René Portet

Relation between membrane phospholipid composition, fluidity and function in mitochondria of rat brown adipose tissue. Effect of thermal adaptation and essential fatty acid deficiency.

Male weanling rats were maintained either at 28 degrees C (thermoneutrality) or at 5 degrees C (cold adaptation). During 9 weeks they were fed either a 2% hydrogenated coconut oil diet deficient in essential fatty acids or a diet containing 2% sunflower oil. The respective incidences of cold adaptation and of EFA deficiency on lipid composition of mitochondrial membranes from brown adipose tissue (BAT) were investigated. Using 1,6 diphenylhexatriene (DPH) as a probe, the parameters of membrane fluidity were estimated by steady-state fluorescence polarization measurements (rs) and by time-resolved fluorescence anisotropy decay (order parameter S). Cold acclimation induced a decrease of phosphatidylcholine to phosphatidylethanolamine (PC/PE ratio), an increase of the total fatty acid unsaturation index (T.U.). EFA deficiency had the same effect as cold on the PC/PE ratio, but decreased T.U. Cold adaptation induced a larger decrease of S than of rs, whereas EFA deficiency only increased rs and did not modify S. In liposomes prepared from mitochondrial lipids, rs values were smaller than in whole mitochondria. Both in cold-adapted and in EFA-deficient rats the variations of rs were correlated with lipid unsaturation. Comparison between BAT thermogenic activity, assessed by GDP binding and proportions of PE and PC showed a high correlation suggesting a change in the membrane occurring with the increase of mitochondrial activity that could be related to phospholipid composition rather than to membrane fluidity.

Some regulatory aspects of thermogenesis in cold-exposed piglets

1. Thermoregulatory mechanisms were studied in weaned piglets fed ad libitum for 3 weeks at 12 or 23 degrees C. 2. Cold-adapted pigs maintained a growth rate and a carcass composition similar to the control ones, while increasing food intake by 20% (P less than 0.05). 3. Lipoprotein lipase activity was increased (P less than 0.05) by cold exposure in white adipose tissue (WAT) and heart. A large enhancement of lipogenesis was observed in WAT and to a lesser extent in the liver while WAT composition did not change significantly in the cold. 4. Cytochrome oxydase activity was increased in liver (30%), perirenal fat (40%) and interscapular muscle (75%) of cold-exposed pigs. 5. Plasma levels of thyroid hormones (TSH, T3, T4) were increased in the cold. 6. Mechanisms and locations of heat production are discussed.

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.

The effects of essential fatty acid deficiency on brown adipose tissue activity in rats maintained at thermal neutrality

1. The consequences of essential fatty acid (EFA) deficiency on the resting metabolism, food efficiency and brown adipose tissue (BAT) thermogenic activity were examined in rats maintained at thermal neutrality (28 C). 2. Weanling male Long-Evans rats were fed a hypolipidic semi-purified diet (control diet: 2% sunflower oil; EFA-deficient diet: 2% hydrogenated coconut oil) for 9 weeks. 3. They were kept at 28 C for the last 5 weeks. Compared to controls, in EFA-deficient rats the growth shortfall reached 21% at killing. 4. As food intake was the same in EFA-deficient and control rats, food efficiency was thus decreased by 40%. 5. Resting metabolism expressed per surface unit was 15% increased. 6. Non-renal water loss was increased by 88%. 7. BAT weight was 28% decreased but total and mitochondrial proteins were not modified. 8. Heat production capacity, tested by GDP binding per BAT was 69% increased in BAT of deficient rats. 9. The stimulation of BAT was established by two other tests: GDP inhibition of mitochondrial O2 consumption and swelling of mitochondria. 10. It is suggested that the observed enhancement of resting metabolism in EFA-deficient rats is, in part, due to an activation of heat production in BAT.

Effect of ambient temperature on lipid metabolism in brown fat during the perinatal period

Abstract 1. 1. Norepinephrine-induced lipolysis, cyclic AMP production and glycerokinase activity were measured, in vitro , in the brown fat of rats born and reared at either 28° or 16°C during the first 3 weeks of life. 2. 2. During the first two postnatal days, lipolytic activity in the tissue was lower than in the foetuses at both ambient temperatures At day 10, increased values of the parameters under consideration were similarly observed in both groups. 3. 3. However, the hormonal regulation of lipolysis seemed to be quite different from that found in adult cold-acclimated rats. 4. 4. At day 21, the cold-induced characteristics of lipid metabolism in brown fat were observed in the 16°C exposed rats, whereas a loss of tissue stimulation occurred in the 28°C exposed ones.

Post-Natal Development of Brown Adipose Tissue in the Rat Bred at 23 °C or 28 °C

AbstractIn view to study the effects of thermal environment on the development and the thermogenic activity of interscapular brown adipose tissue (BAT), young rats born at 23 °C or 28 °C were sacrificed at 1, 3, 7, 11, 14 or 21 days after birth. The rate of increase in animal weight was quite the same at both temperatures up to the 14th day. The development of BAT and its contents in lipids, in water and in noradrenaline indicate that the energetic activity of the tissue is greatly stimulated in rats kept at 23 °C up to the 11th day. It is concluded that in rats bred in the habitual thermal conditions (23 °C), the occurrence of non shivering thermogenesis (NST) is important during the period of ten days after birth; in the following period NST could be progressively replaced by other thermoregulatory processes.

Invivo β-adrenergic induction of the unmasking of the uncoupling protein in rat brown fat

Abstract 1. In 28°C adapted rats (WA) both cold stress and norepinephrine (NE) led to a 4-fold increase of uncoupling protein dependent proton conductance which was abolished by propranolol (PRO). 2. In 4-day warm re-exposed rats (after 10 days at 5°C) (WR) the same uncoupling by cold stress was observed but the NE effect was lower. Uncoupling by cold stress was not abolished by PRO. 3. In WR rats, uncoupling was not due to the involvement of an α-adrenergic pathway. 4. Both β-agonist isoproterenol and β-agonists BRL 35135A and ICI D7114 led to high levels of unmasking. 5. Interscapular brown adipose tissue surgical denervation, which abolished cold stress unmasking both in WA and WR rats, indicates a mediation by direct sympathetic innervation. 6. Depending on the thermal history of the rat, the possibility that unmasking by cold stress could be mediated by different types of β-receptors is discussed.

Time-course variations of effective proton conductance and GDP binding in brown adipose tissue mitochondria of rats during prolonged cold exposure

1. Time-course variations of the thermogenic pathway in rat brown adipose tissue (BAT) mitochondria were examined. 2. Several parameters of mitochondrial energization, protonmotive force and its components pH gradient and membrane potential were investigated. The specific binding of GDP was compared with the effective proton conductance (CmH+) of the membrane. 3. Ten-days cold exposure led to maximal GDP binding and GDP-dependent CmH+. 4. The subsequent relative decrease in GDP binding observed during prolonged cold exposure (40 days) was functional and led to a lower GDP-dependent CmH+. CmH+ showed greater variation than GDP binding. 5. The CmH+ decrease was not due to a masking of active sites of the uncoupling protein. 6. Basal GDP-independent CmH+ was not modified. 7. Results are discussed with reference to the significance of biochemical measures and to the physiological regulation of BAT thermogenesis.

In vitro study of adrenergic stimulation of 32p incorporation into phospholipids of brown adipose tissue of control and cold-acclimated rats

1. 32P-labelled inorganic phosphate incorporation into total and mitochondrial phospholipids was studied, in vitro, on brown adipose tissue (BAT) of control and cold-acclimated rats. 2. It was found that norepinephrine acts as in vivo, on BAT phospholipid metabolism via alpha 1 adrenergic receptors specifically increasing phosphatidic acid and phosphatidylinositol turnover with the same magnitude in both groups. 3. Cold-induced alpha 1 adrenergic desensitization is not as important as cold-induced beta adrenergic desensitization. 4. No specific effect of norepinephrine was seen in mitochondrial phospholipid turnover.