Carbohydrate metabolism has been shown to be affected in a number of ways by different models of hypokinesia. In vivo glycogen levels in the soleus muscle are known to be increased by short-term denervation and harness suspension. In addition, exposure to 7 days of hypogravity also caused a dramatic increase in glycogen concentration in this muscle. The biochemical alterations caused by unloading that may bring about these increases in glycogen storage in the soleus were sought

Henriksen, E. J.

Tischler, M. E.

English

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ORIGINAL NA. GAE C.7
OF POOR QUALIV
IBLE MECHANISM FOR CHANGES IN GLYCOGEN
LISM IN UNLOADED SOLEUS MUSCLE
 D
ik	 enriksen and Marc E. Tisehler
V
Department of Biochemistry
University of Arizona
Tucson, AZ 85724
Six days of tail-cast suspension caused marked
alterations V glycogen metabolism in the soleus
muscles from female rats. The in vivo glycogen
content in the soleus of suspended rats (TCS) was
significantly higher than in the soleus (SOL) from
tail-carted, weight bearing controls (TCWB). In
muscles depleted of glycogen by preincubation with
isoproterenol, glucose metabolism in the SOL from
TCS animals Was found to be significantly more
responsive to insulin (100 uU/ml) than the loaded
SOL.	 Measured parameters included net glycogen
turnover, glucose oxidation, and release of lac-
tate and pyruvate.	 No differential effect due to
insv.lin was observed for pyruvate oxidatior or for
pyruvate incorporation into glycogen. In non-
g.ycogen-depleted muscles, the uptake of 2-deoxy-
glucose In the presence of insulin Was much great-
er in the unloaded SOL than in the loaded muscle.
These results suggest that the accumulation of
glycogen in the SOL of TCS rats may be related to
the increased glucose uptake in the presence of
insulin. Accordingly the levels of glucose-6-
phosphate are higher in fresh SOL of TCS rats and
the ratio of glycogen synthetase d to glycogen
synthetasei is also greater.
Carbohydrate metabolism haz been shown to be
affe^_Led in a number of ways by different models
of hypokinesia. In vivo glycogen levels in the
soleus muscle are known to be increased by short-
term denervation (1) and harness suspension (2).
In addition, exposure to 7 days of hypogravity
also caused a dramatic increase in glycogen con-
centration in this muscle (3). We, therefore,
sought to find the biochemical alterations caused
by unloading that may bring about these increases
in glycogen storage in the soleus.
METHODS
Female Sprague-Dawley rats were subjected to 6
days of tall-cast suspension (4). Muscles wers
excised .rapidly, weighed, and either frozen in
liquid nitrogen or used in incubation studies. In
fresh-frozen tissue,	 glycogen (5,6), glycogen
aynthaae activity (7), and gluc03e-6-phosphate
levels 16) were assayed. Glueo a uptake was de-
termined by the uptake of 2-[ H]deoxy-D-glucose
using [ 14C]inulin to correct for extracellular
space (8). Isoproterenol (5 uM) was used in some
Incubation experlmen,.s to deplete the muscles of
glycogen, in an attempt to reduce dilution effects
on metabolite measurements. Muscles were preincu-
bated in isoproterenol/Krebs-Ringer bicarbonate
buffer, washed in fresh buffer, and then incubated
in buffer wit% and without insulin (10 -4 U/mil).
The musc^es were assayed for glucose incorporation
,into glycogen and the media for lactate and 
RIl^"'vate (6). Glucose oxidation Was measured by 1 CO 2
production from radio}ibeled glucose (9).
	 In a
separate experiment, CO2 production from radio-
labeled pyruvate and pyruvate incorporation Into
glycogen were determined.
RESULTS AND DISCUSSION
In vivo Determinations. Following six-days of
tail-cast suspension, the concentration of glyco-
gen, the percent of total of glycogen synthase.
and the level of glucc3e-6-phosphate were deter-
mined In fresh-frozen tissue.
	 As shown in Table
1, there was a much greater glycogen concentration oD
In the unloaded soleus muscle compared to the CO
Height bearing control. In the unloaded soleus, u1
the percentage of the glycogen synthase enzyme in M
the gluc03e-6-phosphate dependent form increased 1
significantly. In this same group of muscles, the Ln
level of glucose-6-phosphate was much higher than xQD
in the control muscles. 	 No changes were seer for
any of these parameters to the extensor digitorum
longus, a muscle known to be unresponsive to un-
loading Q).
These results suggest that the increased gly-
cogen levels in the unloaded soleus may be due to
an increased activity of the gly^_ogrn synthasa d as
a result of higher glucose-6-ph03phate levels.
In vitro Determinations.	 In an attempt to
explain the above in vivo observations, a number
of in vitro measurements were done.
	 Dramatic
changes in the responsiveness to insulin were
found in the unloaded soleus. As presented in
Table 2, the increase in 2-deoxy-glucose uptake
due to insulin was much greater in the unloaded
soleus as compared to the loaded muscle. Glucose
incorporation into glycogen was also increased to
a significantly greater extent by insulin in the
unloaded soleus. When viewed together with the
higher gluco3e-6-phosphate levels found in vivo
this increased uptake of glucose due to insulin in
the unloaded soleus could explain the Increased
glycogen levels found in this muscle. There Wss
no differential effect of insulin on any of these
parameters in the extensor digitorum longus.
In addition, glucose oxidation, and lactate
and pyruvate release were also increased to a
gr=ater degree due to insulin in the unloaded
soleus muscle (Table 2). However, pyruva a oxida-
tion and pyruvate incorporation into glycogen
(data not shown) were not differentially affected
by insulin in either soleus or extensor digitorum
longus muscles.
Concluding Remarks. From the data presented
in this paper, a possible mechanism to explain the
greater glycogen concentration in the unloaded
soleus muscle can be described. The increase in
the ratio of glycogen synthase. to glycogen syn-
thasei together with an increased uptake of glu-
cose due to insulin, which results in higher in
vivo levels of gluc03e-6-phosphate, may cause in
increase in glycogenesis. In the extensor digi-
torum longus, a muscle unresponsive to unloadinq,
these changes are not seen, and the in vivo con
centratlon of glycogen is the same in the loaded
and unloaded muscles.
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nTable 1. Changes in Fresh-frozen Suspended Soleus
Parameter	 S Difference from Weight-Bearing
Glycogen concentration +77b
Glycogen synthased	(S of total) +23a
Glucose-6-pho3phate +30a
Significance	 of	 difference:	 aP<0.05; bP<0.001.
Table 2.	 Changes in Insulin Response in Suspended
Soleus
Parameter	 S Difference from Weight-Bearing
2-Deoxyglucose Uptake + 74
Glucose Incorporation into Glycogen b +125
Glucose Oxidation + 26
Lactate and Per-uvate Release +417
aDetermined at 10-1 U/ml insulin
b Determined at 10-4 U/ml insulin with glycogen
depleted nuscles.
ACKNOWLEDGMENTS
This work was carried out during the tenure of
an Established Investigatorship from the American
Heart Association to M.E. Tischler. The work was
supported by National Aeronautics and Space Admin-
istration Grant NAGW-227.
ORIGINAL	 v
OF POOR QUALI Y
RUERENCES
1. Moruzzi, E.V., and E. Bergaminl. Effect of
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Oxygen and glucose uptake of disused hirilimb
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stract). 1985.
? Henriksen, E.J., M.E. Tischler, S. Jacob, and
P.H. Cook. Muscle protein and glycogen responses
to recovery from hypogravity and unloading by
tall-cast	 suspension.	 The	 Physiologist
	 28
(suppl.), this Sssue, 1985.
 
4. Jaspers, S.R., and M.E. Tischler. Atrophy and
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5. Hassid, W.Z., and S. Abraham. Chemical proce-
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Possible mechanism for changes in glycogen metabolism in unloaded soleus muscle

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