SPRIET, L. L., R. A. HOWLETT, and G. J. F. HEIGENHAUSER. An enzymatic approach to lactate production in human skeletal muscle during exercise. Med. Sci. Sports Exerc., Vol. 32, No. 4, pp. 756–763, 2000.
Purpose: This paper examines the production of lactate in human skeletal muscle over a range of power outputs (35–250% V̇O2max) from an enzymatic flux point of view. The conversion of pyruvate and NADH to lactate and NAD in the cytoplasm of muscle cells is catalyzed by the near-equilibrium enzyme lactate dehydrogenase (LDH). As flux through LDH is increased by its substrates, pyruvate and NADH, the factors governing the production of these substrates will largely dictate how much lactate is produced at any exercise power output. In an attempt to understand lactate production, flux rates through the enzymes that regulate glycogenolysis/glycolysis, the transfer of cytoplasmic reducing equivalents into the mitochondria, and the various fates of pyruvate have been measured or estimated.
Results: At low power outputs, the rates of pyruvate and NADH production in the cytoplasm are low, and pyruvate dehydrogenase (PDH) and the shuttle system enzymes (SS) metabolize the majority of these substrates, resulting in little or no lactate production. At higher power outputs (65, 90, and 250% V̇O2max), the mismatch between the ATP demand and aerobic ATP provision at the onset of exercise increases as a function of intensity, resulting in increasing accumulations of the glycogenolytic/glycolytic activators (free ADP, AMP, and Pi). The resulting glycolytic flux, and NADH and pyruvate production, is progressively greater than can be handled by the SS and PDH, and lactate is produced at increasing rates. Lactate production during the onset of exercise and 10 min of sustained aerobic exercise may be a function of adjustments in the delivery of O2 to the muscles, adjustments in the activation of the aerobic ATP producing metabolic pathways and/or substantial glycogenolytic/glycolytic flux through a mass action effect.
During exercise, carbohydrate (CHO) is metabolized in the cytoplasm of skeletal muscle cells to produce pyruvate in the glycolytic pathway (Fig. 1). The CHO fuel is provided by the uptake of glucose from the blood and from glycogen stored inside the muscle. Once produced, the pyruvate can be further metabolized in the cytoplasm or transported across the inner mitochondrial membrane and metabolized inside the mitochondria. The most important mitochondrial pathway of pyruvate metabolism is conversion to acetyl-coenzyme A (acetyl-CoA) with the reduction of NAD to NADH in a reaction catalyzed by the pyruvate dehydrogenase (PDH) complex. The acetyl-CoA is then available to enter the tricarboxylic acid (TCA) cycle where reducing equivalents are produced for use in the electron transport chain to generate ATP in the process of oxidative phosphorylation. The oxidative use of 1 mmol glucose or glucosyl unit results in the production of ∼39 mmol ATP. Conversely, the most important cytoplasmic pathway of pyruvate metabolism is conversion to lactate with the oxidation of NADH to NAD in the lactate dehydrogenase (LDH) reaction. When 1 mmol glucosyl unit is metabolized to lactate, it provides either 2 mmol (exogenous glucose) or 3 mmol (muscle glycogen) ATP.
Pyruvate can also combine with glutamate to form 2-oxoglutarate and alanine in the alanine aminotransferase (AAT) reaction in the cytoplasm, and to a minor extent in the mitochondria (Fig. 1). This reaction appears to be important in the early stages of exercise to increase the content of the TCA cycle intermediates but only accounts for ∼2–5% of pyruvate disposal (12,13). Other reactions catalyzed by pyruvate carboxylase and malic enzyme also compete for pyruvate in the cytoplasm but do not appear to be quantitatively important (25), although they have not been studied in human muscle. NADH, the other substrate for LDH, can also be reconverted to NAD in the cytoplasm via the near-equilibrium malate-aspartate and alpha-glycerophosphate shuttle systems (SS), which transfer reducing equivalents to the mitochondria (Fig. 1). The malate-aspartate shuttle appears to be the quantitatively important system (26,32,33).
Determining the exact mechanisms of lactate production in skeletal muscle during all exercise conditions has been difficult and will not be resolved in this paper. Readers are urged to consult the numerous research papers and reviews that have been published on this topic (5–7,14,22). The purpose of this paper is to use an enzymatic approach to examine lactate production over a wide range of exercise power outputs in human skeletal muscle. This approach examines the flux through the key enzymes of glycogenolysis/glycolysis, the SS, and pyruvate metabolism at 35, 65, 90, and 250% V̇O2max. The first 10 min of cycle exercise at each of the aerobic power outputs and 30 s of cycle sprinting at ∼250% V̇O2max are examined. Because the measurements and estimates of enzyme and pathway fluxes are derived from needle muscle biopsy samples, this approach only provides average responses for the fiber type populations that exist in the sampled vastus lateralis muscles. Also, the subjects who volunteered for these studies were active, but not well-trained aerobically.