Control of ventilation in adult rats hypoxic in the neonatal period.

Three groups of 50-day-old (i.e., postpuberty) rats have been studied: controls, rats exposed to 6 days of hypoxia [inspired fraction of O2 (FIo2) = 10% O2] when newborn (Nb-Hypox), and rats exposed to the same level and duration of hypoxia after weaning (Ad-Hypox). Ventilation during normoxic breathing was higher in Nb-Hypox than in controls or Ad-Hypox. The ventilatory response to acute hypoxia (10 min of 10% O2) was about one-half in Nb-Hypox than in the other two groups. Additional measurements performed on Nb-Hypox and controls showed minimal or no differences between the two groups in the ventilatory responses to hyperoxia and hypercapnia, heart rate and blood pressure at various FIO2, and blood biochemistry. Analysis of the Hering-Breuer reflexes, during barbiturate anesthesia, suggested a decreased central inhibition on inspiratory activity in Nb-Hypox, which with a lower sensitivity to inputs from the peripheral chemoreceptors may contribute to the normoxic hyperventilation and the blunted response to acute hypoxia. The ventilatory patterns of Nb-Hypox rats bear numerous similarities with those of high-altitude natives and could suggest that the highlander's ventilatory responses are not genetic characteristics but relate to chronic hypoxia early in life.