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.