Shivering and nonshivering thermogenesis in exercised cold-deacclimated rats

Norepinephrine (NE)-induced increase in oxygen consumption ( \(\dot V_{{\text{O}}_{\text{2}} } \) ) and colonic temperature (Tc) was greater in cold-acclimated rats housed at 4° C for 4 weeks (CA) than warm-acclimated controls housed at 24° C for 4 weeks (WA). On the other hand, shivering activity measured at 4° C was less in CA than in WA, while propranolol administration eliminated the difference between these two groups by enhancing shivering in CA. Wet weight and protein content of interscapular brown adipose tissue (IBAT) were greater in CA than in WA. Following cold acclimation, CA were deacclimated at 24° C for 5 weeks. During deacclimation, half of this latter group were forced to run (15 m·min⁻¹ for 1 h) every day (CD-T) while the remaining rats remained sedentary (CD-S). Shivering activity assessed at 4° C 4 weeks after commencing cold deacclimation was significantly less in CD-T than in CD-S and the difference disappeared following propranolol injection. \(\dot V_{{\text{O}}_{\text{2}} } \) and Tc responses to NE injection measured 1, 2 and 5 weeks after commencing cold deacclimation did not differ between CD-S and CD-T. Although IBAT weight was lighter in CD-T than in CD-S, its total protein content was not different between the latter two groups of rats. These results suggest that a greater degree of NE-independent nonshivering thermogenesis (NST) is retained in rats that are exercised during the process of deacclimation as compared with animals that are sedentary. This difference in NST would not seem to be directly related to BAT thermogenic capacity.