Reduction of the PaCO2 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 (PaCO2) 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 (VE) rose continuously while PaCO2 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 VE or PaCO2. VE increased during CO2 insufflations via a shift to higher tidal volume and lower frequency. CO2 insufflations also increased PaCO2, although not above the pre-exercise level. Within 5 min after each CO2 insufflation, PaCO2 had decreased to match that following the equivalent air insufflation. These results are consistent with a reduced PaCO2 set point or an increased gain of the PaCO2 regulatory system during hyperthermic exercise. Either change in the control of PaCO2 could facilitate respiratory evaporative heat loss by mitigating homeostatic conflict.