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Department of Comparative Biosciences, School of Veterinary Medicine, University of Wisconsin, Madison, Wisconsin 53706
Received 10 June 1996; accepted in final form 9 September 1996.
Dwinell, M. R., P. L. Janssen, J. Pizarro, and G. E. Bisgard. Effects of carotid body hypocapnia during ventilatory acclimatization to hypoxia. J. Appl.
Physiol. 82(1): 118-124, 1997.
Hypoxic
ventilatory sensitivity is increased during ventilatory acclimatization
to hypoxia (VAH) in awake goats, resulting in a time-dependent increase
in expired ventilation (
E). The
objectives of this study were to determine whether the increased
carotid body (CB) hypoxic sensitivity is dependent on the level of CB CO2 and whether the CB
CO2 gain is changed during VAH.
Studies were carried out in adult goats with CB blood gases controlled by an extracorporeal circuit while systemic (central nervous system) blood gases were regulated independently by the level of inhaled gases. Acute E responses
to CB hypoxia (CB PO2 40 Torr) and CB
hypercapnia (CB PCO2 50 and 60 Torr)
were measured while systemic normoxia and isocapnia were maintained. CB
PO2 was then lowered to 40 Torr for 4 h while the systemic blood gases were kept normoxic and normocapnic.
During the 4-h CB hypoxia, E increased
in a time-dependent manner. Thirty minutes after return to normoxia,
the ventilatory response to CB hypoxia was significantly increased
compared with the initial response. The slope of the CB
CO2 response was also elevated
after VAH. An additional group of goats
(n = 7) was studied with a
similar protocol, except that CB PCO2
was lowered throughout the 4-h hypoxic exposure to prevent reflex
hyperventilation. CB PCO2 was
progressively lowered throughout the 4-h CB hypoxic period to maintain
E at the control level. After the 4-h
CB hypoxic exposure, the ventilatory response to hypoxia was also
significantly elevated. However, the slope of the CB
CO2 response was not elevated
after the 4-h hypoxic exposure. These results suggest that CB
sensitivity to both O2 and
CO2 is increased after 4 h of CB
hypoxia with systemic isocapnia. The increase in CB hypoxic sensitivity
is not dependent on the level of CB
CO2 maintained during the 4-h
hypoxic period.
carotid body chemoreceptors; goats; respiratory control
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