Abstract
Mitochondrial matrix inclusions in brown fat from neonatal rats appear to consist of lipid. They are absent after lipid extraction of isolated mitochondria. The inclusions are preferentially bleached when reduced osmium is oxidized with diluted H2O2 or periodic acid. In glutaraldehyde-fixed tissue the inclusions appear in negative image. They are invisible after "preferential staining for ribonucleic acid." Inclusions are unaffected when protein is extracted by digestion with Pronase. No cytochrome oxidase activity appears to be associated with the inclusions. The activity of the inner membrane seems to be rather low in mitochondria from neonatal rats. In brown fat from 3-week-old rats, which has the largest mitochondria with maximally crowded cristae, the activity appears greatly increased, perhaps beyond the increase in inner membrane. The postnatal changes in mitochondrial structure, among which the disappearance of inclusions is the most striking, are probably mediated by cyclic adenosine monophosphate. The nucleotide has no direct effect on isolated mitochondria, and its mechanism of action remains to be elucidated.