The effects of hypercapnia on the metabolic response to steady-state exercise

Studies on isolated muscle and resting man have demonstrated that altering CO2 stores influences intracellular lactate production and/or tissue lactate release. In the present project, subjects (N = 6) performed steady-state exercise for 30 min while inspiring 0, 2, 4, or 6% CO2 and 21% O2. They were tested on eight occasions, four at 50% and four at 65% VO2max. Arterialized venous blood PCO2 increased in proportion to FICO2 (P less than 0.05). Blood pH had a similar but inverse relationship, decreasing from 7.371 to 7.233 (P less than 0.05). The VI increased directly with PCO2 (P less than 0.05), but no differences were found for VO2 or VO2. The R decreased in proportion to PCO2 (P less than 0.05) at both exercise intensities. Blood lactate was reduced (P less than 0.05) with CO2. At 65% VO2max lactate had an inverse linear relationship with blood PCO2 (P less than 0.05). The mean lactate decreased 43% from 3.88 mM . l-1 with 0% CO2 to 2.22 mM . l-1 with 6% CO2. The R shift suggest that carbohydrate metabolism may have been inhibited and lipid metabolism enhanced.