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Effects of hypothermia on ventilation and ventilatory responses to hypercapnia and hypoxia in the golden-mantled ground squirrel and the wistar rat Osborne, Salma (Sally)


In this study I examined the effects of progressive hypothermia on minute ventilation, metabolic rate and the ventilatory responses to hypercapnia and hypoxia in the golden-mantled ground squirrel (Spermoohilus lateral is) and the laboratory rat (Rattus norveqicus). These experiments were designed to test the hypothesis that reductions in minute ventilation with progressive body cooling in this species are independant of seasonal changes associated with hibernation and are the result of and therefore parallel the changes in metabolic rate. Similar experiments were carried on the laboratory rat to test the scope of this hypothesis in a non-hibernating mammalian species. Minute ventilation was measured by pneumotachography and carbon dioxide production was measured as an index of metabolic rate. The "helox-cold" method was used to induce progressive hypothermia from 36 to 27°C body temperature under a constant functional plane of halothane anesthesia chosen to suppress shivering. Progressive hypothermia was studied in the ground squirrels during the non-hibernating season and in the laboratory rat throughout the year. During normothermia, breathing frequency and metabolic rate were approximatley 60% lower in the golden-mantled ground squirrel compared to the rat. In both species. however, hypothermia resulted in proportional decreases in minute ventilation, breathing frequency and metabolic rate. The inspiratory flow rate, an index of respiratory drive was also reduced with decreasing body temperature and showed a similar linear relationship with the ventilatory requirement of each species at any given body temperature. A gradual decrease in duty cycle was observed in both species which was significant only at lower levels of minute ventilation. Breathing remained rhythmic throughout hypothermia although apneic periods occured between breaths at body temperatures below 31°C. The slopes of the ventilatory responses to hypoxia and hypercapnla in the ground squirrel were decreased in proportion to the decreases in minute ventilation and metabolic rate. Ventilatory sensitivity in the rat, however, was not altered. These results demonstrate that ventilation and metabolic rate are tightly coupled during hypothermia. In addition both species decrease their minute ventilation to match reduced metabolic demands by decreasing breathing frequency alone. Tidal volume is not altered by decreases in body temperature, presumably to ensure adequate alveolar ventilation. The temperature coefficient (Q₁₀) determined in the present study for the effect of body temperature on minute ventilation and metabolic rate in the golden-mantled ground squirrel is similar to that obtained during hibernation in the same species (McArthur, 1986 and Webb, 1987). Consequently, the exponential equations defining the drop in minute ventilation and metabolic rate during progressive hypothermia in the present study accurately predict the values observed at 7°C body temperature during hibernation. It is therefore concluded that ventilation at reduced body temperatures is regulated independantly of the physiological processes that are unique to hibernation and is simply coupled to the metabolic demand of the ground squirrel.

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