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 physicalconditioning to the variance in arterialCO₂ tension(Pa_CO2) in the exercising sheep werequantified. We hypothesized that thermal drive contributes tohyperventilation. Four unshorn sheep were exercised at ~30, 50, and70% of maximal O₂ consumption for30 min, or until exhaustion, both before and after 5 wk of physicalconditioning. In addition, two of the sheep were shorn and exercised ateach intensity in a cold (<15°C) environment.T_re andO₂ consumption were measured continuously. Lactic acid and Pa_CO2 weremeasured at 5- to 10-min intervals. Data wereanalyzed by multiple regression onPa_CO2. During exercise,T_re rose andPa_CO2 fell, except at the lowest ExIntin the cold environment. T_reexplained 77% of the variance in Pa_CO2,and ExInt explained 5%. All other variables were insignificant. Weconclude that, in sheep, thermal drive contributes to hyperventilation during exercise.