Muscle in all animals relies upon four potential sources of energy: ATP hydrolysis, phosphagen hydrolysis, fermentations or oxidative metabolism. Although the relative contributions of different fuels varies greatly in different organisms, in none is there a simple reliance on stored ATP. Muscle work therefore requires a balance between rates of utilization and formation of ATP, a provision supplied by one of the three remaining fuels and metabolic pathways. Useful endogenous fuels must be storable at high level, and rapidly mobilizable with minimal perturbation of [ATP] and with minimal end-product effects on pH, charge or osmotic balance. In addition to displaying these properties, good exogenous fuels must be transferable at high rates between depot sites and muscle; actual flux rates of exogenous fuels depend upon respective ATP yields and are lowest for fuels which most amplify the yield of ATP per mol substrate oxidized. Substrate flux rates must be matched with O2 flux rates and with rates of endogenous substrate mobilization in order that the right energy-yielding pathways are activated at the right times. Of various control possibilities, an effective competition for ADP (and possibly Pi) seems at this time to be the dominant strategy for assuring integration of aerobic and anaerobic ATP-yielding pathways.

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