Rates of formation and oxidation of lactic acid in dogs at rest and during moderate exercise

The rates of formation and oxidation of plasma lactic acid were measured in dogs, either resting quietly or running at 6 km/h on the level, by priming and continuous infusion with uniformly labelled 14 C-lactic acid. During running at rates of oxygen consumption almost 5 times the resting value, corresponding to approximately 30% of the maximal effort, the rates of lactic acid formation and disposal were always greater than at rest and doubled on the average. Average plasma lactic acid concentrations during running were greater, equal, or lower than at rest in different dogs. The energy released by formation of the extra lactic acid from glucose during running represented, energetically, less than 0.5% of the extra energy cost of running. Seventy-four percent of the lactate formed was promptly converted to CO 2 , and about 12% of the respiratory carbon was derived from lactic acid in the running dogs. About 10% of the plasma glucose was derived from lactic acid during both rest and activity. At similar plasma lactate concentrations, running dogs had greater rates of formation of lactic acid than resting animals.