Four groups of male New Zealand white rabbits, Oryctolagus cuniculus, were used. Besides the control (Group 1, reared at Ta=25°C) and cold-acclimated (Group 2, reared at Ta=0°°) groups, two others were intermittently exposed, one to 0°C for 3 hr daily (Group 3), and the other to a hypobaric stress of 308 mmHg for 3 hr daily (Group 4) for 3 months. Hematocrit and RBC in Group 4 were 54.0%±0.9 SE and 758×104/mm³±40SE, respectively, significantly higher than those observed in the control.
Rectai (T_re) and mean skin (T_s) temperature, metabolic rate (M) and heart rate (HR) were measured after thermal steady state was obtained at ambient temperature (T_a) of 25°C and5°C. When T_a was lowered to 5°C, T_re of Group 1 fell considerably, while that in Groups 2 and 4 remained unchanged. T_s fell in all animais in cold environment, but the rate of fall was more conspicuous in the control. When exposedto T_a=5°C, M was found to increase in all animals, particularly in Groups 2 and 4 (by 189-216%). However, the extent of shivering seemed to be less in Group 2.
In the curarizedrabbits, infusion of norepinephrine (NE, 3μg/kg·min for 30 min, at T_a=25°C) caused a marked bradycardia. The decrease in HR was more pronounced in the controls and Group 4. NE infusion caused an increase in M in Group 2, but not in the controls. The increase in M was very slight in Group 4. NE did not cause an increase in body temperatures in all groups, but only a small increase in T_s in Group 2. On the contrary, T_re increased after NE infusion in the noncurarized rabbits. These results suggest that the rabbits acclimated to hypoxia also increase their tolerance to cold. However, it seemed possible that the mechanisms involved may be different between the cold-and hypoxia-acclimated groups.
Calorigenic responses to cold and to infhsed NE in Group 3 were simiLar to those in Group 2. The inter mittent exposures to cold also resulted in acclimation to cold in rabbits.