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

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