SHIVERING AND NONSHIVERING THERMOGENIC RESPONSES OF RATS SUBJECTED TODIFFERENT PATTERNS OF HEAT ACCLIMATION

Male Wistar rats were divided into five groups: a control group kept a t an ambient temperature of 24 degrees C for 14 days, and four heat-ac climated groups (two groups subjected to a constant ambient temperatur e of 33 degrees C for 4 days or 14 days (HC-14) immediately preceding the measurement; and another two groups subjected to an ambient temper ature of 33 degrees C for about 5 h once a day for 4 days, or 14 days (HI-14) just prior to the measurement). After the completion of the sc hedule, the rats were placed in a temperature-controlled chamber. Hypo thalamic (T-hy) and interscapular brown adipose tissue (T-BAT) tempera tures, oxygen consumption (VO2), and shivering activity were measured during a gradual fall in temperature of a water jacket surrounding the chamber (T-w) from 30 to 10 degrees C at a constant rate of 0.18 degr ees C/min. During the fall in T-w, VO2 and T-BAT increased significant ly and shivering was induced without associate changes in T-hy in all groups. T-w at the onset of a rise in metabolic heat production (onset of cold-induced thermogenesis) coincided with that at the onset of a rise in T-BAT (onset of BAT thermogenesis), but was significantly high er than that at the onset of shivering. In HC-14 and HI-14 rats, T(w)s at the onset of cold-induced thermogenesis and BAT thermogenesis were significantly higher than those in control rats, whereas T(w)s at the onset of shivering were not different from those in control rats. The onset of cold-induced thermogenesis did not change after the 4-day he at exposure. These results suggest that heat exposure for 14 days, reg ardless of the pattern, shifts the lower critical temperature to a hig h level, and the changes are attributed to an upward shift in the ambi ent temperature at which nonshivering thermogenesis occurs.