Oxidative and Glycolytic Metabolism

Although skeletal muscle fibers, particularly of the fast glycolytic type, are able to utilize their phospho-creatine (PCr) and glycogen stores for sizable bursts of energy turnover at rates well exceeding that of oxidative recovery processes, these stores are small in 

Lactate produced by smooth muscle of swine carotid artery has been shown to be quantitatively derived from extracellular glucose, whereas gly-cogenolysis provides carbohydrates for oxidative metabolism (Lynch and Paul, 1983, 1987). Interestingly, the lactate dehydrogenase (LDH) isoenzyme pattern varies between slow (aorta) and fast (portal vein and bladder) smooth muscle in rat, such that aorta has a higher proportion of the H subunit, which has a higher affinity for lactate and is product-inhibited at lower concentrations of lactate (Malmqvistef flZ., 1991). Thus the slow aortic muscle is more adapted for oxidative metabolism than the faster bladder and portal vein muscles. 

Most studies of muscle energetics in vitro have utilized glucose as substrate. Under in vivo conditions the utilization of metabolic substrate is presumably more varied. Chace and Odessey (1981) showed that metabolism in rabbit aorta can be sustained by fatty acids, ketone bodies, and amino acids, as well as carbohy- 

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Gomez, A. and Ferrer, I. (2009). Increased oxidation of certain glycolytic and energy metabolism enzymes in the frontal cortex in Lewy body diseases. J. Neurosci. Res. 87,1002-1013. 

It is advantageous for aerobic tissues (e.g., heart and brain) to retain a significant amount of pyruvate which can be oxidized in the citric acid cycle and result in the production of ATP. On the other hand, in anaerobic tissue such as in skeletal muscle, where there are occasional high demands for energy and where no ATP production is possible via the citric acid cycle, it is essential for there to be an effective glycolytic metabolism with lactic acid as the end product (Fig. 1). The latter can then be recirculated in the bloodstream. 

Axenically cultured amastigote-like cells of T. cruzi have an essentially glycolytic metabolism. They ferment glucose to sueeinate and aeetate and do not seem to excrete ammonia. Only after they reach stationary phase and transform to epimastigotes do they acquire the ability to oxidize substrates such as amino acids (25). Whether these amastigote-like cells resemble in their earbohydrate metabolism the intracellular stages in the mammalian host remains an open question. 

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