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1 Departamento de Fisiologia, Faculdade de Medicina de Ribeirão Preto, and 3 Departamento Morfologia, Estomatologia, and Fisiologia, Faculdade de Odontologia de Ribeirão Preto, Universidade de São Paulo, 14040-904 Ribeirão Preto, São Paulo, Brazil; and 2 Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
We examined the magnitude of the hypoxic metabolic response in golden-mantled ground squirrels to determine whether the shift in thermoregulatory set point (Tset) and subsequent fall in body temperature (Tb) and metabolic rate observed in small mammals were greater in a species that routinely experiences hypoxic burrows and hibernates. We measured the effects of changing ambient temperature (Ta; 6-29°C) on metabolism (O2 consumption and CO2 production), Tb, ventilation, and heart rate in normoxia and hypoxia (7% O2). The magnitude of the hypoxia-induced falls in Tb and metabolism of the squirrels was larger than that of other rodents. Metabolic rate was not simply suppressed but was regulated to assist the initial fall in Tb and then acted to slow this fall and stabilize Tb at a new, lower level. When Ta was reduced during 7% O2, animals were able to maintain or elevate their metabolic rates, suggesting that O2 was not limiting. The slope of the relationship between temperature-corrected O2 consumption and Ta extrapolated to a Tset in hypoxia equals the actual Tb. The data suggest that Tset was proportionately related to Ta in hypoxia and that there was a shift from increasing ventilation to increasing O2 extraction as the primary strategy employed to meet increasing metabolic demands under hypoxia. The animals were neither hypothermic nor hypometabolic, as Tb and metabolic rate appeared to be tightly regulated at new but lower levels as a result of a coordinated hypoxic metabolic response.
body temperature set point; hypometabolism; ventilation; hypothermia
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