Arterial hypocapnia during exercise in beagle dogs

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.