Discordance between thermogenic activity and expression of uncoupling protein in brown adipose tissue of old rats

To elucidate the reason why thermogenic ability is impaired with age, the relationship between thermogenic activity and gene expression of several proteins including uncoupling protein (UCP) in brown adipose tissue (BAT) was investigated in 24-month-old rats. Compared to young (3-month-old) rats, a significant decrease in BAT/body mass ratio and considerable changes of chemical components in BAT were observed in the old rats. This indicates a marked decrease in the thermogenic capacity in BAT of the old rats. Likewise, the thermogenic activity (which was assessed by guanosine 5'-diphosphate binding to BAT mitochondria) was greatly reduced in the mitochondria recovered from BAT of the old rats. However, the UCP content in the BAT mitochondria was similar in both groups. Conversely, the expression of UCP mRNA was greater in the old rats than in the young rats. These results suggest that the regulation on and after UCP translation may be concerned with impaired BAT thermogenic activity due to aging. Moreover, the mRNA levels of several other proteins including insulin-sensitive glucose transporter concerned in BAT thermogenesis changed with age, suggesting that the mRNA levels may be affected in order to compensate for the decline in the thermogenic activity in the old rats.