The present study examined the physiological mechanisms of the responses of brain tissue
oxygen partial pressure (P(t)O(2)), brain temperature (T(brain)), global oxygen consumption (V(O2)), and respiratory frequency (f(R)) to
hypoxia in non-sedated and non-anesthetized arctic ground squirrels (Spermophilus parryii, AGS) and rats. We found that (1) in contrast to
oxygen partial pressure in blood (P(a)O(2)), the baseline value of P(t)O(2) in summer euthermic AGS is significantly higher than in rats; (2) both P(t)O(2) and P(a)O(2) are dramatically reduced by inspired 8% O(2) in AGS and rats, but AGS have a greater capacity in P(t)O(2) to cope with environmental
hypoxia; (3) metabolic rate before, during, and after hypoxic exposure is consistently lower in AGS than in rats; (4) the respiratory responding patterns to
hypoxia in the two species differ in that f(R) decreases in AGS but increases in rats. These results suggest that (1) AGS have special mechanisms to maintain higher P(t)O(2) and lower P(a)O(2,) and these levels in AGS represent a typical pattern of adaptation of heterothermic species to and a brain protection from
hypoxia; (2) AGS brain responds to
hypoxia through greater decreases in P(t)O(2) and decreased f(R) and ventilation. In contrast, rat brain responds to
hypoxia by less reduction in P(t)O(2) and increased f(R) and ventilation.