Parallel regression of cold-induced changes in ultrastructure, composition, and properties of brown adipose tissue mitochondria during recovery of rats from acclimation to cold.

When cold-acclimated rats are returned to a thermoneutral environment (28 degrees C) after 5 weeks at 4 degrees C, the size of the enlarged interscapular brown adipose tissue decreases during the 1st week. The altered composition of the mitochondria, however, reverts to normal during deacclimation more rapidly than the size of the tissue: a marked decrease in binding of purine nucleotides occurs by 3 days and a decrease in the proportion of polypeptides of molecular weight 25 000-35 000 occurs by 1 day. The very rapid decrease in the polypeptides, compared with the slower decrease in binding of purine nucleotides, known to be to a 32 000 polypeptide, suggests an initial conversion of masked binding sites to another form. Mitochondria isolated from cold-acclimated rats are larger than those of warm-acclimated rats, a difference which appears slowly during acclimation to cold, requiring 7-14 days for full development; during deacclimation the increase in size disappears by 7 days. The ultrastructure of mitochondria isolated from cold-acclimated rats shows inmerices arranged in numerous, narrow, interconnecting parallel sheets in contrast to the sparser broad tubes and bands in mitochondria isolated from warm-acclimated rats. This difference in ultrastructure develops in a biphasic manner during acclimation to cold: an initial appearance (1-12 h) of the parallel arrangement, associated with a transient increase in size, is no longer apparent at 1-3 days and reappears only after 7-14 days. During deacclimation the reversal to the tubular form requires approximately 7 days. During deacclimation the changes in brown adipose tissue mitochondrial composition, structure, size, and properties and in tissue size appear to occur independently.