reactions involved in the degradation of glucose-
6-phosphate to pyruvate.
Glycogenolysis
The integrative nature of energy metabolism
ensures that the activation of muscle contraction
by Ca^2 +and the accumulation of the products of
ATP and phosphocreatinine (PCr) hydrolysis
(ADP, AMP, IMP, NH 3 and Pi) act as stimulators
of glycogenolysis, and in this way attempt to
match the ATP production to the demand. The
control of glycogenolysis during muscle contrac-
tion is a highly complex mechanism which can
no longer be considered to centre only around
the degree of Ca^2 +induced transformation of less
active glycogen phosphorylase bto the more
activeaform, as is suggested in many textbooks.
For some time it has been known that
glycogenolysis can proceed at a negligible rate,
despite almost total transformation of phospho-
rylase to the a form; for example, following
adrenaline infusion (Chasiotis et al. 1983). Con-
versely, an increase in glycogenolytic rate has
been observed during circulatory occlusion,
despite a relatively low mole fraction of the phos-
phorylaseaform (Chasiotis 1983). From this and
other related work, it was concluded that inor-
ganic phosphate (Pi) accumulation arising from
ATP and PCr hydrolysis played a key role in the
regulation of the glycogenolytic activity of
phosphorylasea, and by doing so served as a link
between the energy demand of the contrac-
tion and the rate of carbohydrate utilization
86 nutrition and exercise
(Chasiotis 1983). However, the findings that high
rates of glycogenolysis can occur within 2 s of the
onset of muscle contraction in conjunction with
only a small increase in Piand, more recently, that
glycogenolysis can proceed at a low rate despite
a high phosphorylase aform and Piconcentra-
tion, suggest that factors other than the degree of
Ca^2 +induced phosphorylase transformation and
Piavailability are involved in the regulation of
glycogenolysis (Ren & Hultman 1989, 1990).
Classically, both inosine monophosphate
(IMP) and adenosine monophosphate (AMP)
have been associated with the regulation of
glycogenolysis during exercise (Lowry et al. 1964;
Aragonet al. 1980). IMP is thought to exert its
effect by increasing the activity of phosphory-
lase b during contraction (the apparent Km
(Michaeli’s constant) of phosphorylase bfor IMP
is about 1.2 mmol · l–1intracellular water). AMP
has also been shown to increase the activity of
phosphorylaseb, but it is thought to require an
unphysiological accumulation of free AMP to do
so (the apparent Km of phosphorylase bfor AMP
is about 1.0 mmol · l–1intracellular water). In vitro
experiments have demonstrated that AMP can
bring about a more marked effect on glycogeno-
lysis by increasing the glycogenolytic activity of
phosphorylasea(Lowryet al. 1964). Because 90%
or more of the total cell content of AMP may be
bound to cell proteins in vivo, it has in the past
been questioned whether the increase in free
AMP during contraction is of a sufficient magni-
tude to affect the kinetics of phosphorylase a.
More recent work, however, demonstrates that aTable 6.1The amounts of substrate available and the maximal rates of energy production from phosphocreatine,
carbohydrate and lipid in a 70-kg man (estimated muscle mass, 28 kg).
Amount available Production rate
(mol) (mol · min-^1 )ATP, PCrÆADP, Cr 0.67 4.40
Muscle glycogenÆLactate 6.70* 2.35
Muscle glycogenÆCO 2 84 0.85–1.14
Liver glycogenÆCO 2 19 0.37
Fatty acidsÆCO 2 4000* 0.40- These pathways of substrate utilization will not be fully utilized during exercise.