Postnatal maturation of carotid chemoreceptor responses to O₂ and CO₂ in the cat

This study aimed to characterize neural responses of the carotid chemoreceptors of the maturing cat to natural stimuli and to determine the time course of carotid chemoreceptor development from the neonatal period to adulthood. Carotid sinus nerve (CSN) responses to O₂ and CO₂ were studied in cats at 1, 4, and 8 wk of age and in adult cats (n = 6 at each age). Pentobarbital sodium-anesthetized cats were exposed to three levels of O₂ (arterial PO₂ = 40-45, 80-90, and >300 Torr) at five levels of arterial PCO₂ (22, 35, 48, 63, and 75 Torr) while the moving average of whole nerve output from the CSN was recorded. Ganglioglomerular nerves were sectioned. All cats at every age increased CSN activity during hypoxia. However, the CSN response to hypoxia was not sustained in some immature cats. Of the cats that sustained CSN activity during hypoxia, four of the six 1-wk-old cats showed a biphasic pattern of response, with an initial overshoot followed by a steady level of discharge. Older cats did not exhibit this pattern. CSN sensitivity to hypoxia was weakest in 1-wk-old kittens but increased to nearly adult levels by 4 wk of age. Carotid chemoreceptor responses to CO₂ were also smallest in 1-wk-old kittens and increased with maturation. However, unlike hypoxia responses, CO₂ sensitivity during hypoxia continued to develop between 8 wk and adulthood. O₂-CO₂ interaction did not become significant until after 4 wk of age. Thus, carotid chemoreceptor responses to both O₂ and CO₂ are weak in newborn cats and increase during postnatal development. However, the time courses for the maturation of O₂ vs. CO₂ sensitivity appear to differ.