Effect of locomotor respiratory coupling on respiratory evaporative heat loss in the sheep

During galloping, many animals display 1:1 coupling of breaths and strides. Locomotor respiratory coupling (LRC) may limit respiratory evaporative heat loss (REHL) by constraining respiratory frequency (f). Five sheep were exercised twice each, according to a five-step protocol: 5 min at the walk, 5 min at the trot (trot1), 10 min at the gallop, 5 min at the trot (trot2), and 5 min at the walk. Rectal temperature (T(re)), stride frequency, f, REHL, and arterial CO₂ tension and pH were measured at each step. Tidal volume (VT) was calculated. LRC was observed only during galloping. The coupling ratio remained at 1:1 while VT increased continuously during galloping, causing REHL to increase from 2.9 ± 0.2 (SE) W/kg at the end of trot1 to a peak of 5.3 ± 0.3 W/kg. T(re) rose from 39.0 ± 0.1 °C preexercise to 40.2 ± 0.2 °C at the end of galloping. At the gallop-trot2 transition, VT fell and f rose, despite a continued rise in T(re). Arterial CO₂ tension fell from 36.5 ± 1.1 Torr preexercise to 31.8 ± 1.4 Torr by the end of trot1 and then further to 21.5 ± 1.2 Torr by the end of galloping, resulting in alkalosis. In conclusion, LRC did not prevent increases in REHL in sheep because VT increased. The increased VT caused hypocapnia and presumably elevated the cost of breathing.