R. Portet

Hormonal Stimulation of Substrate Utilization in Brown Adipose Tissue of Cold Acclimated Rats

Summary A study of glucose, glycerol and free fatty acid (FFA) uptake and release in the interscapular brown adipose tissue (BAT) was done on rats acclimated to constant cold (CA), fluctuating cold (Cy) or a control ambient temperature. At thermal neutrality the arteriovenous differences and the blood flow through the tissue were estimated. The effects of norepinephrine (NE) or dibutyryl cyclic-AMP (DBcAMP) were measured. Before infusions an uptake of plasma FFA by BAT was found in CA and Cy rats, and there was a release in the controls. Plasma glucose was taken up by the BAT of the three groups of rats. DBcAMP infusions did not enhance blood flow and lipid metabolism in BAT but increased glucose uptake in all the groups. By the end of a NE infusion, blood flow was increased 2.4-fold in controls and four- to fivefold in CA and Cy rats. Glucose uptake and glycerol release were greatly increased in all the groups. In controls hydrolyzed fatty acids were released in the venous blood, but in CA and Cy rats most of them were retained in the tissue. Therefore, in the controls BAT seems to be essentially a reserve of lipids which can be mobilized by NE. In the two groups of cold acclimated animals, the fatty acids hydrolyzed by NE, which are retained in the tissue, may be the substrate of the NE dependent increase of tissue energetic metabolism.

Lipid biochemistry in the cold-acclimated rat

Abstract 1. 1. The increased lipid metabolism induced by cold acclimation of the rat results in a decrease in body fat stores and in an enhanced turnover of body lipids. 2. 2. As indicated by lipoprotein lipase activity, the capacity for blood lipid uptake is increased in muscle and heart, and above all, in brown fat. 3. 3. A large enhancement of lipogenesis from glucose is observed in the latter tissue and a lesser one in the liver and white fat. 4. 4. The in vivo fatty acid mobilization from white fat by lipolytic hormones is enhanced, but in vitro the effect of these hormones on lipolysis and cyclic AMP production is reduced in brown adipocytes. A precise regulation of fatty acid liberation in brown fat could be postulated, the mechanism of which is unknown. 5. 5. The ability of brown fat to produce heat by means of a decrease in mitochondrial respiratory control due to an increased membrane proton conductance is greatly enhanced by cold acclimation. However, the mechanisms of cyclic nucleotide—or fatty acid—dependent regulation of heat production in brown fat are hypothetical under physiological conditions.

Effects of cold acclimation on the feeding pattern and energetic metabolism of genetically obese Zucker rats.

1. The feeding pattern, growth rate and energetic metabolism were studied in obese Zucker rats of 5-12 weeks of age kept at ambient temperatures of 22 or 10 degrees C. 2. During this period, the increment in obesity of the 22 degrees C-exposed animal was found to be due to diurnal hyperphagia and not to a lowered resting metabolism. 3. In the 10 degrees C-exposed rat the development of non-shivering thermogenesis associated with a lack of enhancement of food intake leads to prevent the obesity.

Effects of diets and cold acclimation on lipoprotein lipase activity and cyclic nucleotide levels in some tissues of rats.

In view to study a possible regulation of lipoprotein lipase (LPL) activity by tissue cyclic nucleotide levels (cAMP and cGMP), parallel determinations of the three factors were performed on brown adipose tissue (BAT), epidymal fat (WAT) and heart of rats. Opposite variations of cAMP levels and LPL activity were only observed in WAT following high fat diet administration. The enhancement of LPL activity in BAT and in heart by cold acclimation or feeding an hyperlipidic diet was never associated with a decrease in cAMP content. The variations of cAMP and cGMP levels were not dependent. It is concluded that there is no effect of cyclic nucleotide levels in the enhancement of LPL activity in these tissues.

Effects of cold acclimation on the activity of lipoprotein lipase in adipose tissues of genetically obese Zucker rats

The activity of lipoprotein lipase (LPL) was studied in interscapilar brown adipose tissue (BAT), epididymal white adipose tissue (WAT) and in the heart of lean and obese adult Zucker rats maintained at 22 degrees C or adapted to cold (10 degrees C). In WAT the specific activity per gram of tissue was lower in obese than in lean rats but the total activity within the tissue was three-fold higher. Cold acclimation did not modify total activity in either lean or obese rats. In BAT, but not in the heart, both specific and total activities were lower in obese than in lean animals. They were enhanced in both tissues following cold acclimation. Six-hour fasting led to a decrease in specific activity in WAT of lean rats but had no effect in obese animals; an increase was observed in BAT and heart of both genotypes. Insulin administration has no effect on activities in WAT in either 22 or 10 degrees C adapted obese rats. Norepinephrine administration stimulates LPL activity in BAT and heart of all groups. It is concluded that the lack of development of obesity previously observed in obese rats following cold acclimation is not due to a decreased capacity of lipid uptake by WAT. It might in part be due to an increased lipid oxidation in BAT.

Brown adipose tissue and thermogenesis in hypophysectomized rats in relation to temperature acclimation

The aim of this work was to test the role of pituitary dependent hormones in cold-induced non-shivering thermogenesis. In the 28°C-acclimated rat, hypophysectomy inhibited body growth and led to an atrophy of thyroid and adrenals. In brown adipose tissue (BAT) some alterations were induced which are usually observed after cold acclimation of the animal: increase in relative weight, decreases in the relative amount of lipids, increases in the amounts of protein and DNA and modification of the proportions of several phospholipid fatty acids; moreover, basal lipolysis, in vitro, was enhanced to the same extent as that following cold acclimation of the normal rat. The in vivo stimulation by norepinephrine (NE) of O2 consumption (test for nonshivering thermogenesis) and of fatty acid release into blood were suppressed.Progressive cold acclimation of the hypophysectomized rats at 15°C led to a hypertrophy of BAT to the same extent as in the sham-operated animals. The in vivo sensitivity to NE was partially restored. The results suggest that hypophysectomy does not suppress the ability to acclimate to moderate cold by means of BAT dependent non-shivering thermogenesis. However, the low ability to produce heat seems to indicate that pituitary or pituitary-dependent hormones are necessary to optimize the cold stimulation of brown fat thermogenesis.

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 Sulzers 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.

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.

The lipid composition of plasma membrane and mitochondrial fractions from epididymal adipocytes of cold-acclimated rats

1. The effects of cold acclimation (5 degrees C) on the lipid composition of plasma membrane and mitochondrial fractions from epididymal adipocytes of rats were studied. 2. The adipocyte plasma membrane fraction of the cold-acclimated rats had lower lipid, phospholipid and cholesterol to protein weight ratios, a lower cholesterol to sphingomyelin molar ratio, and a higher linoleic acid content in the phospholipids than controls. 3. The mitochondrial fraction of the cold-acclimated rat adipocyte had lower ratios of cholesterol to protein (weight), to phospholipid and to cardiolipin (molar), and less sphingomyelin content than did controls. 4. These data, discussed in terms of alterations in physical and biochemical properties, indicate cold-induced changes at the membrane level in rat epididymal adipocytes.

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.