Muscle creatine stores

Effects of Creatine Supplementation on Muscle Creatine Stores... 169... 

This muscle phosphotransferase (EC 2.132) catalyzes the reversible rephosphorylation of ADP to form ATP (/.c., T eq = [ATP][Creatine]/([Creatine phosphate] [ADP]) = 30). In resting muscle, creatine phosphate is synthesized at the expense of abundant stores of ATP intracellular creatine phosphate stores often reach 50-60 mM. If ATP is suddenly depleted by muscle contraction, its product ADP is immediately converted back into ATP by the reverse of the creatine kinase reaction. Depending on the pH at which the enzyme is studied, the kinetic reaction can be either rapid equilibrium random or rapid equilibrium ordered. A-Ethylglycocyamine can also act as a substrate. 

Oral creatine supplements first gained popularity among athletes in the early 1990s following the publication of a Karolinska Institute study that found that subjects who took creatine supplements experienced a significant increase in total muscle creatine content. In theory, increased creatine stores would increase PCr stored in the muscles, which would in turn provide a larger power supply for anaerobic muscle activity and exercise (short bursts of exercise which don t require oxygen). 

PCr acts on anaerobic adenosine triphosphate (ATP), the substance that powers muscle contractions. When ATP powers a muscle contraction, it loses one of its three phosphate molecules, changing from a triphosphate to a diphospate. The phosphate loss converts ATP to ADP (or adenosine diphosphate). Creatine phosphate provides an extra phosphate molecule for the ADP to convert or regenerate quickly back to ATP again and refuel muscle performance. Storing extra creatine in the skeletal muscles theoretically will provide for faster, more frequent ATP conversion. 

Creatine is an amino acid found in meat and fish. It is also produced in the human body by the liver, kidneys, and pancreas. Most of it is stored in skeletal muscle. Of the creatine stored, more than half of it is stored as creatine phosphate, which is involved in the production of energy (by cleavage of the high-energy phosphate bond) for high-intensity activity. It is the lack of creatine phosphate that causes fatigue... 

Approximately 95% of creatine is stored in the muscle. There is also a small amount of creatine ( 5%) found in the brain and testes. About two thirds of the creatine found in the muscle is stored as phosphoereatine (PCr), while the remaining amount... 

ATP stores in muscle are augmented or supplemented by stores of phosphocreatine. During periods of contraction, phosphocre-atine is hydrolyzed to drive the synthesis of needed ATP in the creatine kinase reaction ... 

The enzyme creatine kinase (CK) facilitates the transfer of phosphate and energy to a molecule of ADP to form ATP. Stores of creatine phosphate are sufficient to sustain approximately 15 more seconds of muscle contraction. Because this is a single-step process, it provides ATP very rapidly and is the first pathway for formation of ATP to be accessed. 

During the recovery period from exercise, ATP (newly produced by way of oxidative phosphorylation) is needed to replace the creatine phosphate reserves — a process that may be completed within a few minutes. Next, the lactic acid produced during glycolysis must be metabolized. In the muscle, lactic acid is converted into pyruvic acid, some of which is then used as a substrate in the oxidative phosphorylation pathway to produce ATP. The remainder of the pyruvic acid is converted into glucose in the liver that is then stored in the form of glycogen in the liver and skeletal muscles. These later metabolic processes require several hours for completion. 

Creatine Phosphokinase (CPK) (rabbit skeletal muscle Calbiochem) Resuspended with deionized RNAse free water to a final concentration of 14 mg/ml and stored at —20°. 

Undemutrition decreases the levels of ATP and phospho-creatine in muscle which affects function, but they return to normal levels with adequate energy intake (Table 18.3). The store of glutamine in muscle is also decreased. 

The majority of creatine in the body (over 95%) is stored in skeletal muscle. About two-thirds of this is bound with phosphates, forming the compound creatine phosphate (PCr). 

The urea cycle intermediate arginine can be condensed with glycine to form guanidinoacetate, which in turn is methylated by the methyl donor S-adenosyl methionine to creatine. The creatine is then phosphorylated to form creatine phosphate, a high-energy store found in muscle. 

Type II muscle fibers (white muscle, fast muscle) (Table 6) contribute to rapid muscle contraction by using energy obtained from stored ATP, and ATP production from creatine phosphate and anaerobic glycolysis of glycogen. However, the myoglobin content of these fibers is low [144,145], which may lead to less marked increases in serum myoglobin and creatine phosphokinase (CPK) in patients with ALPE. 

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