Naloxone accelerates the rate of ventilatory acclimatization to hypoxia in awake rats

During ventilatory acclimatization to hypoxia in rats, PaCO2 progressively falls from about 40 torr in normoxia (PIO2 approximately equal to 150 torr) to a new steady-state at about 23 torr in chronic hypoxia (24 or more hours at PIO2 approximately equal to 90 torr). In acute (20 or 60 minutes) hypoxia naloxone treatment caused a hyperventilation greater than that caused by acute hypoxia alone. Following 20 minutes hypoxia, naloxone treated rats had a PaCO2 = 28.6 +/- 0.7 torr (mean +/- 95% confidence limits) which was significantly lower (P less than .001) than the saline treated PaCO2 = 31.0 +/- 0.6 torr. In contrast, in normoxia and at 24 hour hypoxia and at 20 minute return to normoxia following 24 hours hypoxia, naloxone treatment had no effect on PaCO2. We conclude that in the rat about one third of the ventilatory acclimatization to hypoxia is due to a progressively decreasing endogenous opioid-like inhibition of ventilation.