Development of carotid chemoreceptor dynamic and steady-state sensitivity to CO2 in the newborn lamb.

1. The maturation of carotid chemoreceptor steady-state and dynamic responses to CO2 in 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. 2. Steady-state CO2 sensitivity was measured over a range of arterial CO2 pressures (Pa,CO2) at four levels of arterial O2 pressure (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. 3. Steady-state CO2 sensitivity 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 CO2 was increased. 4. Dynamic CO2 sensitivity was measured by producing alternations in end-tidal CO2 levels (etCO2) (alternation amplitude, 1.23 +/- 0.07% (mean +/- S.E.M.); etCO2, 7.56 +/- 0.15%) over 2-8 s at two Pa,O2 levels 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 etCO2 for the control and CO2-loaded breaths. Dynamic CO2 sensitivity was calculated as the slope between these points. 5. Dynamic CO2 sensitivity was greater than steady-state sensitivity in Nx (P < 0.05) and ModHx (P < 0.01, Student's paired t test). Unlike steady-state CO2 sensitivity, there was no significant effect of age or Pa,O2 on dynamic sensitivity (P > 0.39 and P > 0.68, respectively, ANOVA). 6. Our results show that the neonatal lamb possesses a carotid body steady-state CO2 sensitivity within a few days of birth, an age when hypoxia sensitivity is low. This CO2 sensitivity increases with age, perhaps due to the increasing interaction between CO2 and O2. Dynamic sensitivity of the carotid body to CO2 is mature at birth and does not increase with age, as predicted if the response of the carotid body to rapid changes in CO2 is independent of the sensitivity to the partial pressure of O2 (PO2).