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 (Tre), and physical conditioning to the variance in arterial CO2 tension (PaCO2) in the exercising sheep were quantified. We hypothesized that thermal drive contributes to hyperventilation. Four unshorn sheep were exercised at approximately 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 degrees C) environment. Tre and O2 consumption were measured continuously. Lactic acid and PaCO2 were measured at 5- to 10-min intervals. Data were analyzed by multiple regression on PaCO2. During exercise, Tre rose and PaCO2 fell, except at the lowest ExInt in the cold environment. Tre explained 77% of the variance in PaCO2, and ExInt explained 5%. All other variables were insignificant. We conclude that, in sheep, thermal drive contributes to hyperventilation during exercise.