ENERGY SUPPLY
Clearly ATP is used up in large quantities during muscle contraction. Its levels must not be allowed to fall significantly since this will affect detachment of the myosin heads, and contraction will be impaired. (Rigor complexes will be formed). As it is used, ATP is rapidly regenerated from another high energy compound creatine phosphate (CrP), in a reaction catalysed by the enzyme creatine phospho-kinase (CPK). CrP serves as an ATP buffer, but can sustain the ATP levels in an active muscle only for a few seconds. Depleted CrP is later regenerated as the ATP stores build up during rest periods: ADP + CrP
º ATP + Cr.ATP for rapid, powerful contractions can be obtained by anaerobic glycolysis, using glucose from the blood (to produce 2 ATP molecules per glucose molecule) or stored glycogen (to produce 3 ATP molecules per glucose molecule). Pyruvate and NADH + H+ are produced by this process of glycolysis. All the enzymes for this process are present in the cytosol (sarcoplasm / myoplasm), and the rate of diffusion of oxygen from the blood into the muscle is not a limiting factor. ATP produced in this way therefore, although limited in quantity, is rapidly and readily available to the contractile mechanism. If oxygen supplies are inadequate, the pyruvate interacts with the NADH and H+ to form lactic acid, which builds up in the cell and causes fatigue.
In the presence of adequate supplies of O2 much more ATP can be obtained by oxidative phosphorylation, albeit more slowly and less readily available. This involves a progressive breakdown of pyruvate or Free Fatty Acids (FFAs) to CO2 and water. Through oxidative phosphorylation 36 molecules of ATP can be obtained per glucose molecule. Over 140 molecules of ATP can be obtained per FFA molecule. Oxidative burning of fats therefore is clearly a much more efficient way of producing ATP. All the enzymes for this process are located in the mitochondria, and the process is heavily dependent upon the availability of oxygen.
Resting muscles and muscles working at low intensity preferentially burn FFAs. Muscles working at high intensity preferably burn glycogen. Exercise sustained for more than about 30 min also begins to mobilise FFAs as a preferred energy source as the glycogen stores diminish.