Thermoregulatory responses of the inbred heat-tolerant FOK rat to cold

The responses of inbred heat-tolerant FOK rats to cold were compared with t hose of Wistar King A/H (WKAH) and Std:Wistar (WSTR) strains. The fall of c olonic temperature during cold exposure was unexpectedly smaller in FOK tha n in other groups, but the onset of shivering was delayed in FOK. Norepinep hrine (NE)-induced in vivo oxygen consumption and the mitochondrial uncoupl ing protein 1 level of brown adipose tissue (BAT) were not different among the groups, but the cold-induced increases in in vivo oxygen consumption as well as plasma glycerol and free fatty acids were higher in FOR than in ot her groups. In vitro NE-induced oxygen consumption of BAT was less in FOK t han WSTR, but not WKAH. The magnitude of the NE-induced increase in blood f low through BAT as higher in FOK than in other groups. These results sugges t that FOK paradoxically have a high capacity for nonshivering thermogenesi s in spite of their high capacity for heat tolerance, probably due to an in creased Lipid utilization and improved circulation of BAT.