The energy source

So far we have concentrated on the output side of the energy balance equation. It is now time to consider the question, what chemical changes supply the energy for muscular contraction?

Pyruvic acid

Lactic acid

Glycogen

Pyruvic acid

Carbon dioxide + water

37 mois ADPATP per glucose unit

Fig. 9.14. An outline of the breakdown of glycogen with the release of energy (in the form of ATP) in respiration.

Energy for all bodily activities is ultimately derived from the food. Food energy is transported to the muscle as glucose or fatty acids and may be stored there as glycogen (a polymer of glucose). Respiration of these substances within the muscle cells results in the production of adenosine triphosphate (ATP) from adenosine diphosphate (ADP), as is indicated in Fig. 9.14. ATP appears to be the immediate source of energy for a large number of cellular activities.

The 'high energy phosphate' can be transferred from ATP to creatine (Cr), forming creatine phosphate (CrP):

This reaction is catalysed by the enzyme creatine phosphotransferase; it is readily reversible, so that the creatine phosphate can form a short-term 'bank' of high energy phosphate.

This scheme is now familiar to all who study elementary biochemistry, but it is worth examining some of the evidence that it applies to muscle. In 1950 A. V. Hill issued a famous 'challenge to biochemists' in which he said that if ATP really was the immediate source of chemical energy, then it should be possible to demonstrate that ATP was broken down during a contraction in living muscle.

The general method used in the experiments that followed was to use metabolic inhibitors to prevent the resynthesis of high energy phosphate, and then to compare its concentration in two muscles of which only one had been stimulated. The muscles had to be very rapidly frozen after the contraction (by plunging them into liquid propane, for example) so as to prevent any further biochemical change.

R. E. Davies and his colleagues used the substance 1-fluoro-2,4-dini-trobenzene (FDNB) to block the action of creatine phosphotransferase in frog muscles, so that ADP could not be rephosphorylated to ATP by the breakdown of creatine phosphate. In one set of experiments they found that the stimulated muscles lost on average 0.22 fxmoles of ATP per gram of muscle in an isotonic twitch. Now the heat of hydrolysis of the terminal phosphate bond of ATP is about 34 kJ/mole, so the breakdown of 0.22 fxmoles should release about 7.5 X 10~3 J. The work done by the muscle amounted to 1.7 X 10~3J per gram of muscle. This means that the ATP breakdown is more than sufficient to account for the work done in the twitch; the excess energy appears as heat.

Another way of measuring the 'fuel consumption' of the muscle is to measure the differences in creatine phosphate content of stimulated and unstimulated muscles. Here FDNB is not used because one wishes the transfer of phosphate from creatine phosphate to ATP to occur rapidly, as in the normal muscle. The resynthesis of creatine phosphate is prevented by poisoning the muscle with iodoacetate, which blocks one of the enzymic reactions in the breakdown of glycogen, in an atmosphere of nitrogen. Under these conditions there is less creatine phosphate in the stimulated muscle than in the unstimulated one.

It has proved quite difficult to draw up a precise balance sheet for the energy changes in muscle. D. R. Wilkie made some careful measurements on the energy output (heat + work) and creatine phosphate breakdown in frog muscles during a variety of different types of contraction. His results (Fig. 9.15) showed that the energy output is linearly proportional to the breakdown of creatine phosphate, with 46.4 kJ of energy being produced for each mole of creatine phosphate broken down.

However, calorific measurements suggest that the hydrolysis of creatine phosphate should yield only about 32 kJ per mole. The gap between expectation and observation became known as the 'unexplained energy'. More recent results (discussed by Homsher, 1987) suggest that at least part of this is connected with calcium-binding reactions associated with the activation processes in the muscle, but there are still some features of muscle energy balance which are not understood.

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