Ventilatory and metabolic responses to ambient hypoxia or hypercapnia in rats exposed to CO hypoxia.

We have investigated at ambient temperatures (Tam) of 25 and 5 degrees C the effects of ambient hypoxia (Hxam; fractional inspired O2 = 0.14) and hypercapnia (fractional inspired CO2 = 0.04) on ventilation (V), O2 uptake (VO2), and colonic temperature (Tc) in 12 conscious rats before and after carotid body denervation (CBD). The rats were concomitantly exposed to CO hypoxia (HxCO; fractional inspired CO = 0.03-0.05%), which decreases arterial O2 saturation by approximately 25-40%. The results demonstrate the following. 1) At Tam of 5 degrees C, in both intact and CBD rats, V/VO2 is larger when Hxam or CO2 is associated with HxCO than with normoxia. At Tam of 25 degrees C, this is also the case except for CO2 in CBD rats. 2) At Tam of 5 degrees C, the changes in VO2 and Tc seem to result from additive effects of the separate changes induced by Hxam, CO2, and HxCO. It is concluded that, in conscious rats, central hypoxia does not depress respiratory activity. On the contrary, particularly when VO2 is augmented during a cold stress, both V/VO2 during HxCO and the ventilatory responses to Hxam and CO2 are increased. The mechanisms involved in this relative hyperventilation are likely to involve diencephalic integrative structures.