Difference in the mode of acute cold-induced hypothermia between rat and hamster.

Abstract

The present work was undertaken to elucidate the underlying causative factors limiting survival in hypothermic rats (non-hibernator) and hamsters (hibernator). A variety of physiological and biochemical parameters were measured for 28 male Wistar rats (205-286 g) and 26 male golden hamsters (85-118 g) before and during induction of hypothermia to colonic temperature (Tco) of 18°C. With progressive decreases in Tco, parallel falls in Vo2, Vco2, and heart rate were observed. In rats, plasma pH (pH) in arterial blood was significantly lowered from 7.453 at Tco of 38°C to 7.327 at Tco of 18°C (p <0.001). From the observations of increased arterial P~o2 and bound CO2 (bicarbonate ion) concentration, together with the unchanged blood lactate, respiratory acidosis caused by hypoventilation was suggested to be responsible for the decrease in pH. In contrast to rats, in hamsters pH , was almost unchanged during the induction of hypothermia, while nearly complete depletion of blood glucose (-84.6%) and liver glycogen (-99.5%) were observed when Tco of 38°C was lowered to 18°C. It is concluded that the decrease in pH p is a limiting factor for rats to survive the deep hypothermia at Tco of 18°C and the exhaustion of carbohydrates as an energy supply for hamsters. Under severe cold, when heat loss surpasses heat production progressive hypothermia will lead to death of homeothermic animals unless properly rectified. Systematic studies of the physiological changes dung hypothermia are necessary to identify the mechanisms for the failure in homeostasis as well as the inability to recover from it. Although many studies have been made on the underlying potential factors limiting survival in the hypothermic state, the cause of death due to acute hypothermia is controversial and still not completely understood (PREWITT et al.,

Acid-base balance in the developing marsupial: from ectotherm to endotherm

“…The constant pHa (⌬pHa/⌬Tb ϳ0.002 U/°C) of the rats in this study differs from previous studies using unanesthetized rats during forced hypothermia, where ⌬pHa/⌬Tb has been shown to range from Ϫ0.0015 to Ϫ0.009 U/°C (2,3,63,66). Discrepancies may be related to methodologies used such as restraint with potential stress-related effects on blood gases and acid-base balance (63) or shaving, which can alter behavioral adaptations to cold stress (2, 3), such as gross movement, piloerection, and postural adjustments that reduce surface area. In addition, restraint can further confound findings by causing increased V E (15), increased V O 2 with no corresponding increase in V E (55), and increased plasma corticosterone levels, heart rate, and arterial blood pressure (33).…”

“…Thus neither ectothermic nor endothermic joeys conform to the values predicted by the ␣-stat hypothesis (50) and often observed in reptiles (e.g., 10,16,20). The constant pHa (⌬pHa/⌬Tb ϳ0.002 U/°C) of the rats in this study differs from previous studies using unanesthetized rats during forced hypothermia, where ⌬pHa/⌬Tb has been shown to range from Ϫ0.0015 to Ϫ0.009 U/°C (2,3,63,66). Discrepancies may be related to methodologies used such as restraint with potential stress-related effects on blood gases and acid-base balance (63) or shaving, which can alter behavioral adaptations to cold stress (2, 3), such as gross movement, piloerection, and postural adjustments that reduce surface area.…”

Hibernation induces pentobarbital insensitivity in medulla but not cortex

“…Despite evidence for global forebrain depression, torpid hibernators maintain a robust, neuronally regulated cardiorespiratory output (37,25,41), suggesting that respiratoryand cardiovascular-related neurons in the brain stem remain active. The mechanisms that selectively depress the forebrain during hibernation are not well understood and may contribute to hibernators' unique ability to survive ischemia, hypovolemia, and hypothermia (5,12,31,34,66).While euthanizing aroused hibernators for an unrelated study, we noted that pentobarbital sodium, an allosteric modulator of gamma aminobutyric acid type A (GABA A ) receptors, when used at doses sufficient for euthanasia, rapidly immobilized animals yet had no observable effect on respiratory rhythm. Subsequently, we began to investigate the role of GABA and GABAergic synaptic transmission in respiratory control during hibernation.…”

“…Despite evidence for global forebrain depression, torpid hibernators maintain a robust, neuronally regulated cardiorespiratory output (37,25,41), suggesting that respiratoryand cardiovascular-related neurons in the brain stem remain active. The mechanisms that selectively depress the forebrain during hibernation are not well understood and may contribute to hibernators' unique ability to survive ischemia, hypovolemia, and hypothermia (5,12,31,34,66).…”