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Development of Carotid Chemoreceptor Dynamic and Steady‐State Sensitivity to CO 2 in the Newborn Lamb
Author(s) -
Calder N. A.,
Kumar P.,
Hanson M. A.
Publication year - 1997
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1997.187bi.x
Subject(s) - chemoreceptor , hyperoxia , steady state (chemistry) , hypoxia (environmental) , anesthesia , chemistry , zoology , medicine , biology , oxygen , lung , receptor , organic chemistry
1 The maturation of carotid chemoreceptor steady‐state and dynamic responses to CO 2 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 CO 2 sensitivity was measured over a range of arterial CO 2 pressures ( P a,CO 2 ) at four levels of arterial O 2 pressure ( P a,O 2 ): 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 CO 2 sensitivity was present at all ages, and a significant effect of age ( P < 0.001 ) and P a,O 2 ( P < 0.025 ) (ANOVA) was observed. Older lambs were unable to sustain an increase in chemoreceptor discharge during SvHx as CO 2 was increased. 4 Dynamic CO 2 sensitivity was measured by producing alternations in end‐tidal CO 2 levels (etCO 2 ) (alternation amplitude, 1.23 ± 0.07% (mean ± s.e.m. ); etCO 2 , 7.56 ± 0.15%) over 2–8 s at two P a,O 2 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 etCO 2 for the control and CO 2 ‐loaded breaths. Dynamic CO 2 sensitivity was calculated as the slope between these points. 5 Dynamic CO 2 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 CO 2 sensitivity, there was no significant effect of age or P a,O 2 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 CO 2 sensitivity within a few days of birth, an age when hypoxia sensitivity is low. This CO 2 sensitivity increases with age, perhaps due to the increasing interaction between CO 2 and O 2 . Dynamic sensitivity of the carotid body to CO 2 is mature at birth and does not increase with age, as predicted if the response of the carotid body to rapid changes in CO 2 is independent of the sensitivity to the partial pressure of O 2 ( P O 2 ).