +0.30 degrees C in sucrose rats and -0.46 degrees C in controls (P < 0.05); this, together with the biochemical changes, shows persistent activation of BAT by excess calories. Basal HPA measures were not altered. The sucrose group exhibited smaller BAT temperature and HPA responses to restraint on day 10; there was no HPA difference on day 5. We conclude that calorically mediated increases in BAT thermogenesis are independent of basal HPA activity; however, both systems respond concordantly to restraint stress. The diminished response to restraint in both systems in sucrose-fed rats may result from signals indicating increased energy stores.

Immunohistochemical Localization of Leptin and Uncoupling Protein in White and Brown Adipose Tissue

Leptin is synthesized exclusively by adipocytes and acts on the hypothalamus to regulate energy balance. Previous messenger RNA expression studies demonstrated that leptin is expressed in white adipocytes and also in brown adipose tissue, however expression in brown fat is markedly lower than in white fat. This suggests the possibility that leptin expression in brown adipose tissue is due to the presence of white adipocytes that reside within brown adipose tissue, and that brown adipocytes actually do not express leptin. To address this point, we performed immunohistochemistry on paraffin sections and studied leptin protein expression in different depots of white and brown fat of lean and obese (db/db) mice. To establish the cell type expressing leptin, we also assessed the size and organization of lipid droplets, the ultrastructural features of mitochondria, and the presence or absence of uncoupling protein, a brown fat-specific marker. In white adipose tissue of lean and obese (db/db) mice, leptin protein was expressed in adipocytes of various sizes (range examined: 19.67–200 μm), including adipocytes at the multilocular stage of differentiation. Leptin staining was more intense in some depots (retroperitoneal), and appeared to decrease with fasting. In brown adipose tissue of lean animals, multilocular uncoupling protein (UCP)-positive brown adipocytes had typical brown mitochondria and were leptin-negative, both in fed and fasted conditions. At the periphery of the interscapular brown adipose tissue depot, unilocular, UCP-negative adipocytes (mean diameter: 41.55 μm) with white-type mitochondria were observed, and these cells were leptin-positive. In obese (db/db) animals, brown fat was composed mainly of small unilocular, UCP-positive adipocytes (mean diameter: 40.08 μm), which were also leptin-positive. At the periphery of the organ, numerous large, unilocular, UCP-negative adipocytes (mean diameter: 73.65 μm) with white-like mitochondria were present. As expected, these cells were also leptin-positive. In summary, classical brown adipocytes differ from white adipocytes, not only by their morphology and UCP expression, but also by their apparent lack of detectable leptin expression. db/db brown adipocytes, however, were unilocular and leptin-positive. The molecular mechanisms mediating expression of leptin in white but not brown adipocytes of lean animals, and the significant expression of leptin in brown adipocytes of db/db mice will be the focus of future studies.

Immunohistochemical localization of leptin and uncoupling protein in white and brown adipose tissue.

Leptin is synthesized exclusively by adipocytes and acts on the hypothalamus to regulate energy balance. Previous messenger RNA expression studies demonstrated that leptin is expressed in white adipocytes and also in brown adipose tissue, however expression in brown fat is markedly lower than in white fat. This suggests the possibility that leptin expression in brown adipose tissue is due to the presence of white adipocytes that reside within brown adipose tissue, and that brown adipocytes actually do not express leptin. To address this point, we performed immunohistochemistry on paraffin sections and studied leptin protein expression in different depots of white and brown fat of lean and obese (db/db) mice. To establish the cell type expressing leptin, we also assessed the size and organization of lipid droplets, the ultrastructural features of mitochondria, and the presence or absence of uncoupling protein, a brown fat-specific marker. In white adipose tissue of lean and obese (db/db) mice, leptin protein was expressed in adipocytes of various sizes (range examined: 19.67-200 microns), including adipocytes at the multilocular stage of differentiation. Leptin staining was more intense in some depots (retroperitoneal), and appeared to decrease with fasting. In brown adipose tissue of lean animals, multilocular uncoupling protein (UCP)-positive brown adipocytes had typical brown mitochondria and were leptin-negative, both in fed and fasted conditions. At the periphery of the interscapular brown adipose tissue depot, unilocular, UCP-negative adipocytes (mean diameter: 41.55 microns) with white-type mitochondria were observed, and these cells were leptin-positive. In obese (db/db) animals, brown fat was composed mainly of small unilocular, UCP-positive. adipocytes (mean diameter: 40.08 microns), which were also leptin-positive. At the periphery of the organ, numerous large, unilocular, UCP-negative adipocytes (mean diameter: 73.65 microns) with white-like mitochondria were present. As expected, these cells were also leptin-positive. In summary, classical brown adipocytes differ from white adipocytes, not only by their morphology and UCP expression, but also by their apparent lack of detectable leptin expression. db/db brown adipocytes, however, were unilocular and leptin-positive. The molecular mechanisms mediating expression of leptin in white but not brown adipocytes of lean animals, and the significant expression of leptin in brown adipocytes of db/db mice will be the focus of future studies.

Changes induced by fasting and dietetic obesity in thermogenic parameters of rat brown adipose tissue mitochondrial subpopulations.

The effects of starvation on the thermogenic parameters of three different mitochondrial subpopulations in brown adipose tissue (BAT) of both post-cafeteria obese and lean rats were investigated. Tissue from different BAT depots from fed and 24 h starved rats were collected, pooled and three mitochondrial subpopulations were isolated by differential centrifugation; the M1 fraction (1000 g), the M3 fraction (3000 g) and the M15 fraction (15,000 g). Thermogenic parameters were measured in the three mitochondrial subtypes, and uncoupling protein (UCP) mRNA was determined in BAT. The results showed that starvation induced a decrease in mitochondrial turnover in BAT from both lean and obese rats. Moreover, a selective net loss of UCP from the lightest mitochondrial fraction (M15) in lean rats, with a concomitant reduction of UCP mRNA was observed. The reductions did not occur in obese rats and, as a result, a change in UCP distribution between the mitochondrial subpopulations was produced, with an increase in the M1 mitochondrial subtype. The lack of response of UCP to starvation observed in BAT of obese rats compared with the decrease seen in lean animals, is a consequence of a different mitochondrial subpopulation composition and/or a different response of a particular subpopulation to starvation.

Biogenesis of thermogenic mitochondria in brown adipose tissue of Djungarian hamsters during cold adaptation.

After cold exposure, cytochrome c oxidase (COX) activity increased about 2.5-fold within 2 weeks in the brown adipose tissue (BAT) of Djungarian hamsters. The mRNAs for COX subunits I and III and the 12 S rRNA, encoded on mitochondrial DNA (mtDNA), as well as mRNAs for COX subunits IV, Va and mitochondrial transcription factor A, encoded in the nucleus, were unchanged when expressed per unit of total tissue RNA. However, since total tissue RNA doubled per BAT depot, while total DNA remained unchanged, the actual levels of these transcripts were increased within BAT cells. In contrast, the abundance of mRNA for uncoupling protein was increased 10-fold, indicating specific activation of this gene. In addition, the maximal rate of protein synthesis analysed in a faithful in organello system was increased 2.5-fold in mitochondria isolated from BAT after 7 days of cold exposure. We conclude from these data that the biogenesis of thermogenic mitochondria in BAT following cold adaptation is achieved by increasing the overall capacity for synthesis of mitochondrial proteins in both compartments, by increasing their mRNAs as well as the ribosomes needed for their translation. In addition, the translational rate for COX subunits as well as all other proteins encoded on mtDNA is increased. Thus the pool of subunits encoded on mtDNA required for assembly of respiratory chain complexes is provided. By comparison with other models of increased mitochondrial biogenesis, we propose that thyroid hormone (generated within BAT cells by 5'-deiodinase, and induced upon sympathetic stimulation), which is a well known regulator of the biogenesis of mitochondria in many tissues, is also the major effector of these adaptive changes in BAT.

Corticosterone decreases nonshivering thermogenesis and increases lipid storage in brown adipose tissue.

Brown adipose tissue (BAT) contains glucocorticoid receptors; glucocorticoids are required for maintaining differentiated BAT in culture. These studies were performed to determine the effects of corticosterone on BAT thermogenic function and lipid storage. Rats were adrenalectomized and given subcutaneous corticosterone pellets in concentrations that maintained plasma corticosterone constant across the range of 0-20 micrograms/dl or were sham adrenalectomized. All variables were examined 5 days after surgery and corticosterone replacement. Measures of BAT function-thermogenic capacity [guanosine 5'-diphosphate (GDP) binding and uncoupling protein (UCP; a BAT-specific thermogenic protein)] and storage (BAT wet wt, protein, and DNA levels) were made. Plasma hormones (corticosterone, adrenocorticotropic hormone, insulin, 3,3',5-triiodothyronine, and thyroxine were measured. Corticosterone significantly affected BAT thermogenic measures: UCP content and binding of GDP to BAT mitochondria decreased with increasing corticosterone; GDP binding characteristics in BAT from similarly prepared rats examined by Scatchard analysis showed that maximum binding (Bmax) and dissociation constant (Kd) decreased with increasing corticosterone dose. BAT DNA was increased by adrenalectomy and maintained at intact levels with all doses of corticosterone; BAT lipid storage increased dramatically at corticosterone values higher than the daily mean level in intact rats. Histologically, the number and size of lipid droplets within BAT adipocytes increased markedly with increased corticosterone. White adipose depots were more sensitive to circulating corticosterone concentrations than were BAT depots and increased in weight at levels of corticosterone that were at or below the daily mean level of intact rats. We conclude that, within its diurnal range of concentration corticosterone acts to inhibit nonshivering thermogenesis and increase lipid storage.(ABSTRACT TRUNCATED AT 250 WORDS)

Long photophase is not a sufficient stimulus to reduce thermogenic capacity in winter-acclimatized short-tailed field voles (Microtus agrestis) during long-term cold acclimation.

The thermogenic capacity of brown adipose tissue in winter- and summer-acclimatized short-tailed field voles (Microtus agrestis) was investigated by examining changes in mass of brown adipose tissue, the ratio of white adipose tissue to brown adipose tissue, the concentration of the uncoupling protein (thermogenin) in whole depots (micrograms) and in mitochondrial mass (micrograms.mg-1) and the activity of cytochrome c oxidase in the depots (mmol.min-1). The concentration of thermogenin in winter-acclimatized voles (n = 8), per brown adipose tissue depot and per mitochondrial mass, was significantly higher than in summer-acclimatized voles (n = 6). There was no significant difference in the level of cytochrome c oxidase activity between these two groups. Four groups of winter-acclimatized voles (n = 6 in each group) were exposed to 5 degrees C for 10, 20, 50 and 100 days in a 14L:10D photoperiod. Body mass, brown adipose tissue mass, white adipose tissue mass and basal metabolic rate were significantly positively related to the length of time cold exposed up to 100 days. There was a significant inverse relationship between the ratio of white to brown adipose tissue mass and the duration of cold exposure. There was no significant relationship between thermogenin concentration, either per depot or in mitochondrial mass of brown adipose tissue, with the length of time cold exposed. The level of cytochrome c oxidase activity increased significantly from control levels to a maximum after 10 days in the cold but decreased from 10 days onwards. In winter-acclimatized M. agrestis, a 14L:10D photoperiod is not a sufficient stimulus to reduce thermogenic capacity during cold acclimation.(ABSTRACT TRUNCATED AT 250 WORDS)

Surgical removal of brown fat results in rapid and complete compensation by other depots

Interscapular, scapular, and cervical brown adipose tissue (BAT) depots were removed from young male rats (BATX). These depots represented 40% of both the total dissectible mass and cytochrome oxidase content of BAT in these animals. Sham-operated and BATX rats were fed a low (8%) protein diet for periods of 9 or 16 days. Over both periods, body weight gain and energy intake, gain, expenditure, and efficiency were almost identical in sham and BATX groups. Four days after lipectomy the maximal thermogenic response (increase in O2 consumption) to norepinephrine (400 micrograms/kg sc) was 30% lower for BATX rats than for controls, but by day 13 this difference had disappeared. Nine days after lipectomy the total mass and cytochrome oxidase activity of the remaining dissectible BAT was comparable to that of the sham-operated controls, although the protein content was slightly reduced. The specific mitochondrial GDP binding (an index of thermogenic activity) was increased significantly in BATX rats, and total BAT mitochondrial GDP binding was no different from control values. At the end of the experiment (day 16), no regeneration of excised tissue had occurred, but the remaining BAT depots had shown almost complete compensation; the mass and the oxidative and thermogenic capacity of the total dissectible brown fat were virtually identical in both groups.

Involvement of sympathetic nervous system and brown fat in endotoxin-induced fever in rats.

The object of this study was to assess the role of brown adipose tissue (BAT) and the sympathetic nervous system in the rise in heat production associated with endotoxin-induced fever. Oxygen consumption (VO2) was found to be significantly increased (28%) over a 4-h period after two doses of endotoxin (Escherichia coli lipopolysaccharide, 0.3 mg/100 g body wt) given 24 h apart. Injection of a mixed beta-adrenoceptor antagonist (propranolol) reduced VO2 by 14% in endotoxin-treated rats, whereas the selective beta 1- (atenolol) or beta 2- (ICI 118551) antagonists suppressed VO2 by 10%. These drugs did not affect VO2 in control animals. BAT thermogenic activity assessed from measurements of in vitro mitochondrial guanosine 5'-diphosphate (GDP) binding was elevated by 54% in interscapular BAT and by 171% in other BAT depots. Surgical denervation of one lobe of the interscapular depot prevented these responses. Endotoxin failed to stimulate GDP binding in rats fed protein-deficient diets. This may have been because BAT thermogenic activity was already elevated in control rats fed these diets or because endotoxin caused a marked suppression of food intake in the protein-deficient animals. The results indicate that sympathetic activation of BAT is involved in the thermogenic responses to endotoxin and that these can be modified by dietary manipulation.

Development of adipose tissues

Studies on the development of brown and white adipose tissues and their relationship to ordinary connective tissue are the subject of the present review, which is updated to the fall of 1985. Primordial entities described as "primitive organs" have been noted at sites of adipose tissue development by numerous investigators. These "primitive organs" are clearly delineated from surrounding connective tissue and possess a well-defined vascular network in the interstices of which cellular organization takes place. The precursor cells of brown and white adipocytes appear to acquire their distinctive cytogenetic properties at a very early stage in the embryo. These progenitor cells temporarily exhibit structural similarities to fibroblasts and endothelial cells. This resemblance has led to speculation that fibroblasts and/or endothelial cells may be ancestral to adipocytes. However, recent in vivo and in vitro observations suggest that the brown adipose precursor cell is morphogenetically distinct from the white adipose precursor cell on one hand, and from fibroblasts and endothelial cells on the other. In vitro studies have also shown that the pericytic elements present in both brown and white adipose tissue depots in the adult are quiescent precursor cells that may be recruited under certain circumstances. Areas for further inquiry by means of recently developed techniques are indicated.

Activation of brown fat thermogenesis in response to central injection of corticotropin releasing hormone in the rat

Injection of CRF-41 (2–5 nmol) into the third ventricle, or the para ventricular nucleus of anaesthetized rats caused a marked rise in the temperature of the interscapular brown adipose tissue (BAT) depot (peak rise 0.8–1°C) which was inhibited by prior intravenous injection of propranolol. There was also a significant increase in the mitochondrial proton conductance pathway of brown adipose tissue, assessed from the binding of guanosine diphosphate (GDP) to mitochondria isolated from the interscapular (89% above control) and perirenal and para-aortic depots (130%). Acute surgical sympathectomy of interscapular brown adipose tissue immediately prior to injection of CRF significantly attenuated the increase in mitochondrial GDP-binding. Hypophysectomized (HYPX) rats showed a large (180%) increase in GDP-binding of brown adipose tissue 7 days after surgery and this was almost completely prevented by denervation of the interscapular depot prior to hypophysectomy. Acute injection of morphine also reduced the GDP-binding in hypophysectomized, but not in control rats. These data demonstrate that central injection of CRF stimulates thermogenesis in brown adipose tissue, probably by modifying sympathetic outflow. The activation of brown adipose tissue following hypophysectomy was also dependent on the sympathetic innervation and could be due to an increase in release of CRF.

Cold-induced increase in brown fat thyroxine 5'-monodeiodinase is attenuated in Zucker obese rat.

In this study we examined the possibility that the reduced brown adipose tissue (BAT) thermogenesis in the Zucker obese rat may result from a limited capacity for enzymic conversion of thyroxine (T4) to triiodothyronine (T3) in BAT. A total of 34 lean and obese rats, approximately 4 mo old were divided into three treatment groups: group 1 (5 lean and 6 obese) was fed Purina rat chow for 21 days, and group 2 (5 lean and 6 obese) was fed a cafeteria diet for 21 days, and group 3 (6 lean and 6 obese) was fed Purina rat chow and maintained in the cold (8 +/- 1 degrees C) for 7 days. The lean and obese rats in all three groups of animals were matched closely for age and respective body weight. Activity of T4 5'-deiodinase was determined as the rate of T3 production from added T4 under controlled in vitro conditions. Serum T4 and T3 were determined by radioimmunoassay. The rate of T4-to-T3 conversion in BAT was similar in the lean and obese rats maintained at room temperature, whether fed rat chow or a cafeteria diet (approximately 40-50 pmol T3/scapular BAT depot per h). However, expressed per scapular BAT depot, lean rats exposed to cold displayed about a fivefold increase in BAT T3 production (P less than 0.0001), whereas only a small increase was observed in the cold-exposed obese rats. Serum T3 levels tended to be reduced in the Zucker obese rats. Our data indicate a reduced capacity for T3 production in Zucker rat BAT exposed to cold.(ABSTRACT TRUNCATED AT 250 WORDS)

Blood flow to brown fat in lean and obese adrenalectomized Zucker rats.

The Zucker obese rat is characterized by decreased capacity for diet-induced and for nonshivering thermogenesis. This decrease is due, in large part, to reduced thermogenesis in depots of brown adipose tissue, a major source of heat production in rats. Adrenalectomy retards the weight gain observed in the obese rats and also normalizes brown fat guanosine 5'-diphosphate (GDP) binding, an in vitro measure of brown fat thermogenic capacity. This study examined the effect of adrenalectomy on brown fat blood flow, an in vivo measure of the tissue's function, and on norepinephrine-induced O2 consumption (NST) of 11-wk-old obese (fa/fa) and lean (Fa/?) rats. Adrenalectomy had little effect on weight gain, NST, or norepinephrine-stimulated blood flow to brown fat in lean rats. However, adrenalectomy produced profound changes in the obese animals, preventing the weight gain normally occurring in the obese rats and normalizing both NST capacity and norepinephrine-stimulated blood flow to brown fat. These findings provide further support for the importance of brown fat thermogenesis and glucocorticoids in modulating the obesity of the Zucker rat.

Brown adipose tissue lipectomy leads to increased fat deposition in Osborne-Mendel rats.

Inter- and subscapular brown adipose tissue depots were removed from nine female Osborne-Mendel rats. These lipectomized animals and nine sham-operated controls recovered from surgery for 7 days at 25 degrees C and were then placed on a highly palatable liquid diet. All animals were maintained for a 2nd wk at 25 degrees C before being switched to 8 degrees C. After 9 wk in the cold, animals were killed, and the brown adipose tissue was dissected from scapular, cervical, thoracic, perirenal, and axillary regions. Total brown fat pad mass, protein content, brown adipocyte number, citrate synthase activity, and beta-hydroxyacyl CoA dehydrogenase activity in each of the dissected brown fat depots were significantly less than those of the sham-operated controls. Thus there was incomplete metabolic compensation in the remaining brown fat depots after the removal of the scapular brown fat in the lipectomized rats. The mass and lipid content of the retroperitoneal white adipose depot were significantly increased in the lipectomized rats as was their carcass fat content (up 14%). Food intake of the lipectomized rats was slightly but significantly decreased. These data indicate that a reduction in the amount of functional brown fat is accompanied by increased body fat accretion and are thus consistent with the hypothesis that decreased brown adipose thermogenesis can lead to altered energy balance and increased white fat deposition.