Heat Balance during Physical Restraint in Rats

Abstract

By using a gradient layer calorimeter built in our laboratory, heat balance during physical restraint was measured in rats at a calorimeter temperature of 25°C. After restraint, the rate of heat production (M) increased sharply and significantly (from 53.86 W ·m^-2±3.12 SE to 69.12 W ·m^-2+2.86 SE). The high M was maintained for the whole period of 2.5-hr restraint. The rate of heat loss (H_L) increased progressively and reached a high value (from 54.02 W ·m^-2 ±2.78 SE to 67.44 W ·m^-2±2.48 SE) 40 min after restraint. The high value in EL was maintained (67.44-68.12 W ·m-2) and the rate of heat storage (S), which was positive during the first 40 min, tended toward zero for the rest of 2.5-hr restraint. Colonic temperature (T_col), which was increased from 37.40 °C ±0.07 SE to 38.03 °C ±0.16 SE during the first 40 min, increased insignificantly during the following period of restraint. These results indicated that a thermal equilibrium was reached during a prolonged restraint at a calorimeter temperature of 25 °C. After release from restraint, H_L and M decreased progressively. S, which was negative in the first 60 min of post-restraint period, became zero and T_col reached a steady value of 37.78°C±0.09 SE.
The restraint-hypermetabolism was either reduced or completely inhibited by the chemical sympathectomy (6-OHDA, 100 mg/kg i.p.) or bilateral adrenalectomy. An increased activity of the sympathetic nervous system and an increased secretion of catecholamines might be responsible for the restraint-hypermetabolism in rats.
This report has also described a direct calorimeter for measuring heat balance in unanesthetized rats. The results obtained in this study approved the simplicity of the actual operation and the usefulness of the calorimeter in studying thermoregulatory responses in small mammals.