POSTNATAL MATURATION OF CAROTID CHEMORECEPTOR RESPONSES TO 0(2) 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 O-2 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 ex posed to three levels of O-2 (arterial Po-2 = 40-45, 80-90, and >300 Torr) at five levels of arterial Pco(2) (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. O-2-CO2 interaction did not become significant until after 4 wk of age. Thus, carotid chemoreceptor responses to both O-2 and CO2 are weak in newborn cats and increase during postnatal development. However, the time courses for the maturation of O-2 vs. CO2 sensitivity appear to differ.