Developmental changes in cardio-respiratory responses to hypoxia and hypercapnia in anesthetized low-birth-weight rats

The present study compared the developmental changes in the cardio-respiratory responses to hypoxia and hypercapnia between full-term low-birth-weight (LBW) and control rats during the postnatal period. The heart rate (HR), respiratory frequency (fR) and amplitude (aR) were measured during hypoxia (10% O(2) for 10 min) and hypercapnia (5% CO(2) for 10 min) in rats aged 7, 14 and 21 days, anesthetized with urethane. During hypoxia, HR was not significantly modified in the younger rats of both groups. In the older rats, aged 14 and 21 days, HR was markedly diminished, with a more pronounced decrease in LBW rats. The HR recovery was never observed in the older LBW rats. The fR and aR showed an age-related increase in both groups: a biphasic fR pattern observed on day 7 was replaced by a sustained increase on days 14 and 21. In contrast to controls, LBW rats never displayed a fR recovery during reoxygenation. In controls, aR shifted from a biphasic pattern in the younger rats to a sustained increase in the older ones. The LBW rats only displayed a decrease of aR in the younger, while in the older ones, a transient and slight increase preceded this decrease. During hypercapnia, the only significant difference detected between these two groups was that aR increased in LBW rats to a greater extent than in controls on days 14 and 21. Altogether, our results revealed a markedly attenuated cardio-respiratory response to hypoxia in LBW rats, but no such effect in response to hypercapnia.