Metabolism
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Selection of fuels for muscle contraction
Even at rest, skeletal muscle accounts for more than one-fifth of total energy expenditure; obviously this increases greatly with exercise.
Muscle can use a variety of fuels, depending on the intensity of work being
performed, the duration of the exercise and also whether the individual is in
the fed or fasting state:
plasma glucose
muscle glycogen
triacylglycerol from plasma lipoproteins
triacylglycerol from adipose tissue reserves within the muscle
plasma non-esterified fatty acids
plasma ketone bodies
Skeletal muscle contains two types of fibres:
Experiment 1: Is ATP used in muscle contraction?
Rats
were anaesthetised and the gastrocnemius muscle on one hind limb was exposed
and subjected to electrical stimulation for three minutes to cause contractions.
The animals were killed, and both hind limbs were immediately immersed in liquid
nitrogen. The gastrocnemius muscles from both limbs were dissected out, and
the concentrations of ATP, ADP, creatine phosphate and creatine were measured.
| µmol /g muscle | unstimulated (at rest) | after stimulation |
| ATP | 5 | 4.9 |
| ADP | 0.01 | 0.11 |
| creatine phosphate | 17 | 1.0 |
| creatine | 0.1 | 16.1 |
What conclusions can you draw from these results?
Only a very small amount of ATP is apparently consumed (and the fall is accounted for by increased ADP). However, a considerable amount of creatine phosphate is consumed (and again the difference is accounted for by increased creatine).
Muscle contraction actually uses a large amount of ATP, which is required immediately. However, there is a significant time lag before the rate of oxidation of metabolic fuels increases to supply ATP. Remember that the total body pool of ATP is very small, but turns over rapidly.
Creatine phosphate acts as an intermediate store of phosphate to rephosphorylate
ADP to ATP until metabolic activity increases in response to increased demand
for muscle contraction.
This means that the concentration of ATP remains more or less constant, but creatine phosphate is depleted. After the muscle contraction ceases, or when the rate of oxidation of metabolic fuels has increased sufficiently, creatine will be rephosphorylated to creatine phosphate.
Creatine phosphate also acts to shuttle phosphate from the sites of ATP formation (from glycolysis in the cytosol and oxidative phosphorylation in mitochondria) to the sites in the cell where it is required for muscle contraction.
Creatinine is formed by non-enzymic cyclisation of creatine or creatine
phosphate; it is a metabolically useless product, and is excreted in the urine. 
A 70 kg man excretes approximately 16 mmol of creatinine per day; a 70 kg woman approximately 10 mmol / day.
Can you account for this gender difference in creatinine excretion?