small increase in AMP concentration (10mmol ·
l–1) can markedly increase the in vitroactivity of
phosphorylasea(Ren & Hultman 1990). Further-
more, in vivoevidence demonstrating a close
relationship between muscle ATP turnover and
glycogen utilization suggests that an exercise-
induced increase in free AMP and inorganic
phosphate may be the key regulators of glycogen
degradation during muscle contraction (Ren &
Hultman 1990).
Glycolysis
From the preceding discussions it can be seen
that the rate of glycogenolysis is determined
by the activity of glycogen phosphorylase.
However, it is the activity of phosphofructo-
kinase (PFK) that dictates the overall rate of gly-
colytic flux (Tornheim & Lowenstein 1976). PFK
acts as a gate to the flow of hexose units through
glycolysis and there is no other enzyme subse-
quent to PFK that is capable of matching flux rate
with the physiological demand for ATP. Stimula-
tion of glycogen phosphorylase by adrenaline
and/or exercise results in the accumulation of
glucose-6-phosphate demonstrating that PFK is
the rate limiting step in the degradation of
hexose units to pyruvate (Richter et al. 1986).
ATP is known to be the most potent allosteric
inhibitor of PFK. The most important activators
or deinhibitors of PFK are adenosine diphos-
phate (ADP), AMP, Pi, fructose-6-phosphate,
glucose 1–6 bisphosphate, fructose 1–6 and 2–6
bisphosphates and, under extreme conditions,
ammonia. Removal of the ATP-mediated inhibi-
tion of PFK during contraction, together with the
accumulation of the positive modulators of PFK,
is responsible for the increase in flux through the
enzyme during exercise and thereby is responsi-
ble for matching glycolytic flux with the energy
demand of contraction.
Hydrogen ion and citrate accumulation during
contraction have been suggested to be capable
of decreasing the activity of PFK and, thereby,
the rate of glycolysis during intense exercise.
However, it is now generally accepted that the
extent of this inhibition of glycolysis during exer-
cise is overcome in the in vivosituation by the
accumulation of PFK activators (Spriet et al.
1987).Pyruvate oxidation
It has been accepted for some time that the rate
limiting step in carbohydrate oxidation is the
decarboxylation of pyruvate to acetyl-coenzyme
A (CoA), which is controlled by the pyruvate
dehydrogenase complex (PDC), and is essen-
tially an irreversible reaction committing pyru-
vate to entry into the tricarboxylic acid (TCA)
cycle and oxidation (Wieland 1983). The PDC is a
conglomerate of three enzymes located within
the inner mitochondrial membrane. Adding to
its complexity, PDC also has two regulatory
enzymes: a phosphatase and a kinase which
regulate an activation–inactivation cycle. In-
creased ratios of ATP/ADP, acetyl-CoA/CoA
and NADH/NAD+activate the kinase, resulting
in the inactivation of the enzyme. Conversely,
decreases in the above ratios and the presence
of pyruvate will inactivate the kinase, whilst
increases in calcium will activate the phos-
phatase, together resulting in the activation of
PDC. Thus, it can be seen that the increases in
calcium and pyruvate availability at the onset of
contraction will result in the rapid activation of
PDC. These factors, together with the subsequent
decrease in the ATP/ADP ratio as contraction
continues, will result in continued flux through
the reaction (Constantin-Teodosiu et al. 1991).
Following decarboxylation of pyruvate by the
PDC reaction, acetyl-CoA enters the TCA cycle,
resulting in the formation of citrate, in a reaction
catalysed by citrate synthase. The rate of flux
through the TCA cycle is thought to be regulated
by citrate synthase, isocitrate dehydrogenase,
anda-ketoglutarate dehydrogenase. The activity
of these enzymes is controlled by the mitochon-
drial ratios of ATP/ADP and NADH/NAD+.
Good agreement has been found between the
maximal activity of a-ketoglutarate dehydroge-
nase and flux through PDC and the TCA cycle.
The last stage in pyruvate oxidation involves
NADH and FADH generated in the TCA cycle