Thermogenic effect of ACYL-CoA in brown fat

104

ABSTRACTS:

SURVIVAL

groups of awake and hibernating animals exposed to various ambient temperatures (T,), dark or light, and influenced by castration, by administration of triiodothyronine (IT,), testosterone, and desoxycorticosterone. It was observed that in awake hamsters the MF values were about three times higher than those in hibernating individuals and that the MF in awake animals is not affected by cold adaptation. The MF in hibernating hamsters depended on body temperature (Tb). Castration, and administration of rT, or desoxycorticosterone further lowered the MF in hibernating animals, the effects of castration or rT, being more pronounced than those of desoxycorticosterone. On the other hand, pretreatment of castrated animals with testosterone prevented the reduction of MF. The MF values found in castrated animals hibernating in the dark were lower than those in castrated animals hibernating in the light. We conclude that (a) hibernation can be realized at different levels of activity of myoneural junction, (b) neither cold adaptation nor lowered T, can fully account for observed tissue adaptation to hibernation, (c) MF is a good criterion of the degree of the tissue adaptation to hibernation, (d) light affects hibernation not only via gonads, and (e) experimentally induced change of the hormonal state of the hibernator can increase the degree of the tissue adaptation to hibernation without another decrease of T. and Tb. SESSION 7. REGULATION ADIPOSE

TISSUE

OF BROWN

METABOLISM

62. Thermogenic Effect of ACYL-CoA in Brown Fat. BARBARA CANNON, JAN NEDERGAARD, AND ULF SUNDIN (The Wenner-Gren Institute, Nontulkgatan 16, S-11345 Stockholm, Sweden). We have earlier postulated that the immediate substrates for respiration (= heat production) in brown fat mitochondria, acyl-CoA esters, may also be responsible for the necessary uncoupling which must occur upon norepinephrine stimulation of the brown fat cell. This suggestion is based upon experiments where palmitoyl-CoA is found to be competitive with GDP (one of the purine nucleotides capable of limiting respiration in these mitochondria), both with respect to direct binding studies and in swelling experiments (Cannon, Sundin, and Romert FEBS Letf. 74, 43-46 (1977)). We have now extended these studies to include respiratory measurements, and find that palmitoyl-CoA is clearly able to uncouple its own respiration in a manner competitive with added GDP, i.e., also these agents demonstrate the reciprocity of action which is to be expected if acyl-CoA’s are indeed the physiological antagonists to nucleotide binding. We are also studying the further metabolism of acyl-CoA esters in the brown fat mitochondria. Their rate of oxidation is influenced by the diet and by the environmental temperature of the animal, but the limiting step

IN COLD

for oxidation is not the substrate permeases, in contrast to what we have demonstrated to be the case for other substrates. 63. Cationic Regulation of Brown Adipose Tissue Metabolism. JAN NEDERGAARD, EEWA NANBERG,AND BARBARA CANNON (Th6 WennerGren Institute, Norrtullsgatan 16, S-113 45, Stockholm Sweden). It is clear from omission experiments that cations play an important role in the regulation of brown adipose tissue metabolism. We have now tried to establish the biochemical role of some cations. Omission of K+ from the incubation medium for hamster brown fat cells leads to a decrease in the thermogenic effect of norepinephrine. This seems to be due to a direct inhibition of lipolysis as a result of lowered intracellular K+. We have monitored K+ uptake into isolated brown fat cells using rubidium-86; chemically, rubidium closely resembles potassium. using a filtration technique we have measured an apparent Rb+ {K’) volume of 30 $/lo8 cells; from an estimated cytosolic volume a K+ potential of about -60 mV can be calculated. We are presently investigating the effect of hormones on the K+ potential; preliminary results with norepinephrine indicate an immediate decrease in the potential by about 20 mV. Omission of Na+ from the incubation medium also decreases thermogenesis, but this is due to another mechanism, as lipolysis proceeds unaltered, whereas stimulated oxygen consumption is inhibited. In the light of our earlier results (AlShaikhaly, Nedergaard, and Cannon Proc. Nat. Acad. Sci. USA 76, 2350-2353 (1979)) it is likely that Na+ and Ca2+metabolism are closely interrelated, and that Na+ exerts its influence indirectly (as an effect of membrane depolarization) by increasing the cytosolic Ca’+ level through release of Ca2+ from the mitochondria. We have demonstrated, for example, that mitochondrial glycerol-3-phosphate dehydrogenase can be regulated in this way, and increased activity of this enzyme may direct fatty acids toward combustion during thermogenesis. Thus evidence of direct molecular effects of cations can now be assembled and a physiological role for cations in the regulation of brown fat metabolism be established. 64. Cold-Induced Proliferation of Brown Fat Peroxisomes. S. ALEXSON, JAN NEDERGAARD,AND BARBARA CANNON (The Wenner-Gren Institute, Norrtullsgatan 16. S-113 45, Stockholm, Sweden). To be defined as a peroxisome, a cell organelle has to contain at least one hydrogen peroxide-producing oxidase together with catalase. Liver peroxisomes are known to be able to oxidize fatty acids at a fairly high rate, especially after feeding the rats clofibrata, a hypolipidemic drug. In rat brown fat, under thermo-