Thermogenesis associated with spontaneous activity: an important component of thermoregulatory needs in rats.

1. Quantification of the energy cost of spontaneous activity in freely moving lean and obese Zucker rats was performed at 28 degrees C and during acute cold exposure (from 28 to 5 degrees C). 2. An open-circuit metabolic chamber was supplemented with an ultrasensitive ergometric platform equipped with six undirectional accelerometers and with an opto-electronic device for location of the rat's centre of mass. 3. Resting and mean metabolic rates during control and cold-exposure periods were similar in both groups of rats. The 'extra thermogenesis' (ET), i.e. the difference between mean and resting metabolic rate, amounted to 11.7 +/- 1.1 and 8.6 +/- 0.7% of resting metabolic rate at 28 degrees C, and 39.7 +/- 2.9 and 34.1 +/- 2.9% of resting metabolic rate during cold exposure for lean and obese rats, respectively. 4. During the control period obese rats moved 3.71 +/- 0.61 m h-1 and lean rats 8.69 +/- 0.57 m h-1, but during cold exposure the distance moved by obese rats increased 3.58 +/- 0.33-fold whereas that moved by lean rats only increased 1.40 +/- 0.06-fold. The external work performed during spontaneous activity seldom reached 1.0% of the increase in metabolic rate. 5. In obese rats, weight was a good predictor of the distance covered, and cold exposure induced the same percentage increase in both distance and ET. Activity-associated thermogenesis of obese rats was the predominant thermogenic source that substituted for their atrophied brown fat thermogenesis whereas in lean rats with active brown fat these correlations were not found.