Effects of Acute and Chronic Cooling on Cardiorespiratory Depression in Rodents

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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. The acute lowering of core temperature (T(c) = 22 degrees C, hypothermia) compared with normothermia (T(c) = 37 degrees C) and hyperthermia (T(c) = 40 degrees C) was used to examine the underlying differences in the 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 degrees 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 (MO(2)). 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 MO(2) associated with enhanced nonshivering thermogenesis. Cold acclimation had little effect on thermal sensitivity, though 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.