Independence of exercise hypocapnia and limb movement frequency in ponies

The objective of this study was to determine whether changes in limb motion per se influence arterial CO2 partial pressure (PaCO2) during muscular exercise in ponies. Fifteen ponies were studied at rest and during 8 min of treadmill exercise when the work load was constant or when the work load was increased after the 4th min. Five different treadmill settings were selected to provide for a range of metabolic rate achieved with primary changes in either speed or grade (1.8 mph at 3, 8, and 15% grade; or 3 and 6 mph at 3% grade). The ponies exercised either on all four legs or on only the hindlegs. Step frequencies were 49, 66, and 99 at 1.8, 3, and 6 mph, respectively. During all work tasks PaCO2 decreased maximally 30-60 s after the work task was initiated from rest or from a less intense level of exercise. This nadir in PaCO2 was followed by some recovery with a stable level of mild hypocapnia (delta PaCO2) maintained after 3-4 min. The delta PaCO2 was directly related to O2 consumption (VO2) (P less than 0.01). The delta PaCO2-VO2 regression slopes did not differ between speed and grade VO2 changes nor between four- and two-legged exercise (P greater than 0.10). These data suggest that neither frequency of limb movement nor the number of limbs moving are major factors in the PaCO2 (and alveolar ventilation) response to exercise in ponies. We conclude that the apparent difference in PaCO2 regulation during exercise between ponies (hypocapnia) and humans (isocapnia during walking and bicycling) is not related to a species difference in the number of limbs employed in the exercise task.