Shivering and nonshivering thermogenic responses of rats subjected to different patterns of heat acclimation

Male Wistar rats were divided into five groups: a control group kept at an ambient temperature of 24 degrees C for 14 days, and four heat-acclimated groups (two groups subjected to a constant ambient temperature of 33 degrees C for 4 days or 14 days (HC-14) immediately preceding the measurement; and another two groups subjected to an ambient temperature 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 schedule, the rats were placed in a temperature-controlled chamber. Hypothalamic (Thy) and interscapular brown adipose tissue (TBAT) temperatures, oxygen consumption (VO2), and shivering activity were measured during a gradual fall in temperature of a water jacket surrounding the chamber (TW) from 30 to 10 degrees C at a constant rate of 0.18 degree C/min. During the fall in TW, VO2 and TBAT increased significantly and shivering was induced without associate changes in Thy in all groups. TW 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 TBAT (onset of BAT thermogenesis), but was significantly higher than that at the onset of shivering. In HC-14 and HI-14 rats, TWS at the onset of cold-induced thermogenesis and BAT thermogenesis were significantly higher than those in control rats, whereas TWS 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 heat exposure.(ABSTRACT TRUNCATED AT 250 WORDS)