Really does temperature reduction and norepinephrine have similar effects on the energy metabolism in rat brown adipose tissue?

CONTEXT: Heat generation by brown adipose tissue (BAT) in response to temperature reduction seems to be entirely related to sympathetic nervous stimulation. OBJECTIVE: To analyse if temperature reduction and norepinephrine may differently affect the expression of proteins related to energy metabolism in BAT. MATERIALS AND METHODS: Isolated rats BAT was incubated with/without norepinephrine (10-6 mol/L, 24 h at 32 °C and 37 °C). RESULTS: In BAT, 32 °C increased the protein expression levels of carnitine palmitoyltransferase-I and -II, mitochondrial uncoupling protein-1 (UCP-1) and the expression and activity of lactate dehydrogenase. Mitochondrial F1-ATP synthase α-chain expression was decreased at 32 °C compared to 37 °C. Norepinephrine and at 32 °C exposure, UCP-1 expression was increased but cytochrome-c oxidase and F1-ATP synthase α-chain expression was reduced with respect to 37 °C. DISCUSSION: Sympathetic stimulation seems not to be the only factor associated with heat generation. CONCLUSIONS: Temperature reduction by itself exerts some different effects on the expression of proteins related to the energy metabolism than norepinephrine.

Metabolic differences between white and brown fat from fasting rabbits at physiological temperature.

It has been suggested that activated brown adipose tissue (BAT) shows increased glucose metabolic activity. However, less is known about metabolic activity of BAT under conditions of fasting and normal temperature. The aim of this study was to compare the possible differences in energetic metabolism between BAT and white adipose tissue (WAT) obtained from rabbits under the conditions of physiological temperature and 24 h after fasting conditions. The study was carried out on New Zealand rabbits (n=10) maintained for a period of 8 weeks at 23±2 °C. Food was removed 24 h before BAT and WAT were obtained. Protein expression levels of the glycolytic-related protein, glyceraldehyde-3-phosphate dehydrogenase, and pyruvate dehydrogenase were higher in WAT than that in BAT. The expression level of carnitine palmitoyltransferase 1 (CPT1) and CPT2, two fatty acid mitochondrial transporters, and the fatty acid ß-oxidation-related enzyme, acyl CoA dehydrogenase, was higher in BAT than in WAT. Cytosolic malate dehydrogenase expression and malate dehydrogenase activity were higher in WAT than in BAT. However, lactate dehydrogenase expression and lactate content were significantly higher in BAT than in WAT. In summary, this study for the first time, to our knowledge, has described how under fasting and normal temperature conditions rabbit BAT seems to use anaerobic metabolism to provide energetic fuel, as opposed to WAT, where the malate-aspartate shuttle and, therefore, the gluconeogenic pathway seem to be potentiated.