Development of Carotid Chemoreceptor Dynamic and Steady‐State Sensitivity to CO2in the Newborn Lamb

1The maturation of carotid chemoreceptor steady‐state and dynamic responses to CO2in newborn lambs was measured. In total, sixteen fibres (13 lambs) were studied at 3–4 days, nineteen fibres (13 lambs) at 5–9 days and twenty‐one fibres (17 lambs) at 10–24 days after birth.2Steady‐state CO2sensitivity was measured over a range of arterial CO2pressures (Pa,CO2) at four levels of arterial O2pressure (Pa,O2): hyperoxia (Hyp), 115–150 mmHg; normoxia (Nx), 90–105 mmHg; moderate hypoxia (ModHx), 40–60 mmHg; and severe hypoxia (SvHx), 20–35 mmHg.3Steady‐state CO2sensitivity was present at all ages, and a significant effect of age (P< 0.001) and Pa,O2(P< 0.025) (ANOVA) was observed. Older lambs were unable to sustain an increase in chemoreceptor discharge during SvHx as CO2was increased.4Dynamic CO2sensitivity was measured by producing alternations in end‐tidal CO2levels (etCO2) (alternation amplitude, 1.23 ± 0.07% (mean ±s.e.m.); etCO2, 7.56 ± 0.15%) over 2–8 s at two Pa,O2levels only: 80–100 (Nx) and 40–60 mmHg (ModHx). Peak and trough values of the oscillation in chemoreceptor discharge were plotted against maximum and minimum etCO2for the control and CO2‐loaded breaths. Dynamic CO2sensitivity was calculated as the slope between these points.5Dynamic CO2sensitivity was greater than steady‐state sensitivity in Nx (P< 0.05) and ModHx (P< 0.01, Student's paired ttest). Unlike steady‐state CO2sensitivity, there was no significant effect of age or Pa,O2on dynamic sensitivity (P> 0.39 and P> 0.68, respectively, ANOVA).6Our results show that the neonatal lamb possesses a carotid body steady‐state CO2sensitivity within a few days of birth, an age when hypoxia sensitivity is low. This CO2sensitivity increases with age, perhaps due to the increasing interaction between CO2and O2. Dynamic sensitivity of the carotid body to CO2is mature at birth and does not increase with age, as predicted if the response of the carotid body to rapid changes in CO2is independent of the sensitivity to the partial pressure of O2(PO2).