COLD-EXPOSURE INDUCES DIFFERENT UNCOUPLING-PROTEIN THERMOGENIN MASKING UNMASKING PROCESSES IN BROWN ADIPOSE-TISSUE DEPENDING ON MITOCHONDRIAL SUBTYPES/

The effect of cold exposure on thermogenic parameters such as mitochon drial protein content, GDP-binding and uncoupling protein (UCP) levels in different mitochondrial fractions from rat brown adipose tissue ha s been investigated. Rats were exposed from 12 h to 5 days at 4 degree s C, and three mitochondrial fractions were isolated by differential c entrifugation: the M(1) fraction (1000 g), the M(3) fraction (3000 g) and the M(15) fraction (15000 g). Cytochrome c oxidase activity as an index of mitochondrial mass showed an increase during cold exposure. D uring the first 24 h of cold exposure UCP was incorporated specificall y into the M(3) and M(15) mitochondrial fractions, and thereafter UCP appeared in the heaviest M(1) fraction. However, specific GDP binding was increased during the first 24 h in the same way in all subpopulati ons, and this increase continued up to 72 h of cold exposure. Results suggest that different molecular events are involved during acute and chronic adaptation to cold: during the first 24 h of cold acclimatizat ion, thermogenic activity is increased by an unmasking process of the UCP binding sites in the M(1) mitochondrial fraction as UCP levels wer e constant and GDP binding increased, but in the M(3) and M(15) fracti on the increase in thermogenic activity was completely due to an incre ase in GDP binding induced by a specific incorporation of UCP targeted to these mitochondria, Thus thermogenic parameters change in a differ ent way in the brown-fat mitochondrial subpopulations during cold accl imatization.