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I understand that there are several reserves available to muscle that can be tapped for ATP, and that each reserve is being used to some degree at all times.

Are these reaction rates purely due to the presence of reactants which are products of myocyte activity?

Or are there other controlling reactants involved that determine the rate at which each reserve is consumed?

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Well, ATP is replenished from three energy sources, namely: 1. phosphagen system (very fast, but quickly deprived) 2. glycolysis (fast, but slower deprived) 3. oxidative system (slow, 'impossible' to deprive)

When ATP is used, for example for muscle contraction, ADP is formed. The formed ADP can be rephosphorylated to ATP by the enzym creatine kinase, which also requires creatinephosphate (the phosphate attached to the creatine provides the phosphate group for ADP to form ATP). Additionally, ATP can be formed out of 2 ADP molecules by adenylate kinase. The phosphagen system is largely driven by the law of mass action, i.e. the direction of the enzymatic reaction is controlled by the concentration of the products and substrates. In other words; when ATP is low, and ADP is high, the generation of ATP is favored and vice versa. The energy from the phosphagen system is depleted rather quickly tho.

The second system, glycolysis, simply refers to the breakdown of carbohydrates (e.g. glucose) to resynthesize ATP from the energy stored in those carbohydrates. Your muscles contain a buffer of glycogen, approx. 300~ gr for the average Joe (give or take). The glycogen can be broken down to glucose-6-phosphate, which can then enter glycolysis. The glucose-6-phosphate is broken down to 2 pyruvate and yields 3 ATP netto (2 when derived from glucose, rather than glycogen, due to a first enzymatic step which requires 1 ATP). The enzymatic steps of glycolysis are controlled by ATP, AMP, ADP and other factors, factually integrating the energy status of the muscle (primarly through allosteric regulation of enzymes, especially phosphofructokinase).

The third system, the oxidative system, refers to the breakdown of carbohydrates and fatty acids, requiring oxygen to 'burn' them (citric acid cycle). The yield of this is much higher than for glycolysis, but the process is way slower.

In essence, all are regulated by the concentration of substrates and products, as well as through allosteric regulation (binding of a molecule at a different site, inhibiting or activating the enzyme, often by intermediates of the pathways themselves). Additionally, there is some long-term regulation through gene expression (e.g. up- or down-regulating expression of genes involved in these pathways), mostly by hormones.

Edit: Well, I guess this is described in any basic biochemistry book (I'm very fund of the book 'Fundamentals of Biochemistry: Life at the Molecular Level'). If you want to see a description of these energy systems in a more exercise related context (since you were aiming at myocytes) I suggest reading Strength and Condition: Biological Principles and Practical Applications from Marco Cardinale et al., and the NSCA book Essentials of Strength and Conditioning.

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It would be great if you could also provide some references as an edit to this response! – Bez May 26 '14 at 17:31
Added a couple. – pbond May 26 '14 at 17:44

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