Effects of exercise and intermittent cold exposure on shivering and nonshivering thermogenesis in rats.

Abstract

The effects of both exercise training and intermittent cold acclimation on heat production (shivering and nonshivering thermogenesis (NST)) in rats were studied. Warm-acclimated rats (housed at 24 degrees C, WA) and intermittently cold-acclimated rats (exposed daily to -5 degrees C for 2 h, CA) were forced to run (25 m.min-1 for 1 h) every day (WA-T and CA-T). WA and CA left sedentary (WA-S and CA-S) served as controls. Norepinephrine (NE)-induced thermogenic capacity assessed from the increment of oxygen consumption (VO2) and colonic temperature (Tc) were measured 4 weeks after commencing acclimation and exercise training. The thermogenic capacity was greater in CA than in WA. However, in WA, WA-T responded to NE less than WA-S, whereas the response of CA-T and CA-S did not differ. Wet weight of interscapular brown adipose tissue (IBAT) and its protein (and dry matter--regarded to be highly representative of protein) content were larger in CA than in WA. Respective sedentary and exercised groups of rats had similar IBAT protein (and dry matter) content although tissue weight was lighter in WA-T than in WA-S. Lipid content of IBAT was also larger in CA than in WA. IBAT of WA-T had less lipid compared to that of WA-S while no difference was seen between CA-S and CA-T. Shivering activity during acute cold (4 degrees C) exposure was less in CA compared to WA and there was no difference between respective groups of exercised and sedentary rats. Propranolol, a blocker of NE-dependent NST, eliminated the difference in shivering among these four groups. When exposed to severe cold (-10 or -20 degrees C), the fall in Tc of rats fasted for 18 h was greater in WA than in CA. CA-T showed a greater decrease in Tc than CA-S during -20 degrees C exposure while it did not differ during -10 degrees C exposure. On the other hand, Tc of WA-T and WA-S did not differ significantly during either cold exposure period. These results suggest that exercise training in rats housed at 24 degrees C suppresses NE-dependent NST whereas another nonshivering thermogenic mechanism (NE-independent) may compensate this suppression. However, NE-dependent NST of WA-S and WA-T did not parallel protein (dry matter) content of IBAT; no difference existed in IBAT protein between these two groups.(ABSTRACT TRUNCATED AT 400 WORDS)