Brown Fat Activity Research Articles

Brown adipose tissue activity in hypocaloric-diet fed lactating rats.

Hypocaloric diet feeding reduced the mitochondrial protein content and whole tissue GDP-binding in interscapular brown adipose tissue from both virgin and lactating rats. A reduction in brown fat lipoprotein lipase activity was also detected in underfed virgin and lactating animals. These results indicate that lactation in the rat, even though it produces a reduction in brown fat activity, does not impair the capacity of the tissue to respond to a diminished caloric intake by lowering its activity further.

Influence of environmental temperature on energy balance, diet-induced thermogenesis and brown fat activity in 'cafeteria'-fed rats.

1. Young male rats were fed on a pelleted stock diet or a variety of palatable food items ('cafeteria' diet) and housed at 24 degrees or 29 degrees. 2. 'Cafeteria' feeding at the lower temperature stimulated energy intake, gain and expenditure, but reduced energetic efficiency such that over 70% of the excess intake was expended. 3. Housing at 29 degrees suppressed intake and expenditure in animals on both diets, but to a greater extent in 'cafeteria'-fed rats and energetic efficiency was greater than control values at this higher temperature. 4. The thermogenic capacity of brown fat (mitochondrial purine nucleotide binding) was increased by 'cafeteria' feeding, but was suppressed in animals kept at 29 degrees. 5. The results demonstrate that diet-induced thermogenesis is inhibited by high environmental temperatures.

Sequential changes in brown adipose tissue composition, cytochrome oxidase activity and GDP binding throughout pregnancy and lactation in the rat

The sequential appearance of changes in interscapular brown adipose tissue composition, cytochrome oxidase activity and GDP binding was studied throughout pregnancy and lactation in the rat. Brown adipose tissue was hypertrophied during pregnancy because of progressive lipid accumulation, whereas its mitochondrial component and GDP binding to brown fat mitochondria were unchanged. In early lactation (day 5) there was a decrease in the overall GDP binding to brown fat only because of the lower mitochondrial protein content. In late stages of lactation (days 10 and 15), the amount of tissue and its mitochondrial protein content were minimal and the GDP binding per mitochondrial protein decreased substantially. Scatchard analysis in day-15-lactating rats indicated a large decrease in GDP binding sites without any changes in affinity. It is concluded that the diminished thermogenic activity of brown fat in lactation is attained through changes at different structural levels of the tissue occurring in a characteristic sequential trend; first a reduction in its mitochondrial component, and only later, at nid-lactation, a decrease in the specific mitochondrial proton conductance pathway activity.

Meal-Induced Changes in Lipoprotein Lipase Activity in Brown Fat and Other Tissues of Rats

Brown fat thermogenesis is increased after a single test meal. This study was conducted to determine whether lipoprotein lipase activity is higher in brown adipose and other tissues after a single large meal. Rats were trained to eat two large meals per day. Two hours after consuming a test meal, lipoprotein lipase activity was measured in interscapular brown adipose tissue, retroperitoneal and epididymal white adipose tissue, gastrocnemius and soleus skeletal muscles and heart. After a high carbohydrate test meal, lipoprotein lipase activity in white adipose tissue pads was higher (P < 0.05) and that in brown adipose tissue was lower (P < 0.05) than in these tissues from the meal-deprived group. Muscle lipoprotein lipase did not change significantly. A high fat test meal did not significantly alter lipoprotein lipase activity in brown adipose tissue, white adipose tissue, gastrocnemius or soleus when compared to the meal-deprived control, but heart lipoprotein lipase activity was significantly elevated. These findings indicate that after a single test meal lipoprotein lipase activity in brown adipose tissue is not higher than that from the meal-deprived group and therefore, lipoprotein lipase may not play a rate-limiting role in moving free fatty acids into this tissue in the postprandial state.

Dissociation of sympathetic and thermogenic activity in brown fat of Syrian hamsters.

Syrian hamsters (Mesocricetus auratus) were fed a high-fat (HF) diet for up to 16 wk. Sympathetic and thermogenic activities in their brown adipose tissue (BAT) were assessed by measuring norepinephrine content and turnover and mitochondrial GDP binding and cytochrome c oxidase activity. Chronic ingestion of the HF diet resulted in significant increases in carcass lipid and interscapular BAT wet weight by the end of the second week. HF-fed hamsters were slightly hyperphagic during the first 2 wk of HF feeding only. Significant weight gains persisted beyond the period of hyperphagia. Hypertrophy of interscapular BAT after 16 wk on the HF diet was accompanied by increases in protein and DNA content, indicating growth of functional tissue. Norepinephrine turnover and content in BAT were decreased throughout the entire period of HF feeding, regardless of changes in caloric intake or body weight. Mitochondrial cytochrome c oxidase activity and GDP binding were increased after 16 wk on the HF diet, a time when the HF-fed animals were obese but not hyperphagic. These results demonstrate a dissociation of BAT thermogenesis from sympathetic activity in the tissue. It appears that sympathetic nervous system activity in BAT was suppressed by the HF diet, whereas thermogenic activity of the tissue was activated when the hamsters became obese.

The receptors responsible for heat production in brown adipose tissue in the young rabbit

It is known that adrenergic agonists stimulate thermogenesis in the brown fat of the young rabbit but the receptors responsible for mediating the response have not been identified. The infusion of either noradrenaline or isoproterenol (1-2 micrograms . kg-1 X min-1) produced an increase in subcutaneous temperature (0.93 +/- 0.15 and 1.22 +/- 0.10 degrees C, respectively over the interscapular brown fat. At low doses (0.4 microgram . kg-1 X min-1) only isoproterenol was effective. The thermogenic response to isoproterenol was blocked by atenolol, a beta 1-adrenergic antagonist. Neither salbutamol or terbutaline, both beta 2-agonists, produced a temperature increase. Collectively, these data suggest that stimulation of beta 1-adrenoceptor is primarily responsible for the thermogenic activity of brown fat in the rabbit. However, it was found that 53% of the increase in temperature could be blocked by prazosin, an alpha 1-antagonist. Phentolamine was not effective as a blocker. Although a maximal brown fat thermogenic response can be achieved by stimulating the beta-adrenoceptors, the alpha-adrenoceptors appears to play at least an auxiliary role in young rabbit.

The relationship between the thermogenic response of brown adipose tissue and age during noradrenaline infusion in the young rabbit.

This work was undertaken to determine if the thermogenic activity of brown fat decreased with age in young rabbits despite the morphological evidence indicating persistence of the brown adipocytes at 4 weeks of age. Data obtained by infusing five doses of noradrenaline and measuring oxygen consumption were used to construct cumulative dose-response curves for five age groups between 3 and 32 days of age. Blood flow to brown fat and other tissues was measured by the microsphere method at 1 and 3 weeks of age. The noradrenaline-induced increase in oxygen consumption when expressed as a percentage of resting oxygen consumption in millilitres per 100 g of body weight decreased (p less than 0.05) with age. However, the absolute noradrenaline-induced increase in metabolic rate (millilitres per minute) increased with age. Total blood flow to brown fat (millilitres per minute) during noradrenaline infusion was unchanged between 1 and 3 weeks of age, but when the blood flow was expressed in millilitres per minute per gram of tissue flow decreased significantly (p less than 0.05) probably because of infiltration of brown fat with white fat. These data suggest that the amount of brown fat and its thermogenic capacity remain relatively constant between 1 and 3 weeks of age, but as a thermogenic organ, brown fat becomes proportionally less effective with age because of the large increase in body mass.

Energy balance and brown fat activity in rats fed cafeteria diets or high-fat, semisynthetic diets at several levels of intake

The object of the study was to determine the relative effects of hyperphagia and diet composition on energy balance and thermogenic activity in rats fed highly palatable cafeteria diets. Three types of diet were used: a pelleted stock diet, a cafeteria diet composed of a variety of human food items, and semisynthetic diets with nutrient compositions similar to the stock and cafeteria diets. Feeding rats a high-fat semisynthetic diet (similar to the cafeteria diet) at a energy intake equivalent to that of stock-fed controls (approximately 2.5 times maintenance) resulted in greater body energy gains and energetic efficiencies. These effects were probably due to the reduced energy costs of fat synthesis associated with high-fat diets. No effect of dietary composition on body energy gain was seen in animals fed below 2.5 times maintenance. Animals fed four cafeteria food items each day, or the high-fat semisynthetic diet, at 2.5 times maintenance showed significantly greater thermogenic responses to norepinephrine, increased brown adipose tissue (BAT) mass, and greater BAT mitochondrial GDP binding than controls on the same intake. Injection of propranolol reduced oxygen consumption in all groups, but the effect was greater in animals on higher intakes and was highest in the cafeteria groups. Thus, increasing fat intake, either by presenting cafeteria food items or by feeding a high-fat semisynthetic diet at the same level of intake as controls, stimulates the sympathetic nervous system and BAT activity. However, any changes in thermogenesis in these rats would appear to be masked by the reduced energy costs of lipid synthesis, and energy expenditure is only seen to rise when intakes are increased well above the level of controls.

Effects of subdiaphragmatic vagotomy on energy balance and thermogenesis in the rat.

Subdiaphragmatic vagotomy caused chronic gastric distension and hypertrophy, and a reduction in voluntary food intake in rats fed a pelleted stock diet. These effects were minimized by feeding a more digestible semisynthetic diet. Vagotomized rats fed the pelleted diet showed lower rates of oxygen consumption than pair-fed controls, and the rise in metabolic rate (thermic response) following gastric intubation with a carbohydrate meal was diminished. This could be restored to normal by simultaneous injection of insulin. Thermic responses to fat and noradrenaline were normal in the vagotomized group. On the powdered semisynthetic diet, vagotomized rats gained more weight and showed greater efficiency of energy gain than pair-fed controls. The thermic response to a single meal of the semisynthetic diet was depressed in these vagotomized rats, but restored to normal by acute insulin treatment. The activity of the thermogenic proton conductance pathway in brown adipose tissue mitochondria (assessed from purine nucleotide binding) was reduced by vagotomy in animals on both diets, but was restored to normal by chronic insulin treatment, which also slightly raised brown fat activity in sham-operated rats. These results demonstrate that the reduced gastric activity and food intake following vagotomy is dependent on the digestibility and/or composition of the diet. When differences in food intake are abolished by pair feeding, vagotomy reduces thermogenic responses to carbohydrate, probably as a result of impaired insulin release. This may be responsible for the enhanced energetic efficiency and elevated weight and energy gains seen after vagotomy.

Biological distribution and significance of brown adipose tissue.

1. The structure, location, identification and thermogenic function of brown adipose tissue is discussed before describing its distribution in animals. 2. With a few interesting exceptions, brown fat occurs almost exclusively in mammals. 3. This tissue has been positively identified in thirteen orders, but more thorough investigations are required before its absence can be confirmed in the remaining eight mammalian orders. 4. Factors influencing the amount and activity of brown fat seen between and within species are numerous, but some of the most important are body size, diet, environmental temperature, age and reproductive state. 5. The role brown fat, and the effects of impairments in its function, are described in relation to thermoregulation and the control of energy balance and body composition.

Scapular brown fat removal enhances development of adiposity in cold-exposed obese Zucker rats

To investigate the contribution of brown fat (BAT) to the development of obesity in genetically obese Zucker rats (fa/fa), scapular brown fat (SBAT) was removed from obese and lean 4-wk-old females. Eight weeks after surgery there was no regrowth of SBAT. Lipectomy had no effect on body weight gain, food intake, and body composition when rats were housed at 25 degrees C. Lean rats completely compensated for the lipectomy by increasing BAT mass, protein, cellularity, and activity of citrate synthase (CS) in axillary, perirenal, and thoracic depots. beta-Hydroxyacyl-coenzyme A dehydrogenase (HOAD) activity was increased, but compensation was incomplete. In lipectomized obese rats, only BAT protein and cell number were increased sufficiently for complete compensation. In a second experiment SBAT was removed from obese and lean rats, but rats were housed in the cold (10 degrees C) for 8 wk. In lean rats, although compensation was incomplete, it was sufficient to maintain a weight gain and body composition comparable with sham-operated lean rats. In obese rats, where there was little or no compensation for lipectomy, weight gain and fat deposition were greater than observed in sham-operated obese controls. These data support the hypothesis that reducing the amount of functional BAT contributes to the development of increased adiposity.

Increased level of mRNA for the uncoupling protein in brown adipose tissue of rats during thermogenesis induced by cold exposure or norepinephrine infusion.

We have studied by in vitro translation the mRNA population from rat brown fat in two situations where the thermogenic activity of the tissue is stimulated. Both cold exposure of rats and the presence of pheochromocytoma result in marked alterations among the polypeptides synthesized. Notably, in these two situations there was an increased labeling of polypeptides of the size of the heat-shock proteins and an increased labeling of the 32,000-dalton uncoupling protein of brown fat mitochondria, which is responsible for energy dissipation as heat. Stimulation of the thermogenic activity of brown fat thus results in a high level of mRNA encoding the uncoupling protein. Chronic delivery of norepinephrine mimicked the increase of mRNA for the uncoupling protein observed in animals exposed to cold or bearing pheochromocytoma. Thus, a neuromediator such as norepinephrine is able to trigger the induction of the mRNA encoding the mitochondrial uncoupling protein and is able to stimulate mitochondriogenesis.

Influence of alcohol and sucrose consumption on energy balance and brown fat activity in the rat

Voluntary intake of solutions of alcohol (7%) and sucrose (10%) represented 20% and 25% of total metabolizable energy intake, respectively, in young male rats maintained on a stock diet, but total energy intake was similar to that of controls drinking water. Body weight and energy gains were similar for control and sucrose-treated rats but were significantly reduced in the group drinking alcohol, and energy expenditure, corrected for body size (kJ/kg 0.75/day), was elevated in rats drinking alcohol (17% above control) or sucrose (18%). Gross and net energetic efficiencies were markedly depressed by consumption of alcohol but not by consumption of sucrose. Resting oxygen consumption, before and after injection of norepinephrine ( 25 μg 100 g body weight ) was similar for all groups. Brown adipose tissue (BAT) mass and mitochondrial protein did not differ between groups, but the activity of the mitochondrial proton conductance pathway, assessed from the binding of 3H-guanosine diphosphate, was significantly elevated by alcohol and sucrose consumption. Thus, the increased energy expenditure associated with alcohol and sucrose ingestion may involve BAT thermogenesis, but this alone cannot explain the larger effects of alcohol on metabolic efficiency.

Sympathetic and adrenocorticoid influences on diet-induced thermogenesis and brown fat activity in the rat

1. 1. Bilateral adrenalectomy markedly reduced body weight and energy gain and energetic efficiency of adult cafeteria-fed rats but enhanced the thermogenic response to food and stimulated brown fat activity. 2. 2. These changes were totally prevented by replacement of the animals with corticosterone (1 mg/ rat/day). 3. 3. Unilateral denervation of the sympathetic nerves supplying the interscapular brown adipose tissue abolished the enhanced activity resulting from adrenalectomy and inhibited thermogenic activity in brown fat from cafeteria rats with intact adrenals, but had no effect in adrenalectomised animals treated with a high dose of corticosterone.

Energy Balance, Diet-Induced Thermogenesis and Brown Adipose Tissue in Lean and Obese (fa/fa) Zucker Rats after Adrenalectomy

Five-week-old male, lean (+/?) and genetically obese (fa/fa) Zucker rats received either bilateral adrenalectomy or sham operations, and energy balance was measured over the subsequent 21 days. Body weight and energy intake were similar for intact lean and obese rats, but the latter group showed a marked increase in body energy gain and energetic efficiency, and reduced energy expenditure. Adrenalectomy did not significantly influence energy balance in lean rats but caused decreases in food intake and body weight gain in obese rats, and restored their energetic efficiency and body energy gain to the level of lean animals. The lower thermic response to a single meal (40 kJ) in intact obese rats was restored to normal by adrenalectomy. Brown adipose tissue (BAT) mass was larger in sham-operated obese rats, but tissue protein concentration, mitochondrial yield and mitochondrial GDP binding were all markedly reduced in obese rats. BAT mass, composition and GDP binding were almost identical in adrenalectomized obese and all lean rats. These findings demonstrate that the reduced fat deposition in Zucker rats after adrenalectomy is mainly due to the large decrease in the efficiency of energy utilization associated with a restoration of brown fat activity.

The separation and properties of phosphoprotein phosphatase modulators from brown adipose tissue of young rats and the changes in their concentration during development.

The heat-stable modulators of phosphoprotein phosphatase activity have been partically purified from brown adipose tissue. A nonphosphorylatable inhibitor of phosphorylase phosphatase (inhibitor 2) and an activator of phosphohistone phosphatase were similar to the corresponding modulators from muscle and liver in both their physical properties and in their relative effects upon three different preparations of phosphatase. An inhibitor of phosphorylase phosphatase that was only active when phosphorylated was eluted from DEAE-cellulose by 80 mM NaCl (inhibitor 1'). Only a small amount of inhibitor was eluted at 20 mM NaCl (inhibitor 1), which is the concentration that eluted the bulk of the phosphorylatable inhibitor in muscle and liver. Inhibitor 1 and inhibitor 1' had similar physical properties but differed in their activities towards the different phosphatases. It is suggested that they are different forms of the same protein and that inhibitor 1' more closely resembles the native inhibitor. The modulators did not compete with each other and were probably not subunits of a phosphatase complex. However, the direction and timing of the changes in their concentration in brown fat during the developmental period indicate that the inhibitors, at least, perform some useful physiological function in the tissue. The concentration of inhibitor 2 was high before birth and for 10 days after birth, when the tissue was proliferating. The concentration of the phosphorylatable inhibitor was highest immediately after birth and for the next 16 days, which is the period of greatest thermogenic activity of brown fat.

3-Hydroxy-3-methylglutaryl-CoA reductase activity in liver, brown fat, and small intestine of developing rats

3-Hydroxy-3-methylglutarylCoA reductase activity was found to be low in the microsomal fraction of the small intestine, liver, and brown fat in suckling rats. It was high perinatally. Treatment with phosphatase of the microsomal fraction increased activity considerably in gut, somewhat in liver, and not al all in brown fat of infant rats. Activity in the mitochondrial fraction of the small intestine showed no developmental changes. injections of infant rats with triiodothyronine increased and with cortisone decreased hepatic activity but had no effect on gut or brown fat. It is concluded that even though activity decreases in all three tissues after birth, it is regulated differently in liver than in gut and brown fat.

Effects of feeding a palatable "cafeteria' diet on energy balance in young and adult lean (+/?) Zucker rats.

1. The effects of feeding a palatable and varied "cafeteria' diet on energy balance were studied in young (5.5 week) and adult (5.5 month) lean male Zucker (+/?) rats. 2. Estimates of metabolizable energy (ME) intake derived from food composition tables were almost identical to values obtained from bomb calorimetry of foods, urine and faeces, and ME intake was elevated by approximately 73% in all "cafeteria' animals compared to stock-fed controls. 3. "Cafeteria' feeding had no effect on the body-weight of young rats but induced excess weight gains in the older animals and resulted in increased deposition of fat and energy in both groups. Energy expenditure, calculated from ME intake and body-energy gain, was elevated by 77 and 57% in young and adult-cafeteria rats respectively. The energy cost of fat deposition could account for only a small proportion of this increased expenditure. 4. The present results confirm previous findings in another strain of rat and show that the increased energy expenditure (i.e. diet-induced thermogenesis, DIT) which occurs in response to hyperphagia is not restricted to young animals but is also seen in older rats. Measurements of resting oxygen consumption after injections of noradrenaline or a beta-adrenergic antagonist (propranolol), and changes in brown adipose tissue mass are consistent with the suggestion that the DIT of "cafeteria'-fed rats results from sympathetic activation of brown fat.

412 THYPDID REGULATION OF PHOSPHOENOLPYRUVATE CARBOXYKINASE IN THE PERINATAL PERIOD OP THE RAT

Triiodothyronine (T3) is known to induce gluconeogenesis in the liver of adult rats. Hence we enquired whether it could be involved in the steep rise of phosphoenolpyruvate carboxykinase (PEPcK) activity described for the newborn. Two injections of T3 (20 μg/100 g body weight each) to the mother rat on days 20 and 21 of pregnancy resulted in a ten to 50-fold rise in PEPcK activity in fetal liver. At the sane time the relatively high activity in brown fat was decreased by about 50%. T3 levels in the fetuses of injected mothers were 42.75 ± 2.25 ng/ml, compared to 27.3 ± 1.8 in the control fetuses. In suckling rats 24 hours after a single injection of 50μg/100 g body weight essentially the same effects in both tissues were observed. At that time blood levels of insulin and glucagon were found to be elevated. These results are very similar to those described previously for corticosteroids (Endocrinol. 103: 1417-1424, 1978). However, T3 does not lead to a rise in fatty acid synthetase activity in brown fat, while prednisolone does. Thus the mechanisms of action of the two hormones are probably not the same. Our data indicate that T3 may be involved in the neonatal rise in hepatic PEPcK activity. (Supported by B.C. Heart Foundation).

Effect of insulin and prednisolone on cyclic nucleotides and phosphoenolpyruvate carboxykinase activity in brown fat and liver of developing rats.

The concentrations of cyclic AMP and cyclic GMP in brown fat and liver of both suckling and adult rats at fixed times after injection of insulin (2.5 U/100 g body weight) or prednisolone (2.5 mg/100 g body weight) were compared with the activity of phosphoenolpyruvate carboxykinase assayed 24 h after the injections. A stimulus that produced an increase in cyclic AMP content also produced an increase in the enzyme activity. If the content of cyclic GMP was also increased there was no rise in phosphoenolpyruvate carboxykinase activity. A rise in the content of cyclic GMP alone was associated with a reduction in the activity of the enzyme. These preliminary results indicate that cyclic AMP could be involved in the induction of phosphoenolpyruvate carboxykinase and that cyclic GMP may somehow be related to its repression. The known differences in the response of phosphoenolpyruvate carboxykinase activity to insulin and prednisolone in different tissues and at different stages of ontogenic development may thus be linked to differences in the responsiveness of enzymes concerned with the metabolism of cyclic nucleotides.