IDENTIFICATION OF BROWN FAT AND MECHANISMS FOR ENERGY-BALANCE IN THE MARSUPIAL, SMINTHOPSIS-CRASSICAUDATA

The presence of brown adipose tissue (BAT) in marsupials is controvers ial because attempts to identify mitochondrial uncoupling protein (UCP ) have been unsuccessful. Sminthopsis crassicaudata is a small nocturn al marsupial with an interscapular pad of adipose tissue. Electron mic roscopy revealed this tissue to have characteristics typical of BAT. G DP binding and UCP detection by immunoblot confirmed BAT. Expression o f UCP was increased by cold exposure. When animals were placed from 28 to 15 degrees C, body temperature (T-b) decreased by 1.7 degrees C wi thin 30 min and a further 1.0 degrees C by 90 min (P < 0.001) before s tabilizing at these lower levels. When animals were returned to 28 deg rees C, T-b increased within 30 min (P < 0.001) and returned to basal by 120 min. When animals were maintained at 15 degrees C with ad libit um food for 12 days, Tb (P < 0.05), tail width (P < 0.04), and O-2 con sumption (P < 0.01) all decreased. The respiratory quotient increased (P < 0.001), indicating a change from fat to carbohydrate utilization. Food intake was unchanged, and body weight increased on day 1 (P < 0. 01) before returning to baseline on day 3, remaining stable thereafter . These data suggest that although BAT is present in the marsupial S. crassicaudata, it may not be necessary for thermogenesis, at least in the short term. S. crassicaudata utilizes a plasticity in T-b and a ch ange in substrate utilization to maintain energy balance and body comp osition without the need for an increase in metabolic rate or food con sumption and without the need for torpor.