Journal of Applied Physiology, Vol 72, Issue 4 1255-1260, Copyright © 1992 by American Physiological Society
Maturation of steady-state CO2 sensitivity in vagotomized anesthetized lambs
A. H. Jansen, S. Ioffe and V. Chernick
Perinatal Physiology Laboratory, University of Manitoba, Winnipeg, Canada.
The maturation of the respiratory sensitivity to CO2 was studied in three
groups of anesthetized (ketamine, acepromazine) lambs 2-3, 14-16, and 21-22
days old. The lambs were tracheostomized, vagotomized, paralyzed, and
ventilated with 100% O2. Phrenic nerve activity served as the measure of
respiration. The lambs were hyperventilated to apneic threshold, and
end-tidal PCO2 was raised in 0.5% steps for 5-7 min each to a maximum 7-8%
and then decreased in similar steps to apneic threshold. The sinus nerves
were cut, and the CO2 test procedure was repeated. Phrenic activity during
the last 2 min of every step change was analyzed. The CO2 sensitivity
before and after sinus nerve section was determined as change in percent
minute phrenic output per Torr change in arterial PCO2 from apneic
threshold. Mean apneic thresholds (arterial PCO2) were not significantly
different among the groups: 34.8 +/- 2.08, 32.7 +/- 2.08, and 34.7 +/- 2.25
(SE) Torr for 2- to 3-, 14- to 16-, and 21- to 22-day-old lambs,
respectively. After sinus denervation, apneic thresholds were raised in all
groups [39.9 +/- 2.08, 40.9 +/- 2.08, and 45.3 +/- 2.25 (SE) Torr,
respectively] but were not different from each other. CO2 response slopes
did not change with age before or after sinus nerve section. We conclude
that carotid bodies contribute to the CO2 response during hyperoxia by
affecting the apneic threshold but do not affect the steady-state CO2
sensitivity and the central chemoreceptors are functionally mature shortly
after birth.
