Reduction of the Pa_CO2 set point during hyperthermic exercise in the sheep

In animals that rely on the respiratory system for both gas exchange and heat loss, exercise can generate conflict between chemoregulation and thermoregulation. We hypothesized that in panting animals, hypocapnia during hyperthermic exercise reflects a reduction in the arterial CO₂ tension (Pa_CO2) set point. To test this hypothesis, five sheep were subjected to tracheal insufflations of CO₂ or air (control) at 3-4 L min^-1 in 3 min bouts at 5 min intervals over 31 min of exercise. During exercise, rectal temperature and minute ventilation (V_E) rose continuously while Pa_CO2 fell from 35.4±3.1 to 18.6±2.9 Torr and 34.3±2.4 to 18.7±1.5 Torr in air and CO₂ trials, respectively. Air insufflations did not affect V_E or Pa _CO2. V_E increased during CO₂ insufflations via a shift to higher tidal volume and lower frequency. CO₂ insufflations also increased Pa_CO2, although not above the pre-exercise level. Within 5 min after each CO₂ insufflation, Pa_CO2 had decreased to match that following the equivalent air insufflation. These results are consistent with a reduced Pa_CO2 set point or an increased gain of the Pa_CO2 regulatory system during hyperthermic exercise. Either change in the control of Pa_CO2 could facilitate respiratory evaporative heat loss by mitigating homeostatic conflict.