Enhanced Activity of the Purine Nucleotide Cycle of the Exercising

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The Journal of Clinical Endocrinology & Metabolism
Copyright © 2001 by The Endocrine Society
Vol. 86, No. 5
Printed in U.S.A.
Enhanced Activity of the Purine Nucleotide Cycle of the
Exercising Muscle in Patients with Hyperthyroidism
HIROKO FUKUI, SHIN-ICHI TANIGUCHI, YOSHIHIKO UETA, AKIO YOSHIDA,
AKIRA OHTAHARA, ICHIRO HISATOME, AND CHIAKI SHIGEMASA
First Department of Internal Medicine, Tottori University Faculty of Medicine, Yonago 683, Japan
ABSTRACT
Myopathy frequently develops in patients with hyperthyroidism,
but its precise mechanism is not clearly understood. In this study we
focused on the purine nucleotide cycle, which contributes to ATP
balance in skeletal muscles. To investigate purine metabolism in
muscles, we measured metabolites related to the purine nucleotide
cycle using the semiischemic forearm test. We examined the following
four groups: patients with untreated thyrotoxic Graves’ disease (untreated group), patients with Graves’ disease treated with methimazole (treated group), patients in remission (remission group), and
healthy volunteers (control group). To trace the glycolytic process, we
measured glycolytic metabolites (lactate and pyruvate) as well as
purine metabolites (ammonia and hypoxanthine).
In the untreated group, the levels of lactate, pyruvate, and ammonia released were remarkably higher than those in the control
I
T IS WELL known that myopathy often develops as a
complication of hyperthyroidism and is present in 33–
60% of thyrotoxic cases. Even though no muscular symptom
is observed, muscle hypotonia or fatigue can appear during
exercise (1, 2). Muscular symptoms will subside rapidly in
parallel with the recovery of thyroid function. According to
a study that involved 240 Japanese patients with thyroid
myopathy, major symptoms, such as muscle hypotonia or
amyotrophy, were related to the prevalence period and the
age of the patients at the time of thyrotoxicosis (3). Under
thyrotoxic conditions, ATP is promptly depleted, and myopathy easily develops, as the im glycogen content decreases
due to the suppression of glycogenesis and glycogenolysis.
This appears to be due to the same factors that cause glycogen storage disease, which is accompanied by a low capacity to perform exercise.
During vigorous exercise, glycogen is rapidly consumed,
and ATP consumption by the skeletal muscles increases
more than the ATP supply. At that time, the purine nucleotide cycle tries to catch up with the insufficient ATP supply
(Fig. 1) (4). The purine nucleotide cycle consists of three
intermediates, namely, AMP, inosine monophosphate (IMP),
and adenylosuccinic acid. Three enzymes, adenylate kinase,
5⬘-nucleotidase, and AMP deaminase, regulate the purine nucleotide cycle. The purine nucleotide cycle plays a critical role
in removing AMP from the exercising muscles to keep a high
Received October 23, 2000. Revision received January 18, 2001. Accepted February 8, 2001.
Address all correspondence and requests for reprints to; Shin-ichi
Taniguchi, M.D., Ph.D., First Department of Internal Medicine, Tottori
University Faculty of Medicine, Yonago 683, Japan. E-mail: [email protected]
grape.med.tottori-u.ac.jp.
group. Hypoxanthine release also increased in the untreated group,
but the difference among the patient groups was not statistically
significant. The accelerated purine catabolism did not improve after
3 months of treatment with methimazole, but it was completely normalized in the remission group. This indicated that long-term maintenance of thyroid function was necessary for purine catabolism to
recover.
We presume that an unbalanced ATP supply or conversion of muscle fiber type may account for the acceleration of the purine nucleotide
cycle under thyrotoxicosis. Such acceleration of the purine nucleotide
cycle is thought to be in part a protective mechanism against a rapid
collapse of the ATP energy balance in exercising muscles of patients
with hyperthyroidism. (J Clin Endocrinol Metab 86: 2205–2210,
2001)
adenylic acid energy level (ATP⫹0.5ADP/ATP⫹ADP⫹AMP),
which reflects ATP balance within the muscle cell. Skeletal
muscles have a high degree of AMP deaminase activity and a
low degree of 5⬘-nucleotidase activity compared with the myocardium (5). Therefore, skeletal muscles convert AMP to IMP,
which is a membrane-impermeable nucleotide, keeping IMP
within the myocyte and promptly produce AMP through adenylosuccinate via activation of adenylosuccinate synthetase
and adenylosuccinate lyase. The purine nucleotide cycle is
known to work only after vigorous long-term exercises. When
the im ATP content decreases after hard exercise, the AMP
degradation (IMP3inosine3hypoxanthine) progresses to
maintain a high constant intracellular energy level, and simultaneously ammonia production increases due to AMP deamination (AMP3 IMP⫹NH4). Accordingly, increases in hypoxanthine and ammonia, which are membrane-permeable
metabolites, are supposed to be important parameters of purine
catabolic reactions.
In glycogen storage disease types III, V, and VII, which are
due to a disorder of glycogenolysis, AMP deamination is
greatly accelerated because of an insufficient ATP supply
from glycogenolysis. As a consequence, large amounts of
ammonia and hypoxanthine are released into the blood. In
addition to glycogen storage diseases, an enhanced degradation of purine nucleotides is often observed in muscles of
patients with hypoparathyroidism or myopathy associated
with electrolyte disorders such as hypokalemia and hypophosphatemia (6), so enhanced production of purine metabolites could be associated with the impairment of ATP
balance within skeletal muscles.
We presume that myopathy in hyperthyroidism may also
belong to the group of skeletal muscle catabolic disorders
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FUKUI ET AL.
FIG. 1. The glycogenolysis/glycolysis and purine catabolism pathway
in muscle. 1) Adenylate kinase; 2) AMP deaminase; 3) 5⬘-nucleotidase.
caused by an impairment of ATP balance. Based on this
assumption, we used a semiischemic forearm exercise test to
investigate purine nucleotide cycle activity in the skeletal
muscles of patients with hyperthyroidism.
Subjects and Methods
Semiischemic forearm exercise test
As hard exercise is not an appropriate way to evaluate the purine
nucleotide cycle activity in patients with myogenic symptoms, we employed the semiischemic forearm exercise test. This test induces a rapid
decrease in the ATP content of the muscle and stimulates the purine
nucleotide cycle. After resting in the supine position for 30 min, a
manschette tourniquet was placed around the patient’s forearm, and a
pressure equivalent to the mean of the systolic/diastolic pressure was
applied. The patient was then asked to repeat a handgrip exercise at 70%
of maximum using a dynamometers at a rate of one handgrip per s for
2 min. Blood was sampled from the forearm vein to measure lactate,
pyruvate, ammonia, and hypoxanthine, at rest, immediately after exercise, and 10 min thereafter. The samples were kept at ⫺80 C, except
for those used to determine ammonia, which were collected in a frozen
heparinized test tube and measured within a few hours. Serum lactate,
pyruvate, and ammonia were analyzed using an enzymatic method
(COBAS-FARA, Roche, Basel, Switzerland). The procedure used to measure hypoxanthine was similar to those described previously (7, 8). In
brief, samples were centrifuged at 2600 ⫻ g at 4 C for 15 min. Methanol
(1.6 mL) was added to 0.4-mL plasma samples, which were then centrifuged at 29,000 ⫻ g for 5 min. Supernatants (1.0 mL) were decanted
at 40 C, and 0.2 mL 0.3% ammonia was added. The plasma hypoxanthine
concentration was then measured by high pressure liquid chromatography (model 510, Waters Corp., Milford, MA).
To determine the maximum increase (⌬) in each metabolite during
exercise, ⌬lactate, ⌬pyruvate, and ⌬ammonia (immediately after exercise ⫺ at rest) and ⌬hypoxanthine (10 min after ⫺ at rest) were calculated. Although it is reasonable to measure the difference between arterial and venous (A-V) concentrations of the metabolites to examine
muscle metabolism, we measured the changes in the metabolites in the
venous samples instead. In a preliminary study we compared the
changes in A-V concentration difference of ⌬hypoxanthine, ⌬ammonia
and ⌬lactate in the semiischemic forearm with the changes in venous
blood samples taken from the antecubital vein (n ⫽ 5). As shown in Fig.
2, there were no significant differences between the increases in the A-V
difference in ⌬hypoxanthine, ⌬ammonia, or ⌬lactate and their increases
in the antecubital vein. This means the increases in metabolites in blood
JCE & M • 2001
Vol. 86 • No. 5
FIG. 2. Comparison of the changes in A-V concentration difference in
hypoxanthine, ammonia, and lactate with their changes in the venous
sample of control subjects (n ⫽ 5). V, Concentration in the venous
sample; V-A, A-V concentration difference. Note that there were no
significant differences between them.
from the antecubital vein after exercise were almost equivalent to those
in the A-V concentration difference in the metabolites after exercise,
indicating that the changes in the level of metabolites in the venous
sample reflected the metabolism of the skeletal muscles instead of the
A-V concentration difference of the metabolites.
Subjects
Subjects were divided into four groups as follows: the untreated
group (untreated patients with Graves’ disease), the methimazole
(MMI)-treated group (patients with Graves’ disease treated with methimazole), the remission group (patients with Graves’ disease in remission), and the control group (healthy volunteers) (Table 1). We explained the purpose of the study to all subjects and obtained their
informed consent. The 20 patients in the untreated group consisted of
3 men and 17 women with a mean age of 44.4 yr who were complaining
of finger tremors, palpitation, weight loss, and squatting and/or arm-up
limitation in daily life.
Diagnosis of Graves’ disease was confirmed by the elevated free T3
level (24.88 ⫾ 9.22 pmol/L), the undetectable TSH level, and the positive
TSH binding inhibitory Ig and/or thyroid-stimulating antibody. Their
CPK, lactate dehydrogenase, and GOT levels were within normal range
(data not shown). Patients with periodic paralysis were not included.
MMI-treated subjects, basically drawn from the untreated group,
were treated with MMI and performed the semiischemic forearm test
again. Free T4 (17.25 ⫾ 5.41 pmol/L) normalized at 3 months after daily
administration of 30 mg MMI. Patients who were in a hypothyroid state
during MMI treatment were not included in the second trial. Eleven of
the 20 patients in the MMI-treated group who performed the second
exercise test still complained of muscle symptoms. The remission group
included treated patients in whom normalized thyroid function and
absence of thyroid-stimulating antibody and TSH binding inhibitory Ig
lasted for at least 1 yr.
Statistical analysis
The values of ⌬lactate, ⌬pyruvate, ⌬ammonia, and ⌬hypoxanthine in
each group were determined and compared by ANOVA. Differences
with P ⬍ 0.05 were considered statistically significant. The correlation
between lactate and ammonia was also estimated, and the regression
line was calculated.
ENHANCED ACTIVITY OF THE PURINE NUCLEOTIDE CYCLE IN HYPERTHYROIDISM
2207
TABLE 1. Serum free T3, free T4, and TSH concentrations in untreated Graves’ disease, MMI-treated Graves’ disease, remission, and
control groups
Graves’ disease
Untreated (n ⫽ 20)
Treated (n ⫽ 11)
Remission (n ⫽ 13)
44.4 ⫾ 5.99
(f17/m3)
24.88 ⫾ 9.22
60.49 ⫾ 23.17
⬍0.02
37.8 ⫾ 24.7
362 ⫾ 279
44.3 ⫾ 17.8
(f9/m2)
4.30 ⫾ 1.64
17.25 ⫾ 5.41
1.98 ⫾ 0.83
25.8 ⫾ 14.2
211 ⫾ 152
40.8 ⫾ 15.2
(f11/m2)
4.92 ⫾ 0.61
12.87 ⫾ 3.86
1.50 ⫾ 0.42
8.2 ⫾ 4.9
115 ⫾ 23
Age/sex
Free T3 (pmol/L)
Free T4 (pmol/L)
TSH (mU/L)
TBII (%)
TSAB (%)
Values are the mean ⫾
Control (n ⫽ 15)
50.6 ⫾ 20.7
(f11/m4)
5.53 ⫾ 0.92
18.02 ⫾ 5.15
1.86 ⫾ 0.38
Range
4.61– 8.91
11.53–28.31
0.3– 4.0
⫺10 to 10
⬍150
SD.
FIG. 3. Glycolytic metabolite production in the untreated group during exercise. ⌬Lactate and ⌬pyruvate in the
control (n ⫽ 15) and untreated groups
(n ⫽ 20) were estimated (immediately
after exercise ⫺ at rest). The raw values
of lactate were as follows: control group:
at rest, 1.01 ⫾ 0.34 mmol/L; after exercise, 2.44 ⫾ 0.66 mmol/L; and untreated
group: at rest, 1.00 ⫾ 0.19 mmol/L; after
exercise, 4.59 ⫾ 2.67 mmol/L. The
raw values of pyruvate were as follows:
control group: at rest, 67.01 ⫾ 23.03
␮mol/L; after exercise, 114.47 ⫾ 44.59
␮mol/L; and untreated group: at rest,
47.02 ⫾ 16.45 ␮mol/L; after exercise,
133.72 ⫾ 60.54 ␮mol/L.
Results
Release of glycolytic metabolites after exercise in the
untreated group
As expected, handgrip forearm exercise under semiischemic conditions resulted in poor ATP supply from aerobic
glycolysis and induced anaerobic glycolysis, which produces
lactate and pyruvate. After performing the handgrip exercise
for 2 min, lactate (3.58 ⫾ 2.62 vs. 1.94 ⫾ 1.20 mmol/L; P ⬍
0.05) and pyruvate (90.0 ⫾ 57.4 vs. 32.1 ⫾ 20.7 ␮mol/L; P ⬍
0.05) were significantly higher in the untreated group than
in the control group (Fig. 3, A and B).
Purine metabolic products after exercise in the
untreated group
Then we estimated purine metabolic products. After performing the handgrip exercise for 2 min, the level of ammonia released was significantly higher in the untreated group
(80.6 ⫾ 81.3 vs. 18.3 ⫾ 18.9 ␮mol/L; P ⬍ 0.05) than in the
control group. The level of hypoxanthine released was also
increased in the untreated group, but there was no significant
difference between the untreated group and the control
group (5.85 ⫾ 5.25 vs. 3.45 ⫾ 3.08 ␮mol/L; P ⫽ 0.184; Fig. 4,
A and B).
Changes in muscle metabolic products in association with
the recovery of thyroid function
To study the direct effect of thyrotoxicosis on purine nucleotide catabolism, the semiischemic forearm exercise test
was carried out in both the MMI-treated and remission
groups. Figure 5 shows the responses of glycolytic and purine metabolites in the untreated, MMI-treated, and the remission groups. Lactate release did not decrease significantly
in the MMI-treated group. In the remission group, the release
of lactate markedly decreased, but the reduction was not
significant either (Fig. 5A). The release of pyruvate and ammonia decreased in the MMI-treated group, but the reduction was not significant. However, in the remission group,
the levels of pyruvate and ammonia normalized (Fig. 5, B and
C). We also measured the level of hypoxanthine in the remission group (3.87 ⫾ 2.99 ␮mol/L), and it was almost identical to that in the control group (data not shown).
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JCE & M • 2001
Vol. 86 • No. 5
FIG. 4. Purine metabolite production
in the untreated group during exercise.
⌬Ammonia and ⌬hypoxanthine in the
control (n ⫽ 15) and untreated groups
(n ⫽ 20) were estimated (⌬ammonia,
immediately after exercise ⫺ at rest;
⌬hypoxanthine, 10 min after exercise ⫺
at rest). The raw values of ammonia
were as follows: control group: at rest,
28.87 ⫾ 13.61 ␮mol/L; after exercise,
69.55 ⫾ 59.30 ␮mol/L; and untreated
group: at rest, 22.76 ⫾ 7.13 ␮mol/L; after exercise, 87.67 ⫾ 79.28 ␮mol/L. The
raw values of hypoxanthine were as follows: control group: at rest, 1.43 ⫾ 1.64
mmol/L; after exercise, 4.66 ⫾ 3.57
mmol/L; and untreated group: at rest,
2.69 ⫾ 3.08 mmol/L; after exercise,
7.36 ⫾ 5.21 mmol/L.
FIG. 5. Change in glycolytic and purine metabolite production by treatment of thyrotoxicosis. ⌬Lactate, ⌬pyruvate, and ⌬ammonia in the
untreated (n ⫽ 20), MMI-treated (n ⫽ 11), and remission (n ⫽ 13) groups. The raw values of lactate (A)were as follows: untreated group: at
rest, 1.00 ⫾ 0.19 mmol/L; after exercise, 4.59 ⫾ 2.67 mmol/L; MMI-treated group: at rest, 1.08 ⫾ 0.21 mmol/L; after exercise, 4.36 ⫾ 1.59 mmol/L;
and remission group: at rest, 0.98 ⫾ 0.30 mmol/L; after exercise, 4.36 ⫾ 1.59 mmol/L. The raw values of pyruvate (B)were as follows: untreated
group: at rest, 1.00 ⫾ 0.19 mmol/L; after exercise, 4.59 ⫾ 2.67 mmol/L; MMI-treated group: at rest, 1.08 ⫾ 0.21 mmol/L; after exercise, 4.36 ⫾
1.59 mmol/L; and remission group: at rest, 0.98 ⫾ 0.30 mmol/L; after exercise, 4.36 ⫾ 1.59 mmol/L. The raw values of hypoxanthine (C) were
as follows: untreated group: at rest, 1.00 ⫾ 0.19 mmol/L; after exercise, 4.59 ⫾ 2.67 mmol/L; MMI-treated group: at rest, 1.08 ⫾ 0.21 mmol/L;
after exercise, 4.36 ⫾ 1.59 mmol/L; and remission group: at rest, 0.98 ⫾ 0.30 mmol/L; after exercise, 4.36 ⫾ 1.59 mmol/L.
Correlation between ⌬lactate and ⌬ammonia
To estimate the relationship between glycolysis and the
purine nucleotide cycle, we compared ⌬lactate and ⌬ammonia in these groups. A significant positive correlation between ⌬lactate and ⌬ammonia was found in all groups (Fig.
6). Therefore, ammonia production is supposed to be closely
related to lactate production. When regression lines were
calculated and compared among all groups, the slopes of the
regression line in the untreated, MMI-treated, remission and
control groups were 28.04, 34.5, 14.3, and 9.9, respectively.
Based on these results, as the slope value of the regression
line in the untreated group was much higher than that in the
control group, ammonia production seemed to be much
more accelerated than that of lactate in hyperthyroidism.
Therefore, purine catabolism through the activation of the
purine nucleotide cycle appeared to be more accelerated than
ENHANCED ACTIVITY OF THE PURINE NUCLEOTIDE CYCLE IN HYPERTHYROIDISM
2209
FIG. 6. Correlation between ⌬lactate
and ⌬ammonia. The regression lines of
the untreated (f), MMI-treated (䡺), remission (F), and control (E) groups were
calculated, and each regression formula
is shown.
glycolysis in hyperthyroidism. Such an enhancement of purine catabolism persisted for at least 3 months during MMI
treatment, and it normalized after long-term maintenance of
thyroid function. Simultaneously, we also estimated the correlation between ⌬lactate and ⌬hypoxanthine in each of the
four groups, but there was no significant correlation between
them (data not shown). This discrepancy between ⌬ammonia
and ⌬hypoxanthine seemed to be a feature peculiar to patients with Graves’ disease.
Discussion
It is well known that patients with hyperthyroidism
present muscle hypotonia, fatigue, and myopathy. Several
studies pointed out that hyperthyroidism alters the ATP
energy balance of skeletal muscle by increasing glycolysis or
glycogenolysis (9 –11). In the present study we focused on
purine nucleotide metabolism of muscles, which is regulated
by the purine nucleotide cycle in hyperthyroidism and also
contributes to ATP energy balance.
In this study an elevation of serum lactate and pyruvate
was observed after forearm exercise in patients with hyperthyroidism. Lactate and pyruvate are the metabolic products
of glycolysis/glycogenolysis, and the glycolysis/glycogenolysis process is supposed to be accelerated in hyperthyroidism. This was not surprising, as the consumption of ATP
in hyperthyroidism is known to be much greater than that in
the normal state. Under semiischemic conditions, aerobic
glycolysis tends to be suppressed, and instead, anaerobic
glycolysis starts to work to catch up with the reduction of
aerobic ATP supply. Then lactate, an anaerobic glycolytic
metabolite, should be produced and released into the
blood flow. The enhancement of muscle glycolysis in hyperthyroidism has been demonstrated by Kruk et al. (12).
They observed that the lactate level was higher in hyperthyroidism than in healthy controls. These results indicated that in hyperthyroidism the muscle ATP content
decreased, and this decrease was followed by enhanced
glycolysis/glycogenolysis.
Furthermore, an elevation of hypoxanthine and ammonia
after forearm exercise was observed in thyrotoxic patients.
This indicated that the activity of the purine nucleotide cycle
was clearly accelerated in hyperthyroidism. In general, an
acceleration of purine catabolism occurs due to the disturbances of the glycolytic systems similar to that in muscle
phosphorylase deficiency (glycogen storage disease type V)
(7). This is because ADP is rapidly converted to AMP by adenylate kinase when the ATP supply is disturbed by impairment of the glycolytic system. Additionally, once the purine
nucleotide cycle is activated (IMP3inosine3hypoxanthine),
hypoxanthine and ammonia production is augmented by
AMP deamination.
We observed a significant increase in ammonia after exercise in patients with hyperthyroidism. However, the difference in hypoxanthine release between hyperthyroidism
and healthy controls was not significant, although the average value appeared to be higher in thyrotoxic patients. This
result was puzzling, because accelerated purine catabolism
should produce hypoxanthine in parallel with ammonia as
metabolites. In glycogen storage disease type V, which accompanies the acceleration of the purine nucleotide cycle,
hypoxanthine production strongly correlates with ammonia
production (7). We propose the following possibilities to
explain this discrepancy. When IMP is converted to hypoxanthine, the salvage process (IMP to AMP) also works,
which influences the release of hypoxanthine independently
of the release of ammonia (13). Although the conversion of
IMP to inosine is regulated by 5⬘-nucleotidase (Fig. 1), this
enzyme is controlled by the intracellular pH. In hyperthyroidism, semiischemic exercise rapidly increases im lactate,
and accumulated lactate reduces the im pH level. Such a pH
reduction may suppress 5⬘-nucleotidase activity, and the
conversion of IMP to inosine would be suppressed; therefore,
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FUKUI ET AL.
the subsequent hypoxanthine production may also be decreased. Another explanation is that thyrotoxicosis directly
suppresses 5⬘-nucleotidase activity, and then hypoxanthine
production is suppressed. To elucidate the hypoxanthine
result, it will be necessary to measure 5⬘-nucleotidase activity
and IMP and inosine production in the exercising muscle in
patients with hyperthyroidism.
It is now clear that both glycolysis and purine nucleotide
catabolism are remarkably accelerated in hyperthyroidism.
There are several explanations for the mechanisms involved
in the acceleration of purine metabolism. In hyperthyroidism, a large amount of ATP is supposed to be consumed
during exercise compared with that in healthy controls, and
the ATP supply from the glycolytic system is rapidly depleted, which automatically advances to purine catabolism
(14). Other studies pointed out changes in the composition of
muscle fiber type induced by hyperthyroidism. Fitts et al.
reported that type II fibers (anaerobic glycolytic muscle) in
rat muscles were increased by the administration of T3 and
T4 (15). The content of type I fibers in hypothyroidism is
higher than that in healthy subjects, and it is decreased by T4
administration (16). Moreover, Celsing et al. found that type
I fibers could be converted to type II fibers in skeletal muscles
of patients with hyperthyroidism (17). It is well known that
AMP deaminase activity of type II fibers is higher than that
of type I fibers. Therefore, the whole AMP deaminase activity
in muscle could be enhanced by the conversion of type I to
type II fibers in hyperthyroidism. Moreover, it was reported
that the im glycogen content in hyperthyroidism was remarkably lower than that in healthy controls. Celsing et al.
demonstrated that the glycogen content was markedly lower
in type I fibers in hyperthyroidism using biopsy specimens
of muscles (17). Taken together, we presume the following
possibilities for enhanced purine catabolism in hyperthyroidism: 1) an increase in ATP consumption due to the augmented basal metabolic rate; 2) the acceleration of AMP
deaminase activity by the conversion of muscle fiber type;
and 3) a poor supplementation of ATP due to the low im
glycogen content. As mentioned above, purine catabolism
contributes to the ATP energy balance in skeletal muscle. The
acceleration of purine catabolism results in the rapid conversion of AMP to IMP, which maintains the im energy
charge. Thus, acceleration of purine catabolism may be a
protective adjustment of skeletal muscles of patients with
hyperthyroidism to avoid a rapid collapse of ATP energy
balance.
It seemed important to determine whether the improvement of hyperthyroidism was associated with the normalization of glycolysis and purine nucleotide catabolism. As
shown in Fig. 5, purine catabolism normalized in parallel
with the recovery of thyroid function. All responses of metabolites, except lactate, completely improved in the remission group. This result again confirms that purine metabolism of skeletal muscles is directly affected by thyrotoxicosis.
In this respect it is noteworthy that a positive correlation
between lactate and ammonia was observed and that the
regression line did not improve after 3 months of treatment
with MMI (Fig. 5, untreated vs. MMI-treated group). This
result suggests that purine catabolism (AMP3 IMP ⫹ NH4)
is more accelerated in hyperthyroidism than in anaerobic
glycolysis (glycogen3lactate). The maintenance of thyroid
function for 2 months was not enough to improve the accelerated purine catabolism in skeletal muscles. However,
the values of the regression line in the remission group were
almost identical to those in healthy controls, indicating that
long-term maintenance of thyroid function is necessary to
improve the purine metabolic change in muscles in hyperthyroidism. Complete recovery of fiber type change or glycolysis in hyperthyroidism may require a long-term maintenance of thyroid function. This observation is compatible
with clinical findings, as muscular symptoms in patients
with Graves’ disease tend to continue even after the complete
recovery of thyroid function by MMI treatment.
In conclusion, our study revealed that glycolysis and purine catabolism were remarkably accelerated in hyperthyroidism, purine catabolism would normalize in association
with a long-term maintenance of thyroid function, and thyrotoxic myopathy could be closely related to the acceleration
of purine catabolism.
Acknowledgment
We thank Prof. Kazumitsu Hirai for his continuous encouragement.
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