Figure 3.10 The role of ATP in muscle contraction.
It was noted above that the small amount of ATP in the body turns over rapidly, and ADP is rapidly rephosphorylated to ATP However, neither the small amount of ATP in muscle nor the speed with which metabolic activity can be increased, and hence ADP can be rephosphorylated, matches the demand for ATP for rapid or sustained muscle contraction. Muscle contains about four times more creatine phosphate than ATP; as shown in Figure 3.11, this acts as a reservoir or buffer to maintain a supply of ATP for muscle contraction until metabolic activity increases. Creatine phosphate is sometimes called a phosphagen because it can be used to rephosphorylate ADP to
Creatine is not a dietary essential; as shown in Figure 3.12, it is synthesized from the amino acids glycine, arginine and methionine. However, a single serving of meat will provide about 1 g of preformed creatine, whereas the average daily rate of de novo synthesis is 1—2 g. Both creatine and creatine phosphate undergo a non-enzymic reaction to yield creatinine, which is metabolically useless and is excreted in the urine. Because the formation of creatinine is a non-enzymic reaction, the rate at which it is formed, and hence the amount excreted each day, depends mainly on muscle mass, and is therefore relatively constant from day to day in any one individual. This is commonly exploited in clinical chemistry; urinary metabolites are commonly expressed per mole of creatinine, or the excretion of creatinine is measured to assess the completeness of a 24-hour urine collection. Obviously, simple concentration of urinary metabolites is not a useful measurement, as the concentration will depend on the volume of urine excreted, and in turn this depends on fluid intake and fluid losses from the body. There is normally little or no excretion of creatine in urine; significant amounts are excreted only when there is breakdown of muscle tissue.
Because of its role as a phosphagen in muscle, creatine supplements are often used as a so-called ergogenic aid, to enhance athletic performance and muscle work output. In subjects who have an initially low concentration of creatine in muscle, supplements of 2—5 g of creatine per day do increase muscle creatine; however, in people whose muscle creatine is within the normal range, additional creatine has little or no effect.
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Figure 3.11 The role of creatine phosphate in muscle.
There is control over the uptake and retention of creatine in muscle cells. There is little or no evidence that supplements of creatine have any beneficial effect on muscle work output or athletic performance, although obviously people whose muscle creatine was initially low will benefit.
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