Effect of oral creatine supplementation on skeletal muscle

Effect of oral creatine supplementation
phosphocreatine
resynthesis
on skeletal
muscle
P. L. GREENHAFF,
K. BODIN,
K. SODERLUND,
AND E. HULTMAN
Queens Medical Center, Department
of Physiology
and Pharmacology,
University
Medical
NG7 2UH Nottingham,
United Kingdom;
and Department
of Clinical Chemistry,
Karolinska
Institute,
Huddinge
University
Hospital,
S-141 86 Huddinge,
Sweden
energy metabolism;
recovery
CREATINE (Cr) is a naturally
occurring compound found
principally in skeletal muscle, and in-its free and phosphorylated forms it plays a pivotal role in the regulation
and homeostasis of skeletal muscle energy metabolism
(5, 6, 24, 31). It is now generally accepted that the
maintenance of phosphocreatine (PCr) availability is
important to the-continuation of muscle force production (19,23). Endogenous synthesis of Cr occurs in liver,
kidney, and pancreas. However, it has been known for
some time that oral ingestion of Cr in the form of meat
and fish or supplements will add to the whole body Cr
pool (7,8,18). It has recently been shown that ingestion
of 20-30 g Cr/day for several days can lead to a > 20%
increase in human skeletal muscle total Cr content, of
which N 20-30% is in the form of PCr (16). It also
appears that muscle Cr uptake is augmented if submaximal exercise is performed during the period of supplementation (16). Further recent evidence of our own (11,
17) and others (1) demonstrates that Cr ingestion can
significantly increase the amount of work that can be
performed during repeated bouts of maximal exercise. It
was postulated in these studies that the ergogenic effect
of Cr ingestion may be attributable to an increased
muscle Cr content, accelerating PCr resynthesis between exercise bouts. As a result, the required rate of
ADP rephosphorylation
would have been sustained
longer during contraction. This suggestion was sup0193-1849/94
$3.00
Copyright
ported by the lower accumulation of plasma ammonia
and hypoxanthine, which was observed during exercise
after Cr ingestion.
The aim of the present experiment was therefore to
investigate the effect of oral Cr ingestion on muscle PCr
resynthesis after a contraction-induced
depletion of
muscle PCr stores.
METHODS
Eight male subjects volunteered
to take part in the present
experiment.
All undertook
some form of recreational
exercise,
but none was highly trained. Their physical characteristics
were as follows (mean * SE): age 29.1 t 1.6 yr; weight 80.0 t
4.7 kg, and height 184 t 3 cm. Before the commencement
of
the study, all subjects gave voluntary consent to take part in
the experiment;
all were informed of the experimental
procedures to be undertaken
and were aware that they were free to
withdraw
from the study at any point. The study was approved
by the Ethics Committee
of the Karolinska
Institute,
Huddinge, Sweden.
Each subject reported to the laboratory
in a “normal”
fed
state on the morning
of the study, having abstained
from
strenuous physical exercise the previous day, and had their
nude body weight recorded. The experiment
began with each
subject lying in a semi-supine position on a bed with both legs
flexed over one end at an angle of 90’. The leg to be investigated on this visit was chosen at random and was attached via
an ankle strap to a strain gauge built into the frame of the bed.
The subject was then asked to perform three maximal voluntary contractions
to determine the maximal voluntary isometric force of the knee extensors. The isometric force produced
was measured with a strain gauge (AB Bofors, Karlskoga,
Sweden) and, after amplification
(direct current amp Medelec
AD6; Medelac, Surrey, UK), was displayed on an oscilloscope
and recorded on ultraviolet
paper (Medelec). The leg was then
prepared for electrical
stimulation,
as described previously
(20). Briefly, the muscles of the anterolateral
portion of the
thigh were stimulated
to contract with square wave impulses
of 0.5 ms duration,
at a frequency of 50 Hz, and at a voltage
sufficient to elicit maximal contraction.
Approximately
35% of
the musculature
that extends the knee is activated in this way
(20). Stimulation
was intermittent
with 20 trains of 1.6 s
stimulation
being separated by rest periods of 1.6 s. The total
contraction
time was therefore
32 s. Before initiation
of
stimulation
(30 s), a cuff surrounding
the proximal portion of
the thigh was inflated (250 mmHg) to occlude limb blood flow
and remained inflated until a muscle biopsy had been obtained
from the vastus lateralis immediately
after the final contraction (4). This stimulation
protocol was chosen because it has
been previously shown to result in almost total degradation
of
muscle PCr stores (12). The cuff was then deflated, and
further muscle biopsy samples were obtained after 20,60, and
120 s. During
this time, subjects remained
resting in a
semi-supine position on the bed.
Subjects reported back to the laboratory
10 days later for
the second part of the study. However, for the 5 days preceding
o 1994 the American
Physiological
Society
E725
Downloaded from http://ajpendo.physiology.org/ by 10.220.32.247 on September 30, 2016
Greenhaff,
P. L., K. Bodin,
K. Soderlund,
and E.
Hultman.
Effect of oral creatine supplementation
on skeletal
muscle phosphocreatine
resynthesis.
Am. J. Physiol.
266
(Endocrinol.
Metab. 29): E725-E730,
1994.-Biopsy
samples
were obtained
from the vastus lateralis
muscle of eight
subjects after 0, 20, 60, and 120 s of recovery from intense
electrically evoked isometric contraction.
Later (10 days), the
same procedures
were performed
using the other leg, but
subjects ingested 20 g creatine (W/day
for the preceding 5
days. Muscle ATP, phosphocreatine
(PCr), free Cr, and lactate
concentrations
were measured, and total Cr was calculated as
the sum of PCr and free Cr concentrations.
In five of the eight
subjects, Cr ingestion substantially
increased muscle total Cr
concentration
(mean 29 t 3 mmol/kg dry matter, 25 t 3%;
range 19-35 mmol/kg dry matter, 15-32%) and PCr resynthesis during recovery (mean 19 t 4 mmol/kg dry matter, 35 t
6%; range 11-28 mmol/kg
dry matter,
23-53%).
In the
remaining
three subjects, Cr ingestion
had little effect on
muscle total Cr concentration,
producing
increases of 8-9
mmol/kg dry matter (5-7%), and did not increase PCr resynthesis. The data suggest that a dietary-induced
increase in
muscle total Cr concentration
can increase PCr resynthesis
during the 2nd min of recovery from intense contraction.
School,
E726
PHOSPHOCREATINE
RESYNTHESIS
CREATINE
INGESTION
‘60 A
1
1
MO.
E
W
g
MO.
0
E
5
130-
p
lzo.
llO-
100
1
Pre-Ingestion
30
G
Post-Ingestion
B
03
l 7
i
02
01
04
RESULTS
Mean body weight before Cr ingestion was 80.0 t 4.7
kg. After Cr ingestion, body weight had increased to
81.6 t 4.8 kg (P < 0.05). A body weight increase was
observed in seven of the eight subjects.
Table 1 shows mean muscle ATP, PCr, free Cr, total
Cr (sum of PCr + free Cr), and lactate concentrations
for all eight subjects during recovery from contraction,
Table 1. Muscle ATP, PCr, free Cr, total Cr, and lactate
concentrations in muscle biopsy samples
Seconds
0
20
Before
ATP
PCr
Free Cr
Total Cr
Lactate
20.6
8.8
115.3
125.5
96.3
+
2
2
+
2
0.4
0.6
3.1
3.3
4.3
ATP
PCr
Free Cr
Total Cr
Lactate
19.0
8.9
134.12
142.9
102.6
2 0.6
2 1.0
3.0
t 2.3
+ 4.4
120
Cr Ingestion
22.0
27.7
96.3
124.0
82.0
After
60
2
+
+
2
2
0.5
2.6
4.7
3.3
7.7
21.6
49.4
74.4
123.9
70.12
+
2
+
+
22.0
52.2
90.7
142.8
75.5
2
+
t
+
t
0.4
3.5
2.3
3.2
5.7
22.5
62.0
60.2
122.12
49.7
_+ 0.4
+ 4.3
2 2.8
3.4
f 5.6
0.7
2.1
4.0
2.4
6.8
22.5
70.12
73.0
143.0
49.9
k 0.6
3.3
+ 2.4
+ 2.2
+ 8.0
Cr ingestion
20.2
24.6
118.6
143.2
89.12
2 0.5
+ 2.4
+, 3.9
+ 2.4
6.8
Values are means * SE; n = 8 subjects.
Muscle ATP, phosphocreatine (PCr),
free creatine
(Cr), total Cr, and lactate concentrations
(mmol/kg
dry matter)
in muscle biopsy samples obtained
at 0, 20, 60,
and 120 s after intense electrically
evoked isometric
contraction,
both
before and after oral Cr ingestion.
-20
d
Increase
.
.
1’0
in
TCr
2'0
after
Cr
-
ingestion
3'0
(mmol/kg
4'0
dm)
Fig. 1. A. individual
values for muscle total creatine
(TCr) concentration before
(CI) and after (N) creatine
(Cr) ingestion.
Subjects
have
been numbered
1-8, based on their initial muscle TCr content,
which
was calculated
to be equal to sum of PCr and free Cr. dm, dry matter.
B: individual
increases
in muscle
TCr after Cr ingestion
for same
subjects
in A, plotted
against
change
in PCr resynthesis
during
recovery
after
Cr ingestion.
Values
on y-axis
were calculated
by
subtracting
PCr resynthesis
during
2 min of recovery
before
Cr
ingestion
from corresponding
values after Cr ingestion.
before and after Cr ingestion. As expected, electrical
stimulation resulted in a decline in muscle ATP, almost
total degradation of muscle PCr, and marked increases
in muscle free Cr and lactate. All variables returned
toward resting values during recovery. On average, Cr
ingestion resulted in an - 15% increase in muscle total
Cr concentration. PCr resynthesis was similar during
the 1st min of recovery when comparing subjects before
and after Cr ingestion. However, during the 2nd min,
the mean rate of PCr resynthesis was increased by
-42% after Cr feeding. Despite this trend, the mean
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their second visit, each ingested 4 x 5 g (total 20 g) creatine
monohydrate
(Chemi, Linz, Austria). Subjects were instructed
to ingest a single 5-g dose in the early morning, at noon, in the
late afternoon, and in the evening. They were also instructed
to totally dissolve each 5-g dose in warm-hot
tea or coffee
before ingestion.
Upon reporting
back to the laboratory,
subjects underwent
exactly the same experimental
procedures
that they had performed
10 days previously,
but on this
occasion their other leg was electrically
stimulated
and biopsied.
Upon removal from the muscle, all biopsy samples were
immediately
frozen by plunging the biopsy needle into liquid
nitrogen. The time delay between the insertion of the biopsy
needle and freezing of the sample ranged from 3 to 5 s. All
samples were freeze dried and stored at -80°C until analyzed
at a later date.
After fat extraction with petroleum
ether, a portion of each
freeze-dried
muscle sample was dissected free of all visible
blood and connective tissue and was pulverized. The powdered
muscle was used for the spectrophotometric
determination
of
ATP, PCr, free Cr, and lactate concentrations
(15). Muscle
total Cr concentration
was calculated by summing PCr and
free Cr concentrations.
Unless stated otherwise,
values presented
in the text,
Tables 1 and 2, and Figs. 1 and 2 refer to means t SE.
Comparison
of metabolite
levels during recovery before and
after Cr ingestion was performed by using repeated-measures
two-way analysis of variance with respect to treatment
(with/
without
Cr) and time (O-120 s). When a significant
F value
was achieved (P < 0.05), Fisher’s test for pairwise comparisons was used to locate the difference between paired means
across treatments.
On all occasions, statistical difference was
declared at P < 0.05.
AFTER
PHOSPHOCREATINE
RESYNTHESIS
AFTER
CREATINE
Table 2. Muscle ATP, PCr, free Cr, total Cr, and lactate concentrations
samples obtained from responders and nonresponders
Before
0
Cr Supplementation,
60
20
(mmollkg
s
dry matter)
After
120
0
E727
INGESTION
in muscle biopsy
Cr Supplementation,
20
s
60
120
Responders
ATP
PCr
Free Cr
Total Cr
Lactate
20.12
7.8
111.6
119.4
95.7
+
+
k
2
0.2
0.9
4.1
3.5
8.3
22.2
26.3
93.0
119.2
88.3
2
+
*
4
2
0.7
0.3
5.4
3.5
7.3
20.9
42.9
76.4
119.3
77.8
t
f
t
+
+
0.4
2.4
4.3
3.4
8.5
22.2
55.1
62.3
117.4
54.7
+
k
+
+
+
0.5
2.3
3.8
3.7
7.2
18.4
7.4
138.7
146.12
110.3
iI 1.0
+ 0.9
+ 2.7”f
2.1$
2 5.0*
19.7
22.12
124.6
146.7
101.8
+ 0.6
3.3
+ 5.0-F
+ 2.0$
2 5.8
21.0
50.8
95.4
146.12
85.4
* 0.6
+ 3.0
+ 4.1*
2.1$
AI 7.8
22.3
71.8
74.12
145.8
53.2
+ 0.8
k 4.7*
4.4*
2 1.7$
+ 12.0
19.8
10.6
127.9
138.6
92.2
+ 0.2
IL 1.6
+ 5.2
f 4.2-f
* 3.2
20.9
28.0
110.6
138.6
72.12
t 1.0
2 3.9
~fr 3.6
f 4.4.f
6.8
23.2
54.0
84.4
138.4
62.5
+_ 1.2
+ 3.6
f 7.8
f 4.4”f
+ 9.4
22.9
67.2
71.1
138.3
44.5
+_ 1.0
+ 4.8
t 0.5
t 4.3t
+ 9.3
Nonresponders
21.4
10.2
120.2
130.4
97.12
+ 0.7
?.z0.1
f 4.8
* 4.7
3.5
21.6
29.7
100.8
130.4
79.7
+_ 0.8
f 5.4
+ 10.1
+ 4.9
t 13.6
22.5
58.12
72.0
130.0
59.9
f 0.4
4.4
+ 0.6
f 5.0
+ 4.2
23.1
73.5
56.6
130.1
41.3
f
+
+
t
2
PCr concentration
was not significantly
different at the
end of recovery when comparing treatments.
Figure
lA shows
the change in muscle total Cr
concentration
with Cr feeding for each subject. Subjects
have been numbered one to eight, based on their initial
muscle total Cr concentration.
As can be seen, five
subjects experienced
a marked 29 t 3 mmol/kg
dry
matter (25 t 3%) increase in total Cr concentration
(subjects l-4
and 7), which ranged from 19 to 35
mmol/kg dry matter or 15 to 32% of the initial total Cr
content. In particular,
the four subjects with the lowest
initial total Cr concentration
(120 mmol/kg dry matter;
subjects l-4) experienced the most dramatic increase in
total Cr (25-35 mmol/kg dry matter), which was equivalent to 20-32% of their initial total Cr concentration.
The remaining three subjects (subjects 5, 6, and S), each
had an initial total Cr concentration
of > 125 mmol/kg
dry matter, and each experienced
a relatively
small
increase in total Cr concentration
with Cr ingestion
(7-9 mmol/kg
dry matter),
which was equivalent
to
- 5% of their initial total Cr concentration.
Figure 1B shows, for each subject, the increase in
muscle total Cr after Cr ingestion plotted against the
change in PCr resynthesis
during the 2 min of recovery
after Cr ingestion
(the latter being PCr resynthesis
before Cr ingestion - PCr resynthesis
after Cr ingestion). As can be seen, the same five subjects who
experienced a substantial
increase in muscle total Cr
concentration
with Cr ingestion
also showed an increased rate of PCr resynthesis
during recovery after Cr
ingestion (mean 19 t 4 mmol/kg dry matter, 35 t 6%;
range 11-28
mmol/kg
dry matter,
23-53%).
Conversely, the three subjects who had a < 10 mmol/kg dry
matter increase in muscle total Cr concentration
with
feeding showed very little change or even a lower rate of
PCr resynthesis
during recovery after Cr feeding.
As in Table 1, Table 2 shows mean muscle metabolite
concentrations
during recovery: but on this occasion
subjects have been categorized into the following
two
groups: those who demonstrated
a marked increase in
0.5
7.6
4.0
4.9
5.2
muscle total Cr after oral Cr supplementation
(responders, n = 5) and those who showed only a small change
(nonresponders,
n = 3). For the sake of clarity, mean
muscle PCr and free Cr concentrations
during recovery
in the group of responders
have been plotted in Fig. 2.
Figure 2 shows data obtained before (open symbols) and
after (closed symbols) Cr feeding, and metabolite levels
are represented
as millimoles per kilogram dry matter
and millimoles per liter intracellular
water. Figure 2
illustrates
that muscle free Cr concentration
declined
during recovery on both occasions but that the free Cr
concentration
was higher throughout
recovery after Cr
ingestion. As expected, PCr resynthesis
began soon after
exercise, and concentrations
were almost identical during the first 40 s of recovery when comparing
treatments. However, during the remainder of recovery, the
rate of PCr resynthesis
was greater after Cr ingestion,
resulting in the mean muscle concentration
being 30%
higher at the end of recovery (P < 0.05). Table 2 also
demonstrates
that muscle lactate concentration
was
significantly
higher immediately after contraction
in the
group of responders.
With the exception of the reported small increase in
muscle total Cr concentration,
dietary Cr supplementation had no influence on muscle metabolite concentrations in the nonresponders.
DISCUSSION
The major finding of the present experiment is that
oral Cr ingestion markedly influences muscle Cr uptake
in those individuals who have a total muscle Cr concentration of close to or < 120 mmol/kg dry matter before
ingestion (Fig. lA), and these same individuals demonstrate an accelerated rate of PCr resynthesis after 1 min
of recovery from intense muscular contraction (Fig. 1B).
Cr ingestion has for some time been known to result
in an increase in the body Cr pool in humans (7, 8, 18).
Only recently has it been shown that the ingestion of a
dose similar to that used in the present experiment can
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ATP
PCr
Free Cr
Total Cr
Lactate
E728
PHOSPHOCREATINE
(s)
Fig. 2. Phosphocreatine
(PCr;
O,O)
and free creatine
(Cr; q I,~)
concentrations
measured
in muscle biopsy samples
obtained
after 0,
20,60, and 120 s of recovery
from intense contraction
before (0,~)
and
after (~,m) Cr ingestion.
Values
(means
IT SE) are represented
as
mmol/l intracellular
water and mmol/kg
dry matter
(dm). Differences
between
treatments
are indicated
as follows: **P < 0.01, *P < 0.05.
result in a substantial increase in muscle total Cr
concentration, of which -2O-30% was in the form of
PCr (16). In agreement with the present experiment, it
was found that the greatest increases were in those
individuals with the lowest prefeeding levels and that
muscle total Cr concentration did not increase above
- 155 mmol/kg dry matter with ingestion. Although not
commented on at the time, the largest increases were
also found in those individuals with a total muscle Cr
concentration close to or < 120 mmol/kg dry matter.
The total Cr concentration of human skeletal muscle
has been shown to be 124.4 t 11.2 mmol/kg dry matter
(mean -+ SD of 81 biopsy samples; see Ref. 15) and to
follow a normal distribution ranging from - 90 to 170
mmol/kg dry matter (13). A muscle total Cr concentration of - 120 mmol/kg dry matter should therefore not
be viewed as appreciably low. The present results demon-
AFTER
CREATINE
INGESTION
strate that “Cr loading” is achievable in normal healthy
individuals. However, they also suggest that increases
may be most dramatic in vegetarians, who obviously will
have a low dietary Cr intake and have been shown to
have a reduced total body Cr pool (9), and in patients
with diseased or atrophied skeletal and heart muscle,
where low total Cr concentrations have been observed
(10, 22). We are unclear about the factors that will
dictate muscle Cr content under normal dietary conditions. A comparison of the subjects who participated in
the present study showed no obvious differences in
lifestyle or eating habits.
As far as we are aware, the present experiment is the
first to show that an increase in muscle Cr concentration, resulting from dietary Cr supplementation, can
accelerate the rate of muscle PCr resynthesis during
recovery from exercise. We have recently demonstrated
that a regimen of Cr ingestion similar to that used in the
present experiment can significantly increase performance during repeated bouts of 400 and 1,000 m
running (17) and can reduce the decline in muscle
torque production seen during repeated bouts of maximal isokinetic contraction in humans (11). More recently, these findings relating to exercise performance
have been confirmed by others (1). The authors showed
that Cr ingestion significantly improved work capacity
during the 4th-6th s of ten 6-s sprints when each sprint
was interspersed with 30 s of rest. The depletion of
muscle PCr stores is generally considered to be one of
the limitations to muscle force production during maximal exercise (19, 23). In the above study (ll), it was
suggested that the increase in work capacity observed
after Cr ingestion may have been attributable to the rate
of PCr resynthesis being accelerated during recovery
between exercise bouts, possibly as a result of Cr
availability displacing the equilibrium reaction catalyzed by creatine kinase (CK). Thus it was postulated
that, after Cr ingestion, subjects began each bout of
exercise with PCr levels at a relatively higher level and
thereby may have delayed the depletion of muscle PCr
stores during exercise. In general, PCr resynthesis
follows an exponential curve after intense muscle contraction (14, 25, 26) and the half-time for resynthesis in
mixed-fibred human skeletal muscle is - 30-40 s (14). It
is generally accepted that the resynthesis of PCr during
recovery is mediated by mitochondrial
membranebound CK, thus linking oxidative ATP production to
cytoplasmic PCr resynthesis (5, 24, 31). Factors that
will undoubtedly influence the rate of resynthesis will
include free ATP, ADP, H+, and Cr concentrations, due
to their role in the CK equilibrium reaction. The in vitro
Michaelis constant (K,) values of CK for ATP and ADP
are relatively low, being -0.6 and 1 mmol/l, respectively (3). Conversely, the K, of CK for Cr is comparatively high, being close to 19 mmol/l. It is known that
muscle free Cr can range in concentration from - 13
mmol/l intracellular water at rest (16) to - 40 mmolll
intracellular water after maximal exercise (29). Thus,
during the initial stages of recovery from maximal
exercise, when the rate of mitochondrial ADP rephosphorylation to ATP will be at its highest, it is unlikely
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Recovery
RESYNTHESIS
PHOSPHOCREATINE
RESYNTHESIS
CREATINE
E729
INGESTION
matter of PCr were still present in the muscle. Thus the
pattern of PCr resynthesis in our previous study (11)
would have probably been more similar to that recorded
between 60 and 120 s in the present experiment.
On the basis of the lactate data shown in Table 2,
another explanation for the increase in exercise performance could be that anaerobic glycolysis made a greater
contribution to ATP production during exercise after Cr
ingestion ( - 10 mmol/kg dry matter). However, the
similarity in blood lactate concentrations in our previous study (11) and the lower lactate accumulation
observed after Cr ingestion by Balsom et al. (1) is not in
agreement with this suggestion. In the present experiment, muscle force production was not measured with
sufficient precision to enable it to be accurately related
to lactate accumulation.
In conclusion, the results demonstrate that oral Cr
ingestion markedly increases the muscle total Cr concentration of those individuals who have a concentration of
close to or < 120 mmol/kg dry matter before ingestion.
Furthermore, these same individuals show an accelerated rate of PCr resynthesis in the 2nd min of recovery
from intense muscular contraction, which has depleted
muscle PCr stores.
Present
addresses:
K. Bodin, Dept. of Clinical
Physiology,
Karolinska Institute,
Huddinge
University
Hospital,
S-141 86 Huddinge,
Sweden;
K. Soderlund,
Department
of Physiology
III, Karolinska
Institute,
Box 5626,114
86 Stockholm,
Sweden.
Address
for reprint
requests:
P. L. Greenhaff,
Queens
Medical
Center,
Dept.
of Physiology
& Pharmacology,
University
Medical
School, Nottingham
NG7 2UH, UK.
Received
22 October
1993; accepted
in final
form
22 December
1993.
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B. Sjodin,
and
E. Hultman.
Creatine
supplementation
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highintensity
intermittent
exercise.
Stand.
J. Med. Sci. Sports
3:
143-149,1993.
2. Belitzer,
V. A. La regulation
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Enzymalogia
6: 1-5, 1939.
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H. U. Methods
of Enzymatic
Analysis
(2nd ed.).
London:
Academic,
1965.
4. Bergstrom,
J. Muscle
electrolytes
in man. Determination
by
neutron
activation
analysis
on needle biopsy specimens.
A study
on normal
subjects,
kidney
patients
and patients
with cronic
diarrhoea.
Stand. J. Clin. Lab. Invest. Suppl. 68: l-110,
1962.
5. Bessman,
S. P., and A. Fonyo.
The possible role of mitochondrial bound creatine kinase in regulation
of mitochondrial
respiration. Biochem.
Biophys.
Res. Commun.
22: 597-602,
1966.
6. Bessman,
S. P., and P. J. Geiger.
Transport
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in
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that the rate of PCr formation by mitochondrial
CK will
be dependent
on the availability
of free Cr, as its
concentration
will be far in excess of the K, value.
However,
as PCr resynthesis
continues
and as the
muscle free Cr concentration
declines toward 19 mmol/l,
it is suggested that free Cr concentration
may then
begin to be a determinant
of the rate of PCr resynthesis.
The results
of the present experiment
support
this
suggestion.
Figure 2 demonstrates
that, in those subjects who responded to Cr feeding during the initial 20 s
of recovery, the rates of PCr resynthesis
were almost
identical when comparing
values obtained before and
after Cr ingestion. This, based on the above explanation,
might be expected, as the free Cr concentration
was at
all times in excess of 27 mmol/l intracellular
water.
However,
as recovery proceeded beyond 60 s, it can be
seen that Cr feeding was associated with a higher rate of
PCr resynthesis,
producing a 30% higher PCr concentration at the end of recovery (P < 0.05). This, we suggest,
was a result of Cr ingestion maintaining the muscle free
Cr concentration throughout the second half of recovery
higher than the K, of CK for Cr (19 mmol/l) and
thereby sustaining a high flux rate through the CK
reaction in favor of PCr resynthesis and ADP formation.
The latter, in turn, will have provided mitochondria
with substrate to maintain a high rate of ATP formation. The role of Cr as an acceptor of mitochondrial ATP
has been discussed in a series of previously published
papers (2,5,6,24,31).
The rate of muscle PCr resynthesis during recovery
from intense contraction has been suggested to be at
least partly limited by intracellular pH (14, 27). However, it is unlikely that a difference in intracellular pH
between treatments could explain the faster PCr resynthesis observed after Cr ingestion in the present study,
as lactate accumulation was greater after Cr ingestion
(Table Z), ind’ica t ing that, if anything, muscle pH would
have been lower during recovery after Cr ingestion.
As previously stated, data from this laboratory have
shown that Cr ingestion can significantly aid performance during repeated bouts of maximal isokinetic
exercise in humans, when each bout of exercise is
separated by 60 s of recovery. Figure 2 demonstrates
that, in those subjects who responded to Cr feeding in
the present study, muscle PCr concentrations were not
statistically different after 60 s of recovery when comparing pre- and postingestion values. It could be argued
therefore that the increase in exercise performance that
we have previously observed could not be attributable to
an accelerated rate of PCr resynthesis during the 60 s of
recovery between exercise bouts. However, it perhaps
should be made clear that, after 60 s of recovery, the
mean muscle PCr concentration was 8 mmol/kg dry
matter greater after Cr ingestion (P = 0.09, Fig. 2) and
that this increase was similar to that observed in resting
skeletal muscle after Cr feeding (16). Furthermore, it is
unlikely that the exercise protocol previously used would
have depleted PCr stores to the same extent as the
model used in the present experiment. Tesch et al. (30)
demonstrated that, after 30 maximal voluntary isokinetic contractions at 3.14 radians/s, 36 mmol/kg dry
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16.
PHOSPHOCREATINE