Development of deep hypothermia in rats with limited motor activity

Changes of the main organism functions (breathing frequency, heart rate and shivering) were investigated under hypothermia in two groups of rats. Animals of the first group were fixed rigidly on the special platform with fixing of head and limbs, and those of the second one--the rats, were placed in a punched cylindrical chamber, inside which they could move freely forward and back. In 2.5-3.0 hours after anaesthesia the rats were placed in a refrigerator (-5 degrees C) until they stop breathing. Cessation of breathing of the first group rats occurred in 1.7 +/- 0.3 hours from the beginning of cooling at body temperature 17.3 +/- 0.6 degrees C and the brain temperature 15.7 +/- 0.5 degrees C. In the second group, a prolonged activation of the frequency of breathing, heart rate and intensity of electrical activity of muscles during 2.5-3.0 hours, was observed. Only in 4.5-5.0 hours, the breathing stopped at rectal temperature 12.3 +/- 1.1 degrees C and the brain temperature 12.9 +/- 0.9 degrees C. In these animals, the time of survival in the cold environment increased considerably and the temperature thresholds of the termination of breathing were lowered. Thus, the activation in the thermo-regulative muscle tone and in shivering muscles provides the most effective resistance against cooling of rats, reducing a surface of heat, dissipation and keeping the temperature of internal areas of body.