Thermal drive contributes to hyperventilation during exercise in sheep

The etiology of exercise hypocapnia is unknown. The contributions of exercise intensity (ExInt), lactic acid, environmental temperature, rectal temperature (T(re)), and physical conditioning to the variance in arterial CO₂ tension (Pa(CO₂)) in the exercising sheep were quantified. We hypothesized that thermal drive contributes to hyperventilation. Four unshorn sheep were exercised at ~30, 50, and 70% of maximal O₂ consumption for 30 min, or until exhaustion, both before and after 5 wk of physical conditioning. In addition, two of the sheep were shorn and exercised at each intensity in a cold (<15°C) environment. Tre and O₂ consumption were measured continuously. Lactic acid and Pa(CO₂) were measured at 5- to 10- min intervals. Data were analyzed by multiple regression on Pa(CO₂). During exercise, T(re) rose and Pa(CO₂) fell, except at the lowest ExInt in the cold environment. T(re) explained 77% of the variance in Pa(CO₂), and ExInt explained 5%. All other variables were insignificant. We conclude that, in sheep, thermal drive contributes to hyperventilation during exercise.