Journal of Applied Physiology, Vol 57, Issue 6 1885-1893, Copyright © 1984 by American Physiological Society
Independence of exercise hypocapnia and limb movement frequency in ponies
H. V. Forster, L. G. Pan, G. E. Bisgard, C. Flynn, S. M. Dorsey and M. S. Britton
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.
