Effect of 12, 24 and 72 hours fasting in thermogenic parameters of rat brown adipose tissue mitochondrial subpopulations.

The aim of the present work was to study the effects of various durations of fasting (12, 24 and 72 hours) on brown adipose tissue (BAT) thermogenic parameters--cytrochrome-c-oxidase (COX) activity, GDP-binding activity and uncoupling protein (UCP1) content--and also on morphological features of different mitochondrial subpopulations, obtained by differential centrifugation--M1 (1000 g), M3 (3000 g) and M15 (15,000 g) fractions. The mitochondrial subpopulations showed morphological differences and a different distribution of UCP1 levels and of GDP-binding in all experimental groups. Starvation induced a decrease in the average size for all mitochondrial subtypes. The main changes induced by fasting in thermogenic parameters were observed in the M15 subtype. After the first 24h of starvation, there was a significant decrease of UCP1 levels only in the lightest mitochondrial subpopulation. However, the 72h fasted situation reflected a tendency to increase UCP1 content and UCP1/COX ratio together with a significant decrease of GDP-binding/UCP1 ratio, thus indicating more masked GDP-binding sites. Important fasting-induced changes in both morphological and biochemical parameters in BAT mitochondrial subtypes reflect their role in the physiological response of BAT to starvation.