Glycerokinase activity in human brown adipose tissue

Brown adipose tissue (BAT) is known to be responsible for heat production in newborn and adult hibernating mammals. In rats and mice, BAT has been demonstrated to possess a much higher glycerokinase activity than white adipose tissue (WAT). It has been speculated that this high activity may cause the futile cycle of triglyceride breakdown and resynthesis to be activated, thus contributing to heat production. However, at present very little information is available regarding the location, function, and quantitative importance of BAT in adult human subjects. Our objective in this study was to locate BAT in human subjects and to characterize it biochemically, especially with respect to the enzyme glycerokinase. We have looked for histologically identifiable BAT in 32 human subjects and found it in 12 subjects. Most of the BAT samples were obtained from perirenal adipose depots in children undergoing surgery. Some of the samples were almost totally comprised of BAT cells, whereas others were a mixture of BAT cells and WAT cells. The glycerokinase activity per gram of tissue was higher in BAT than in WAT in all the subjects where the above comparison was made. The activity per mg protein or per microgram DNA was higher in most BAT samples. In one pure BAT specimen, the basal lipolytic rate and the lipoprotein lipase activity were measured and they were both higher in BAT than in the WAT obtained from the same patient. These results show that human brown adipose tissue possesses an enzymatic profile very similar to that of rodent brown adipose tissue.     

Involvement of the sympathetic nervous system in lipolytic activity in brown adipose tissue of cold acclimated rats

In cold acclimated rats, in vitro, NE led to a significant increase in release of FFA and glycerol in denervated IBAT. In vivo, study of arteriovenous differences showed that the denervated BAT loses its full capacity to utilize FFA and glycerol released by NE. After denervation an increase of blood flow in Sulzer's vein was observed. This effect appeared immediately after intervention whereas the effect on fat metabolism appeared later. In cold acclimated rats, the sympathetic nervous system appears to be an important regulator of fatty acid metabolism in BAT.     

Glycerokinase activity in brown and white adipose tissues of cold-adapted obese Zucker rats

Glycerokinase activity was measured in the brown and white adipose tissues compared with that in the liver obese Zucker rats adapted or not adapted to cold. In white adipose tissue total activity was low but higher in the fa/fa rats than in the Fa/ones; cold adaptation did not modify this activity. In brown adipose tissue specific activity was higher than in white; specific activity was twice as high in the fa/fa rats than in the Fa/-. Cold-adaptation induced an increase in the activity in the Fa rats and a decrease in the fa/fa rats. The results are discussed with regard to the cold-induced increase in the energetic efficiency of the tissue.     

Decrease of oxidative activities in brown adipose tissue mitochondria of cold acclimated rats on short term exposure to heat stress

Exposure of rats to the cold (4-5 degrees C) caused large (2-3-fold) increases in the mass of interscapular brown adipose tissue (BAT), its mitochondrial content and the basal metabolic rate of the animals. The rate of substrate oxidation by BAT mitochondria also increased about 3-fold. When cold-acclimated animals were exposed to heat (37 degrees C), the BMR decreased by half in 3 h, the earliest time interval tested. Mitochondrial substrate oxidation, as well as substrate-dependent H2O2 generation, showed a proportionate decrease in rates. In these mitochondria, activities of cytochrome c reductases, but not dehydrogenases with NADH, alpha-glycerophosphate and succinate as substrates, also showed a significant decrease. The concentration of cytochromes aa3 and b, but not cytochrome c, also decreased in BAT mitochondria from 12-h heat-exposed animals, while the change in concentration of cytochrome b alone was found as early as 3 h of heat exposure. These results identify the change in cytochromes as a mechanism of regulation of oxidative activities in BAT mitochondria under conditions of acute heat stress.     

Effects of lipolytic and antilipolytic drugs on metabolism of adenosine 3':5'-monophosphate in brown adipose tissue of cold acclimated rats.

The intracellular level of adenosine 3':5'-monophosphate (cyclic AMP) and its stimulation in vitro by norepinephrine were studied in brown and white adipocytes from rats adapted to constant or fluctuating cold. Cold acclimatization had no effect on the basal cyclic AMP intracellular content in both tissues, but the level in brown adipocytes was four-fold higher than in the white ones. Addition of norepinephrine in the incubation medium doubled the cyclic AMP content of white adipocytes from control or fluctuating-cold-adapted rats, and enhanced four-fold in constant-cold-adapted rats. In brown adipocytes norepinephrine increased cyclic AMP levels in the first two groups, but had no effects in constant-cold-adapted rats. In the two tissues of control and fluctuating-cold-adapted rats the norepinephrine action was increased by phentolamine and decreased by propranolol. The lack of response to norepinephrine of brown adipocytes from constant cold-adapted rats was not due to the predominance of the alpha component of hormone receptors. Antilipolytic drugs (nicotinic acid, insulin and prostaglandin E2) inhibited the action of norepinephrine on white adipocytes; only prostaglandin E2 had an effect on brown ones.     

Changes in thyroid hormone and insulin status of the brown adipose tissue of cold acclimated rats on short term exposure to heat

Acclimation of rats to cold caused 45% increase in the concentration of triidothyronine (T3) and 35% increase in the concentration of thyroxine (T4) in serum. Exposure of cold-acclimated rats to heat (12 hr, 37 degrees C) failed to decrease the concentrations of thyroid hormones in circulation. The concentration of T3 in brown adipose tissue (BAT) increased almost 10-fold on cold acclimation. Iodothyronine deiodinase activity also registered 3-fold increase. Exposure of cold-acclimated animals to heat caused decrease in the concentration of T3 in BAT without appreciably affecting T4 concentration. In liver tissue, the changes in hormone concentrations were quite small compared to those in BAT. On thyroidectomy or when fed with propyl thiouracil, rats could not survive exposure to the cold. The concentration of insulin in circulation showed small increase, while that in the tissues showed significant decrease on acclimation of rats to the cold. The concentration of the hormone in BAT registered significant increase on exposure of cold-acclimated animals to heat (12 hr, 37 degrees C). The increase in liver was marginal. The temperature-dependent response of T3 indicates an important role for this hormone in rapid physiological response in BAT.     

Triiodothyronine deiodinating activity in brown adipose tissue after short cold stimulation test in trained and untrained rats

Interscapular brown adipose tissue (IBAT) activity is controlled by sympathetic nervous system, and factors that influence thermogenesis appear to be centrally connected to the sympathetic outflow to IBAT. Cold exposure produces a rise in BAT temperature, which is associated with an increased thyroid activity, elevated serum levels of 3,5,3'-triiodothyronine (T3), and an increased rate of T3 production. This study evaluated the effect of swimming training on 5'-triiodothyronine deiodinase (5'-D) activity in IBAT under normal environmental conditions and after short (30 min) cold exposure (TST stimulation test). 5'-D activity is lower in trained rats at basal condition, and TST increases 5'-D in IBAT of both untrained and trained rats. However, this increase is lower in trained rats. Training reduces the deiodinating activity in normal environmental conditions as well as after short cold exposure. Probably, other compensatory mechanisms of heat production are active in trained rodents.     

Morpholigic and physiologic repercussions of partial removal of brown adipose tissue in rats acclimated to different temperatures]

Rats were chronically acclimated to 28 degrees C or 5 degrees C or submitted to daily variations of ambiant temperature. Ten or thirty days after removal of about 40% of the total brown adipose tissue (whole interscapular and 25% of abdominal tissues), the weight of the different pads of brown adipose tissue, thyroid and adrenals were determined. In all the groups, there was a large decrease of brown adipose tissue weight for the first ten days due to the shock following the operation. Then, the brown adipose tissue weight was restaured and, only in constant cold accliclimated rats, compensative hypertrophies of axillary and thoracic brown adipose tissue were found. Adrenals weight was significantly increased after the operation; in the two groups of cold acclimated rats, that increase was still significant one month later. However, the corticosterone production rate was not increased. These results are discussed in relation to the physiolocical role of brown adipose tissue in cold acclimated animals.     

Non-shivering thermogenesis and brown adipose tissue activity in essential fatty acid deficient rats.

The effects of essential fatty acid (EFA) deficiency on energetic metabolism and interscapular brown adipose tissue (BAT) activity were examined in the cold acclimated rat. Weanling male Long-Evans rats were fed on a low fat semipurified diet (control diet, 2% sunflower oil; EFA deficient diet, 2% hydrogenated coconut oil) for 9 weeks. They were exposed at 5 degrees C for the last 5 weeks. In EFA deficient rats, compared to controls, growth retardation reached 22% at sacrifice. Caloric intake being the same in the two groups, it follows that food efficiency was decreased by 40%. Resting metabolism in relation to body surface area was 25% increased. Calorigenic effect of norepinephrine (NE) in vivo (test of non-shivering thermogenesis) underwent a marked decrease of 34%. BAT weight was 21% decreased but total and mitochondrial protein content showed no variation. A 26% increase in purine nucleotide binding per BAT (taken as an index of thermogenic activity) was observed, suggesting that the enhancement in resting metabolism observed was mainly due to increased BAT thermogenesis. However, BAT mitochondria respiratory studies which are more direct functional tests showed a marked impairment of maximal O2 consumption of about 30% with palmitoyl-carnitine or acetyl-carnitine (both in presence of malate) or with alpha-glycerophosphate as substrate. It is likely that this impaired maximal BAT oxidative capacity may explain the impaired NE calorigenic effect in vivo. A possible increase in mitochondrial basal permeability is also discussed.     

Effect of cold on lipid peroxidation in the brown adipose tissue and liver of rats

1. Lipid peroxidation in the interscapular brown adipose tissue (iBAT) and liver was studied in rats acclimated to room (23±1 °C) and low temperature (5±1 °C, 42 days), as well as in animals exposed to 5±1 °C for 24 h; in addition, the tissue metallothionein (MT) and iron were determined.     

Variations of rat brown adipose tissue composition during cold acclimatization.

The modifications in weight and composition (lipids, proteins, water) of rat interscapular brown adipose tissue (BAT) were studied along the six first weeks of cold exposure and acclimatization. The variations of noreponephrine content was also investigated. During the first day of cold exposure, the major part of tissue lipids was released. During the following two days there was a fall in lipid and norepinehprine contents and uptake of water. Then, until the end of the first week a rapid repletion occurred. At that moment the relative pass of the tissue and the amounts of its principal components reached values which are not changed during the following weeks. We can conclude that the adaptative changes in the levels of BAT essential components are carried out at the end of the first week of cold exposure, long time before the non shivering thermogenesis is entirely effective.     

Assessment of brown adipose tissue activity in rats by 99mTc-sestamibi uptake

Brown adipose tissue (BAT) physiology and imaging have recently attracted considerable attention. BAT is characterized both by enhanced perfusion and increased mitochondrial activity. (99m)Tc-sestamibi is a lipophilic cationic tracer that concentrates in mitochondria. Data on the accumulation of (99m)Tc-sestamibi in BAT are currently lacking. This study investigates the in vivo (99m)Tc-sestamibi uptake in rat BAT. (99m)Tc-sestamibi was administered in male Wistar rats of various age and body size. (99m)Tc-sestamibi uptake was measured in vitro in BAT and white fat (WF) together with cytochrome c oxidase activity. Both (99m)Tc-sestamibi uptake and cytochrome c oxidase activity were higher in BAT than in WF (P<0.05). (99m)Tc-Sestamibi uptake in both BAT and WF was negatively related to body weight (r = -0.96 and -0.89, respectively) as was the BAT/WF uptake ratio (r = -0.85). These data show a higher (99m)Tc-sestamibi uptake in BAT compared to WF, in agreement with the high mitochondrial content and respiratory activity of the former. The strong negative correlation between (99m)Tc-sestamibi uptake in BAT and body weight (negative allometry), is in accordance to increased needs of thermogenesis in smaller animals. Implications of increased (99m)Tc-sestamibi uptake in BAT in radionuclide imaging are also discussed.