Effect of Acute and Chronic Cooling on Cardiorespiratory Depression in Rodents

Abstract

The cardiovascular and ventilatory responses of the Wistar rat (a non-hibernator) and the Syrian hamster (a hibernator) to acute and chronic cold exposure were investigated. Acute lowering of core temperature (T_c = 22°C, hypothermia) compared with normothermia (T_c = 37°C) and hyperthermia (T_c = 40°C) was used to examine the underlying differences in extent of cold adaptation. In euthermic rats, acutely induced hypothermia resulted in a pronounced reduction in heart rate (f_H reduced by 55%; P < 0.01), a modest but significant elevation of mean arterial blood pressure (mABP increased by 16%; P < 0.05), and a marked reduction in respiratory frequency (f_R reduced by 64%; P < 0.01). All parameters returned to baseline values on returning T_c to 37°C, with a modest overshoot on acute hyperthermia. These data are consistent with the depressive effect of low temperature on biological rate functions and increased vagal tone in the cold, while matching f_R to a lowered metabolic rate (MO₂). Cold acclimation had little effect on this pattern of response, suggesting that any adaptive increase in thermogenesis is limited. Euthermic hamsters also showed a significant reduction in f_H on acute cooling (74%; P < 0.01). In contrast to rats, hamsters developed a significant decrease in mABP (52%; P < 0.01) and maintained a relatively high f_R (4%; n.s.). These data suggest a resetting of the baroreflex and relative hyperventilation, consistent with an elevated MO₂ associated with enhanced non-shivering thermogenesis. Cold acclimation had little effect on thermal sensitivity, although the response curves were displaced to produce a relative hypertension and tachycardia at a given T_c. These data suggest a reduced cardiorespiratory coupling in the hibernator compared with the non-hibernator.