normal (Bergström et al. 1967). This elevation in
muscle glycogen above normal is referred to as
glycogen supercompensation. Interestingly, the
effectiveness of the carbohydrate supplement to
speed muscle glycogen recovery during the slow
phase is directly related to the plasma insulin
response to the supplement (Ivy 1991).
Rapid phase of glycogen synthesis
after exercise
After exercise that is of sufficient intensity and
duration to deplete the muscle glycogen stores,
the activity of glycogen synthase is increased.
Glycogen synthase is the rate-limiting enzyme in
the glycogen synthesis pathway. Its activity is
strongly influenced by the muscle glycogen
concentration (Danforth 1965; Bergström et al.
1972; Adolfsson 1973). Generally, the percentage
of glycogen synthase in its active I-form is
inversely related to the muscle glycogen concen-
tration. That is, as the muscle glycogen concen-
tration declines the percentage of glycogen
synthase in the I-form increases. Conversely, as
the glycogen concentration increases the percent-
age of glycogen synthase in its inactive D-form
increases.
An exercise-induced increase in glycogen syn-
thase activity can catalyse the rapid restoration of
glycogen only if adequate substrate is available.
Thus, an additional factor that makes possible
the rapid increase in muscle glycogen after exer-
cise is a protracted increase in the permeability of
the muscle cell membrane to glucose (Holloszy &
Narahara 1965; Ivy & Holloszy 1981; Richter et al.
1982). The increase in glucose transport induced
by muscle contractile activity, however, reverses
rapidly in the absence of insulin, with most of
the effect lost within several hours (Cartee et al.
1989). This rapid decline in muscle glucose trans-
port appears to be inversely related to the muscle
glycogen concentration (Cartee et al. 1989;
Richteret al. 1984). Thus, the increase in mem-
brane permeability to glucose, together with the
activation of glycogen synthase, allows for an
initial rapid insulin-independent resynthesis of
muscle glycogen following exercise.
Slow phase of muscle glycogen synthesis
after exercise
After the direct, insulin-independent effect of
exercise on glucose transport subsides, it is
rapidly replaced by a marked increase in the sen-
sitivity of muscle glucose transport and glycogen
synthesis to insulin (Garetto et al. 1984; Richter
et al. 1984; Cartee et al. 1989). The magnitude of
the increased insulin sensitivity induced by exer-
cise can be extremely high, and result in muscle
glucose uptake and glycogen synthesis with
insulin concentrations that normally have no
detectable effect on either process (Cartee et al.
1989; Price et al. 1994). Furthermore, this increase
in sensitivity can be sustained for a very long
period of time, and does not appear to reverse
completely until glycogen supercompensation
has occurred (Garetto et al. 1984; Cartee et al.
1989). These findings therefore suggest that an
increase in muscle insulin sensitivity is a pri-
mary component of the slow phase of glycogen
synthesis.
Although the percentage of glycogen synthase
in the I-form may be as high as 80% immediately
after exercise-induced glycogen depletion, as
glycogen levels are normalized the percentage of
glycogen synthase I decreases back to the pre-
exercise level or lower in a negative feedback
manner (Danforth 1965; Bergström et al. 1972;
Adolfsson 1973; Terjung et al. 1974). However,
during the slow synthesis phase of glycogen,
glycogen synthase appears to be transformed
into an intermediate form which has a depressed
activity ratio, but enhanced sensitivity to activa-
tion by G6P (Kochan et al. 1981). Thus, a second
important component of the slow phase of glyco-
gen synthase appears to be an increase in the sen-
sitivity of glycogen synthase to G6P.
Another possible mechanism that might
contribute to an increase in glycogen synthe-
sis during the slow insulin-dependent phase of
recovery is an increase in GLUT-4 concentration.
Recently, Ren et al. (1994) reported an increase in
GLUT-4 protein in skeletal muscle of rats after a
single prolonged exercise session. The increase in
GLUT-4 protein was accompanied by a propor-