Intermittent hypoxia augments carotid body and ventilatory response to hypoxia in neonatal rat pups

Carotid bodies are functionally immature at birth and exhibit poor sensitivity to hypoxia. Previous studies have shown that continuous hypoxia at birth impairs hypoxic sensing at the carotid body. Intermittent hypoxia (IH) is more frequently experienced in neonatal life. Previous studies on adult animals have shown that IH facilitates hypoxic sensing at the carotid bodies. On the basis of these studies, in the present study we tested the hypothesis that neonatal IH facilitates hypoxic sensing of the carotid body and augments ventilatory response to hypoxia. Experiments were performed on 2-day-old rat pups that were exposed to 16 h of IH soon after the birth. The IH paradigm consisted of 15 s of 5% O-2 (nadir) followed by 5 min of 21% O-2 (9 episodes/h). In one group of experiments (IH and control, n = 6 pups each), sensory activity was recorded from ex vivo carotid bodies, and in the other ( IH and control, n = 7 pups each) ventilation was monitored in unanesthetized pups by plethysmography. In control pups, sensory response of the carotid body was weak and was slow in onset ( similar to 100 s). In contrast, carotid body sensory response to hypoxia was greater and the time course of the response was faster ( similar to 30 s) in IH compared with control pups. The magnitude of the hypoxic ventilatory response was greater in IH compared with control pups, whereas changes in O-2 consumption and CO2 production during hypoxia were comparable between both groups. The magnitude of ventilatory stimulation by hyperoxic hypercapnia (7% CO2-balance O-2), however, was the same between both groups of pups. These results demonstrate that neonatal IH facilitates carotid body sensory response to hypoxia and augments hypoxic ventilatory chemoreflex.