The fluidity and organization of mitochondrial membrane lipids of the brown adipose tissue of cold-adapted rats and hamsters as determined by nitroxide spin probes.

A detailed study of lipid fluidity and organization in the mitochondria of the brown adipose tissue from warm- and cold-adapted rats (nonhibernators) and hamsters (hibernators) is made in order to delineate any relationship between lipid properties and the ability to lower body temperature after cold-adaptation. Complete phospholipid analyses are presented; the data are very similar for cold- and warm-adapted rats, and for cold- and warm-adapted hamsters, but the rat lipids have a higher degree of unsaturation than those of the hamsters. Spin probe analogs of stearic acid and cholestane were used to investigate at the molecular level the fluidity and order of the mitochondrial lipids. Studies were made on intact mitochondria, and in liposomes and oriented multibilayers of extracted lipids. In no case was evidence found for a phase transition in the lipids, or for a relationship between the lipid fluidity in brown adipose tissue mitochondria and the ability to survive at lowered body temperatures. The spin probes generally had a decreased mobility in mitochondria relative to extracted lipids. The electron spin resonance spectra were analyzed to include order- and time-dependent phenomena by a recent stochastic method. The results show that more approximate analyses for order parameters and correlation times can yield incorrect conclusions. As segmental motion decreases in rate, order parameters will be overestimated. Decreasing rates of pseudoisotropic motion lead to incorrect estimates of rotational correlation times. Either of the above can result in the inference of an artifactual phase transition in the lipids.