Effects of fasting and training on pyruvate dehydrogenase activation during exercise

Transcriptional regulation of pyruvate dehydrogenase kinase 4 in skeletal muscle during and after exercise

The pyruvate dehydrogenase complex (PDC) has a key position in skeletal muscle metabolism as it represents the entry of carbohydrate-derived fuel into the mitochondria for oxidation. PDC is regulated by a phosphorylation-dephosphorylation cycle, in which the pyruvate dehydrogenase kinase (PDK) phosphorylates and inactivates the complex. PDK exists in four isoforms, of which the PDK4 isoform is predominantly expressed in skeletal and heart muscle. PDK4 transcription and PDK4 mRNA are markedly increased in human skeletal muscle during prolonged exercise and after both short-term high-intensity and prolonged low-intensity exercise. The exercise-induced transcriptional response of PDK4 is enhanced when muscle glycogen is lowered before the exercise, and intake of a low-carbohydrate high-fat diet during recovery from exercise results in increased transcription and mRNA content of PDK4 when compared with intake of a high-carbohydrate diet. The activity of pyruvate dehydrogenase (PDH) is increased during the first 2 h of low-intensity exercise, followed by a decrease towards resting levels, which is in line with the possibility that the increased PDK4 expressed influences the PDH activity already during prolonged exercise. PDK4 expression is also increased in response to fasting and a high-fat diet. Thus, increased PDK4 expression when carbohydrate availability is low seems to contribute to the sparing of carbohydrates by preventing carbohydrate oxidation. The impact of substrate availability on PDK4 expression during recovery from exercise also underlines the high metabolic priority given to replenishing muscle glycogen stores and re-establishing intracellular homeostasis after exercise.

Effects of fasting on endurance exercise

Early studies agree that fasting is detrimental to overall physical performance and to endurance performance in humans; however, a study in rats reported an ergogenic effect where time to exhaustion was increased by a glycogen-sparing effect of elevated free fatty acids in blood resulting from a 24-hour fast. Later studies on humans have also found a detrimental effect of fasting on exercise endurance, with the exception of 1 study which found no difference. The discrepancy between humans and rats could not be explained by level of glycogen sparing, mode of exercise, duration of the fast, physiological differences or level of training. The intensity of exercise, and a potential placebo effect of fasting, are possible reasons for the conflicting results. Despite reduced endurance performance, fasted humans are able to exercise and maintain blood glucose homeostasis; the specific cause of an earlier onset of fatigue during a single bout of exercise in the fasted state remains unclear.

Acetyl group accumulation and pyruvate dehydrogenase activity in human muscle during incremental exercise

The changes in the muscle contents of CoASH and carnitine and their acetylated forms, lactate and the active form of pyruvate dehydrogenase complex were studied during incremental dynamic exercise. Eight subjects exercised for 3-4 minutes on a bicycle ergometer at work loads corresponding to 30, 60 and 90% of their VO2max. Muscle samples were obtained by percutaneous needle biopsy technique at rest, at the end of each work period and after 10 minutes of recovery. During the incremental exercise test there was a continuous increase in muscle lactate, from a basal value of 4.5 mmol kg-1 dry weight to 83 mmol kg-1 at the end of the final period. The active form of pyruvate dehydrogenase complex increased from 0.37 mmol acetyl-CoA formed per minute per kilogram wet weight at rest to 0.80 at 30% VO2max, 1.28 and 1.25 at 60 and 90% VO2max, respectively. Both acetyl-CoA and acetylcarnitine increased at the two highest work loads. The increase of acetyl-CoA was from 12.5 mumol kg-1 dry weight at rest to 27.3 after the highest work load and for acetylcarnitine from 6.0 mmol kg-1 dry weight to 15.2. The CoASH and free carnitine contents fell correspondingly. There was a close relationship between acetyl-CoA and acetylcarnitine accumulation in muscle during exercise, with a binding of approximately 500 mol acetyl groups to carnitine for each mole of acetyl-CoA accumulated. The results imply that the carnitine store in muscle functions as a buffer for excess formation of acetyl groups from pyruvate catalyzed by the pyruvate dehydrogenase complex.

Heterogeneity of response to exercise of rat muscle pyruvate dehydrogenase complex

Muscle glucose uptake is greatly stimulated by moderate exercise, but full oxidation of the glucose to CO2 depends on the activity of the pyruvate dehydrogenase (PDH) complex. Our aim was to determine how PDH complex in different muscle groups responds to varying periods of moderate exercise. Rats were run on a motor-driven treadmill for 5-30 min and muscle PDH complex activity was determined in heart, diaphragm and red quadriceps muscles after isolation of mitochondria in the presence of inhibitors of PDH complex interconversion. In heart and diaphragm muscle, exercise caused an increase in PDH complex activity after 5 min, but this was followed by a significant decrease in activity as exercise progressed. In red quadriceps muscle, PDH complex activity was reduced after 5 min of exercise and was decreased further as exercise continued. We conclude that increased duration of exercise can lead to reduced PDH complex activity in rat muscles. We propose that this is a consequence of elevated fatty acid oxidation, the products of which stimulate PDH kinase. This implies that increased glycolysis to lactate and increased fatty acid oxidation can simultaneously provide energy for contracting muscle.