Postnatal maturation of carotid body and type I cell chemoreception in the rat

The site of postnatal maturation of carotid body chemoreception is unclear. To test the hypothesis that maturation occurs synchronously in type I cells and the whole carotid body, the development of changes in the intracellular Ca²⁺ concentration responses to hypoxia, CO₂, and combined challenges was studied with fluorescence microscopy in type I cells and compared with the development of carotid sinus nerve (CSN) responses recorded in vitro from term fetal to 3-wk animals. Type I cell responses to all challenges increased between land 8 days and then remained constant, with no multiplicative O₂-CO₂ interaction at any age. The CSN response to hypoxia also matured by 8 days, but CSN responses to CO₂ did not change significantly with age. Multiplicative O₂-CO₂ interaction occurred in the CSN response at 2-3 wk but not in younger groups. We conclude that type I cell maturation underlies maturation of the CSN response to hypoxia. However, because development of responses to CO₂ and combined hypoxia-CO₂ challenges differed between type I cells and the CSN, responses to these stimuli must mature at other, unidentified sites within the developing carotid body.