Unloading does not increase brown-adipose-tissue activity in rat pups

Our previous study has shown that chronic hindlimb suspension leads to an i ncrease in both the thermogenic capacity and the activity in brown adipose tissue (BAT) of adult rats (Yamashita, H., Ohira, Y., Wakatsuki, T., Yamamo to, M., Kizaki, T., Oh-ishi, S., Sate, Y. and Ohno, H. (1995). J. Appl. Phy siol. 78: 384-387). In order to examine if unloading also increases the BAT activity in rat pups, the hindlimbs in the suspended pups were unloaded by tail suspension beginning on postnatal day 4 and suspended until day 21. T he thermogenic activity (which was assessed by guanosine 5'-diphosphate bin ding to BAT mitochondria) was markedly lower in 21-day-old suspended pups t han in 21-day-old control pups, although there was no difference in uncoupl ing protein 1 (UCP1) content or UCP1 mRNA expression in the BAT mitochondri al fraction between both pups. Likewise, there was no disparity in either a drenal or thymus mass between the control and suspended pups throughout the experiment. These results suggest that, in contrast to adult rats, chronic hindlimb suspension leads to a decrease in the thermogenic activity in BAT of rat pups possibly for reason that pups are less susceptible to the stre ss of unloading.