Journal of Applied Physiology, Vol 61, Issue 2 599-602, Copyright © 1986 by American Physiological Society
Arterial hypocapnia during exercise in beagle dogs
P. S. Clifford, J. T. Litzow and R. L. Coon
Previous investigators have assumed that during exercise there is a tight
coupling of ventilation with CO2 delivery to the lungs such that arterial
blood remains isocapnic. We measured arterial blood gases in a group of 10
beagle dogs in which arterial blood sampling could be accomplished via
exteriorized carotid artery loops and in six of the same dogs following
chronic pulmonary denervation. Samples were taken at rest, at 15-s
intervals during the first minute of unrestrained treadmill exercise at 5.0
km/h, 0% grade, and then at 2 and 3 min at the same work load. Mean resting
arterial PCO2 for the control dogs was 37.1 Torr. At the onset of exercise
arterial PCO2 fell progressively to a nadir of 34.6 Torr during the 30- to
45-s sampling period. Samples at 2 and 3 min remained significantly
hypocapnic (PCO2 = 34.8 Torr). The arterial PCO2 and pH responses to
exercise in the lung-denervated dogs were not significantly different from
those of the control dogs, although arterial PO2 was lower at rest and
during exercise following denervation of the lungs. The arterial hypocapnia
exhibited in intact beagle dogs at the onset of exercise persists into the
steady state and suggests that there is not a tight coupling of ventilation
with pulmonary CO2 delivery. The similarity of the response in
lung-denervated dogs suggests that intrapulmonary receptors with afferents
in the vagi are not the primary mediators of the ventilatory response to
exercise.
