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

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Abstract
When cold-acclimated rats were returned to a thermoneutral environment (28.degree. C) after 5 wk at 4.degree. C, size of enlarged interscapular brown adipose tissue decreased during the 1st wk. Altered composition of mitochondria reverted to normal during deacclimation more rapidly than size of tissue: a marked decrease in binding of purine nucleotides occurred by 3 days and a decrease in proportion of polypeptides of MW 25,000-35,000 by 1 day. Rapid decrease in polypeptides, compared with slower decrease in binding of purine nucleotides, known to be to a 32,000 polypeptide, suggested an initial conversion of masked binding sites to another form. Mitochondria isolated from cold-acclimated rats were larger than those of warm-acclimated rats, a difference which appeared slowly during acclimation to cold, requiring 7-14 days for full development; during deacclimation the increase in size disappearred by 7 days. Ultrastructure of mitochondria isolated from cold-acclimated rats showed 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 developed 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, was no longer apparent at 1-3 days and reappearred after 7-14 days. During deacclimation reversal to tubular form required .apprx. 7 days. During deacclimation, changes in brown adipose tissue mitochondrial composition, structure, size and properties and in tissue size occurred independently.

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