School of Biomedical Sciences, University Medical School, Queen's Medical Centre, Nottingham NG7 2UH, United Kingdom
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ABSTRACT |
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This
study investigated the effect of insulin on plasma and muscle creatine
accumulation and limb blood flow in humans after creatine
administration. Seven men underwent a 300-min euglycemic insulin clamp
combined with creatine administration on four separate occasions.
Insulin was infused at rates of 5, 30, 55, or 105 mU · m2 · min
1,
and on each occasion 12.4 g creatine was administered. During infusion
of insulin at rates of 55 and 105 mU · m
2 · min
1,
muscle total creatine concentration increased by 4.5 ± 1.4 (P < 0.05) and 8.3 ± 1.0 mmol/kg
dry mass (P < 0.05), and plasma creatine concentrations were lower at specific time points compared with the 5 mU · m
2 · min
1
infusion rate. The magnitude of increase in calf blood flow
(plethysmography) was the same irrespective of the rate of insulin
infusion, and forearm blood flow increased to the same extent as the
three highest infusion rates. These findings demonstrate that insulin
can enhance muscle creatine accumulation in humans but only when
present at physiologically high or supraphysiological concentrations.
This response is likely to be the result of an insulin-mediated
increase in muscle creatine transport rather than creatine delivery.
phosphocreatine; muscle fatigue; exercise
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INTRODUCTION |
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CREATINE, in its free and phosphorylated forms, plays an important role in the regulation of muscle energy metabolism and fatigue development (22, 35). Increasing dietary creatine intake from 1-2 g/day (in a normal healthy, meat-eating individual) to 20 g/day for a period of 4-6 days has been shown to increase muscle total creatine (TCr) concentration by ~20%, with large interindividual variation (5-30%; 15, 18, 20). It has also been reported that this regimen of creatine supplementation can sustain muscle ATP resynthesis and augment performance during repeated bouts of maximal exercise (1, 5, 6, 10, 16). However, it appears that the ergogenic and metabolic effects of creatine ingestion are dependent on the magnitude of the increase in muscle TCr during supplementation. Specifically, it has been suggested that an increase in muscle TCr content in excess of 20 mmol/kg dry muscle (dm) is required to exert an ergogenic effect on muscle power output (6) and postexercise phosphocreatine (PCr) resynthesis (15). Indeed, this may be one reason for the reported lack of effect of creatine supplementation on exercise performance (30, 32).
Recently, Green et al. (13) reported that ingestion of creatine in combination with a carbohydrate-containing solution increased muscle TCr by >25%. This was 60% greater than the increase observed when creatine alone was ingested. Moreover, the authors demonstrated that the ingestion of creatine in conjunction with carbohydrate increased muscle TCr in all subjects by >20 mmol/kg dm. However, only half of the subjects who ingested creatine alone had an increase of this magnitude. In accordance with published animal experiments (19, 23, 24), the authors proposed that the increase in muscle creatine accumulation originated from carbohydrate-mediated insulin release, which would stimulate sodium-dependent muscle creatine transport. In the study of Green et al., subjects ingested 94 g of carbohydrate (in the form of glucose and simple sugars) with each 5-g dose of creatine to achieve physiologically high plasma insulin concentrations during the 1st h after ingestion. However, the quantity of ingested carbohydrate proved to be close to the limit of palatability. The concentration of insulin necessary to stimulate muscle creatine accumulation in humans is presently unknown. On the basis of previously published work involving a mouse muscle cell line (31), isolated rat skeletal muscle (19), and human volunteers (13), it is hypothesized that a concentration close to the upper physiological limit would be required to promote this response.
Insulin has also been reported to stimulate muscle blood flow (2), a response that cannot be observed in vitro. Therefore, it could be concluded that the increase in muscle creatine accumulation observed after creatine and carbohydrate consumption in humans may have at least partly resulted from an insulin-mediated increase in muscle blood flow and thereby muscle creatine availability. In this context, submaximal exercise performed before creatine ingestion has been shown to augment muscle creatine accumulation (18).
The aims of the present study were, first, to determine whether insulin per se could stimulate in vivo creatine accumulation in human skeletal muscle. The second aim was to determine at what plasma insulin concentration a response, if any, was evident, and the final aim was to determine whether an insulin-mediated increase in muscle blood flow was associated with any observed increase in muscle creatine accumulation.
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MATERIALS AND METHODS |
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Subjects. Seven moderately active,
healthy, nonvegetarian men (age 25.9 ± 3.0 yr, weight 72.6 ± 3.4 kg, height 1.81 ± 0.03 m, and body mass index 22.2 ± 1.0 kg/m2) participated in the
present study, which was approved by the University of Nottingham
Medical School Ethics Committee. The training status of each subject
was assessed by means of a training diary. All exercised on a regular
basis for 1 h on no more than three occasions each week. Before
taking part in the study, all subjects underwent routine medical
screening and completed a general health questionnaire. All gave their
informed consent to take part in the study and were aware that they
were free to withdraw from the experiment at any point.
Study protocol. Subjects reported to
the laboratory after an overnight fast on four occasions, each
separated by 2 wk, having abstained from meat, alcohol, and strenuous
exercise for 24 h before each visit. On arrival, subjects rested in a
supine position on a bed while a cannula was placed in an antecubital
vein in the left forearm for the infusion of insulin and glucose. A
second cannula was placed retrogradely in a superficial vein on the
dorsal surface of the left hand. During each experiment, this hand was kept in a hand-warming unit (air temperature 55°C) to arterialize the venous drainage of the hand (12). After this, a nasogastric tube
was placed in the stomach. The position of the distal end of the tube
was confirmed by aspirating fluid from the stomach and checking its pH.
The proximal end of the tube was then fixed.
Each experimental visit consisted of a 300-min euglycemic insulin
(human Actrapid) clamp (8), combined with oral and nasogastric creatine
feeding (Fig. 1). During each clamp, blood
glucose concentration was maintained at 4.5 ± 0.3 mmol/l via
infusion of a 20% glucose solution. The insulin infusion rate varied
between treatments, being either 5, 30, 55, or 105 mU · m2 · min
1,
and the order of administration was randomized. The lowest infusion rate was aimed at maintaining a plasma insulin concentration comparable to that observed after an overnight fast. The 55 and 105 mU · m
2 · min
1
infusion rates were aimed at producing physiologically high and supraphysiological serum insulin concentrations, respectively. As
illustrated in Fig. 1, insulin infusion started at
t = 0 min. After an equilibration
period of 60 min, subjects ingested 5 g of creatine monohydrate
(Experimental & Applied Sciences, Denver, CO) dissolved in 250 ml warm
water. A nasogastric tube was then used for enteral infusion of an
isotonic creatine solution (100 mmol/l, 2.5 ml/min) from
t = 75 min until the end of
the experiment. In a pilot study, these procedures had been found to
increase plasma creatine concentration to ~1 mmol/l within 60 min of
creatine administration and to maintain a steady-state plasma
concentration at this level thereafter. The total quantity of creatine
administered during each treatment was 12.4 g.
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Blood sampling and analysis. During
each experimental visit, 25 ml of arterialized venous (a-v) blood were
obtained at 5-min intervals for monitoring blood glucose concentration
(YSI 2300 STATplus, YSI, Yellow Springs, OH). At 15-min intervals, 5 ml of a-v blood were collected into lithium heparin containers and, after
centrifugation (3,000 rpm for 10 min), the plasma was removed and
stored frozen at 20°C. Creatine and creatinine
concentrations were determined on these samples at a later date
according to the method of Dunnett et al. (9) with high-performance
liquid chromatography (System Gold 507, Beckman, Bucks, UK). At 30-min intervals, a further sample of 5 ml of a-v blood was collected and
allowed to clot, and, after centrifugation, the serum was stored frozen
at
20°C. Insulin was measured on these samples at a later
date with a radioimmunoassay (Diagnostic Products, Los Angeles, CA).
Blood flow measurements. Forearm and calf blood flow were measured simultaneously at 20-min intervals by venous occlusion plethysmography with a mercury-in-rubber strain gauge (36). Changes in gauge resistance were recorded with commercial computer hardware and software (Maclab, Mega System, New Zealand and Macintosh LC, Cupertino, California). The average slope of five occlusions was used to calculate forearm and calf blood flow as milliliters per 100 ml tissue per minute.
Muscle sampling and analysis. Muscle
biopsy samples were obtained from the vastus lateralis muscle
immediately before and at the end of each insulin clamp with the
percutaneous needle biopsy technique (4). To enable true resting muscle
PCr and free creatine concentrations to be achieved, biopsy samples
were frozen in liquid nitrogen 1 min after removal from the limb (33). Samples were then freeze-dried and stored at 80°C for
subsequent metabolite analysis. After removal of visible blood and
connective tissue, muscle samples were powdered, and ATP, PCr, and free
creatine concentrations were determined spectrophotometrically with the method of Harris et al. (17). To reduce the variance in nonmuscle constituents, PCr and creatine concentrations were adjusted with the
highest ATP concentration from each pair of samples (18). TCr was
calculated by adding PCr and free creatine concentrations. In addition,
1.5-3.5 mg of muscle powder from each biopsy were solubilized in
NaOH (0.1 mol/l) by heating at 80°C for 20 min, and the neutralized
extract was then used for the spectrophotometric determination of
muscle glycogen concentration (17).
Statistical analysis. A two-way ANOVA (time and treatment effects, Minitab, Clecom, Birmingham, UK) was performed to detect differences in plasma creatine, serum insulin, and blood flow responses between treatments. When comparisons of muscle metabolites across treatments were made, metabolite concentrations were initially analyzed with a balanced analysis for repeated measures (ANOVA, Minitab, Clecom). If significance was achieved, a Student's paired t-test was used to locate differences between treatments. Relationships between variables were examined by computing the Pearson product-moment correlation coefficient (r). The total area under the plasma creatine-time curve was calculated with the least squares method. Statistical significance was declared at P < 0.05, and all values are means ± SE.
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RESULTS |
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Serum insulin. Infusion of insulin at
rates of 5, 30, 55, and 105 mU · m2 · min
1
resulted in steady-state serum insulin concentrations of 12 ± 0.2, 56 ± 2, 109 ± 5, and 199 ± 5 mU/l, respectively, within 60 min of the onset of infusion (Fig. 2). The
total volumes of 20% glucose solution infused to maintain these
circulating insulin concentrations were 75 ± 60, 800 ± 60, 1,075 ± 60, and 1,180 ± 80 ml, respectively.
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Plasma creatine. Basal plasma creatine
concentrations were similar between treatments (75 ± 8 µmol/l).
As illustrated in Fig. 3, plasma creatine
concentration increased markedly during the first hour after creatine
administration on each treatment (P < 0.001) and remained elevated at a steady state thereafter. However, plasma creatine concentrations were significantly lower at specific time points when insulin was infused at rates of 55 (P < 0.05) and 105 mU · m2 · min
1
(P < 0.05), when compared with the
infusion rate of 5 mU · m
2 · min
1.
Furthermore, the total area under the plasma creatine-time curve (196 ± 20, 173 ± 12, 160 ± 16, and 156 ± 13 mmol · l
1 · min
at the insulin infusion rates of 5, 30, 55, and 105 mU · m
2 · min
1,
respectively) was negatively correlated with the infused dose of
insulin (r =
0.885,
P < 0.01).
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Blood flow. The forearm and calf blood
flow values immediately before and at the end of each insulin clamp are
presented in Table 1. Before insulin
infusion, neither forearm nor calf blood flow differed between
experimental treatments. Infusion of insulin at rates of 30, 55, and
105 mU · m2 · min
1
resulted in similar increases in forearm blood flow of ~65% within 100 min, but no response was observed at the 5 mU · m
2 · min
1 infusion rate. This
insulin-mediated increase in forearm blood flow was maintained for the
remainder of each infusion period, such that forearm blood flow was
higher at the end of the 30, 55, and 105 mU · m
2 · min
1
treatments when compared with the 5 mU · m
2 · min
1
infusion rate (Table 1). The calf blood flow response was similar for
all treatments; it gradually increased over the initial 120 min to
~40% above baseline (P < 0.001),
which was then sustained until the end of the experiment (Table 1).
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Muscle metabolites. Muscle ATP, PCr,
free creatine, TCr, and glycogen concentrations before and after
insulin and creatine administration are presented in Table
2. Before administration, muscle metabolite
concentrations were not different between treatments. After
administration, not significant changes were observed in muscle PCr and
ATP concentrations within and across treatments. Infusion of insulin at
rates of 5, 30, and 55 mU · m2 · min
1
had no effect on muscle free creatine concentration. However, free
creatine increased by 6.9 ± 1.9 mmol/kg dm
(P < 0.05) during infusion of
insulin at a rate of 105 mU · m
2 · min
1,
but this increase was not significantly different compared with the
infusion rate of 5 mU · m
2 · min
1.
No change in muscle TCr concentration was observed when insulin was
infused at rates of 5 and 30 mU · m
2 · min
1.
As Fig. 4 illustrates, significant
increases were observed at the 55 and 105 mU · m
2 · min
1
infusion rates, and the magnitude of increase was greatest at the
highest infusion rate (P < 0.05).
There was a strong correlation between the dose of insulin infused and
the change observed in muscle TCr concentration
(r = 0.977, P < 0.001).
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Similar to TCr, muscle glycogen concentration increased significantly
when insulin was infused at rates of 55 and 105 mU · m2 · min
1
(Table 2). However, the magnitude of the increase was not different between these treatments. As expected, there was a strong association between the dose of insulin infused and the observed increase in muscle
glycogen (r = 0.966, P < 0.001).
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DISCUSSION |
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We have recently demonstrated that creatine accumulation can be
substantially augmented in human skeletal muscle when creatine is
ingested in conjunction with large quantities of simple carbohydrates (13). It was speculated that insulin, released as a consequence of
carbohydrate ingestion, was responsible for this enhancement of muscle
creatine transport. This hypothesis was supported by findings from
animal studies showing that insulin could increase whole body creatine
retention in vivo (24) and could stimulate cellular creatine uptake in
vitro (19). Over 90% of creatine enters skeletal muscle
by binding to a specific transporter protein (35). This process is
saturable, is sodium dependent, and allows creatine to enter the muscle
against a concentration gradient (25, 31, 34). It has been proposed
that insulin facilitates sodium-dependent creatine transport by
increasing muscle sodium-potassium pump activity (7, 21, 28, 31). In
support of this hypothesis, Odoom et al. (31) demonstrated that
insulin, insulin-like growth factor I, triiodothyronine, and amylin,
all of which are known to stimulate sodium-potassium activity,
increased creatine accumulation in a muscle cell line. Furthermore,
ouabain, an inhibitor of sodium-potassium ATPase activity, was shown to
reduce cellular creatine uptake in vitro (11, 31). All of these
studies, however, employed animal or muscle cell line models, and all
used supraphysiological concentrations of insulin to augment cellular
creatine transport. As a result of this, there is currently little, if
any, published information concerning the effects of insulin on muscle
creatine accumulation in humans. The principal aim of the present study therefore was to examine the relationship between insulin availability and muscle creatine accumulation in humans. We were able to demonstrate that insulin augmented muscle creatine accumulation but only when present at high (~100 mU/l) or supraphysiological (~200 mU/l) concentrations. In line with this finding, we were able to show that
the plasma creatine concentration was lower at several time points
after administration when insulin was clamped at 100 and 200 mU/l (55 and 105 mU · m2 · min
1),
indicating that muscle creatine transport was increased under these conditions.
Previous studies have demonstrated that it takes 4-6 days of
creatine supplementation at a rate of 20 g/day to increase muscle TCr
concentration by ~20 mmol/kg dm (15, 18, 20). It is therefore not
surprising that the changes in muscle TCr concentrations in the present
study, where a total dose of 12.4 g was administered, were considerably
lower than those previously reported. However, extrapolating from the
present data what the increase in muscle TCr might have been if 20 g of
creatine had been administered for a period of 5 days, we can estimate
that at the 5 mU · m2 · min
1
insulin infusion rate (control) an increase of ~20 mmol/kg dm would
have occurred. This fits well with previous studies involving 5 days of
creatine supplementation at a rate of 20 g/day (13, 15). Similarly, the
estimated 33 mmol/kg dm increase in muscle TCr content at the 55 mU · m
2 · min
1
insulin infusion rate is in good agreement with our previous work
showing an increase of 33.4 ± 3.4 mmol/kg dm after the ingestion of
5 g creatine with 94 g carbohydrate four times daily for 5 days (13).
This also confirms that the effect of carbohydrate on muscle creatine
transport that we previously observed was insulin mediated. Indeed, the
area under the plasma creatine-time curve was negatively correlated
(r =
0.887,
P < 0.05) with the rate of insulin
infusion in the present study. It is likely that the 67 mmol/kg dm
increase in muscle TCr concentration predicted for the highest insulin
infusion rate in the present study is an overestimate. The maximal
increase is normally no more than 35 mmol/kg dm (13, 18), and human
skeletal muscle appears to have an upper limit of ~160 mmol/kg dm
(18). In support of this conclusion, it has been demonstrated that the
presence of a high creatine concentration will ultimately downregulate
muscle creatine transport in isolated skeletal muscle (26), and this is
probably achieved by downregulation of creatine transporter expression
(27, 34). The time course of muscle creatine transporter downregulation
during creatine supplementation in humans is currently unknown.
In contrast to several previous studies (16, 18), the increase in
muscle TCr concentration observed when insulin was infused at 55 and
105 mU · m2 · min
1
was not accompanied by a significant increase in muscle PCr
concentration. At the highest insulin infusion rate, muscle free
creatine concentration increased by 6.9 ± 1.9 mmol/kg dm (2.3 mmol/l intracellular water). Given that the Michaelis-Menten constant
(Km ) of creatine
kinase for creatine is known to be close to 19 mmol/l (3), it would appear that the increase in muscle free creatine concentration (from
16.2 to 18.5 mmol/l) during the 4 h of creatine administration was
insufficient to significantly increase creatine kinase flux. Additionally, it is likely that any small increase in PCr concentration that may have occurred because of the equilibrium nature of the creatine kinase reaction would have been too small to detect with current methods.
Harris et al. (18) demonstrated that 1 h of submaximal exercise
performed before creatine ingestion increased muscle creatine concentration by ~10% more than that seen in individuals ingesting creatine in the absence of exercise. The authors postulated that this
response was attributable to an exercise-induced increase in muscle
blood flow increasing muscle creatine delivery. Therefore, a second aim
of the present experiment was to investigate whether insulin could
indirectly enhance muscle creatine accumulation by stimulating muscle
blood flow, and, thereby, muscle creatine delivery. We demonstrated
that insulin did increase calf blood flow, but the magnitude of this
increase was the same irrespective of the rate of insulin infusion.
Forearm blood flow increased during insulin infusion rates of 30, 55, and 105 mU · m2 · min
1.
Again, the magnitude of this increase was not different between infusion rates. It is therefore unlikely that an increase in muscle blood flow was responsible for the increases in muscle creatine accumulation seen in the present study. Indeed, given that the Km and maximum
velocity for muscle creatine transport have been reported to be in the
region of 20-110 µmol/l and 100-600 µmol/l, respectively
(11, 25, 29, 31), it is highly unlikely that creatine availability
limited creatine transport in the present study, where plasma creatine
concentration was maintained at a steady state of ~800 µmol/l (Fig.
3). The same is true of more conventional regimens of muscle creatine
loading (18, 20). It would seem, therefore, that the increase in muscle
creatine accumulation after submaximal exercise observed by Harris et
al. may have been attributable to an exercise-induced increase in muscle insulin sensitivity. In support of this suggestion, Green et al.
(14) demonstrated that the ingestion of 94 g of simple carbohydrates in
combination with 5 g of creatine appeared to eliminate the stimulatory
effect of exercise on muscle creatine transport. This is presumably a
result of the marked increase in serum insulin concentration
overshadowing any effect of exercise on insulin sensitivity.
In conclusion, the present study demonstrates that insulin can enhance muscle creatine accumulation in human skeletal muscle but only when present at high or supraphysiological concentrations. Furthermore, it would appear that this effect is achieved by an insulin-mediated augmentation of sodium-dependent creatine transport, rather than an insulin-mediated increase in muscle creatine delivery. These findings will be of interest to individuals wishing to maximize muscle creatine accumulation in an attempt to improve exercise performance, particularly because the magnitude of the increase in exercise performance and postexercise PCr resynthesis appears to be related to the extent of muscle creatine retention during supplementation (6, 15). The present findings also suggest that ingesting creatine with anything other than large quantities (~100 g) of simple carbohydrates will be no more effective at increasing muscle creatine retention than ingesting creatine alone.
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ACKNOWLEDGEMENTS |
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This work was funded by grant support from Experimental and Applied Sciences, Golden, CO.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests: G. Steenge, School of Biomedical Sciences, Univ. Medical School, Queen's Medical Centre, Nottingham NG7 2UH, UK.
Received 10 March 1998; accepted in final form 21 August 1998.
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REFERENCES |
---|
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---|
1.
Balsom, P. D.,
B. Ekblom,
K. Söderlund,
B. Sjödin,
and
E. Hultman.
Creatine supplementation and dynamic high-intensity intermittent exercise.
Scand. J. Med. Sci. Sports
3:
143-149,
1993.
2.
Baron, A.
Hemodynamic actions of insulin.
Am. J. Physiol.
267 (Endocrinol. Metab. 30):
E187-E202,
1994
3.
Bergmeyer, H. U.
Methods of Enzymatic Analysis (2nd ed.). London: Academic, 1965.
4.
Bergström, J.
Muscle electrolytes in man. Determined by neutron activation analysis on needle biopsy specimens. A study on normal subjects, kidney patients and patients with chronic diarrhoea.
Scand. J. Clin. Lab. Invest.
14:
1-110,
1962.
5.
Birch, R.,
D. Noble,
and
P. L. Greenhaff.
The influence of dietary creatine supplementation on performance during repeated bouts of maximal isokinetic cycling in man.
Eur. J. Appl. Physiol.
69:
268-270,
1994.
6.
Casey, A.,
D. Constantin-Teodosiu,
S. Howell,
E. Hultman,
and
P. L. Greenhaff.
Creatine ingestion favorably affects performance and muscle metabolism during maximal exercise in humans.
Am. J. Physiol.
271 (Endocrinol. Metab. 34):
E31-E37,
1996
7.
Clausen, T.,
and
P. G. Kohn.
The effect of insulin on the transport of sodium and potassium in rat soleus muscle.
J. Physiol. (Lond.)
265:
19-42,
1977[Medline].
8.
DeFronzo, R. A.,
J. D. Tobin,
and
R. Andres.
Glucose clamp technique: a method for quantifying insulin secretion and resistance.
Am. J. Physiol.
237 (Endocrinol. Metab. Gastrointest. Physiol. 6):
E214-E223,
1979
9.
Dunnett, M.,
R. C. Harris,
and
C. E. Orme.
Reverse-phase-ion-pairing high-performance liquid chromatography of phosphocreatine, creatine and creatinine in equine muscle.
Scand. J. Clin. Lab. Invest.
51:
137-141,
1991[Medline].
10.
Earnest, C. P.,
P. G. Snell,
R. Rodriguez,
A. L. Almada,
and
T. L. Mitchell.
The effect of creatine monohydrate ingestion on anaerobic power indices, muscular strength and body composition.
Acta Physiol. Scand.
153:
207-209,
1995[Medline].
11.
Fitch, C. D.,
and
R. P. Shields.
Creatine metabolism in skeletal muscle. 1. Creatine movement across muscle membranes.
J. Biol. Chem.
15:
3611-3614,
1966.
12.
Gallen, I. W.,
and
I. A. Macdonald.
Effect of two methods of heating on body temperature, forearm blood flow, and deep venous oxygen saturation.
Am. J. Physiol.
259 (Endocrinol. Metab. 22):
E639-E643,
1990[Abstract].
13.
Green, A. L.,
E. Hultman,
I. A. Macdonald,
D. A. Sewell,
and
P. L. Greenhaff.
Carbohydrate ingestion augments skeletal muscle creatine accumulation during creatine supplementation in humans.
Am. J. Physiol.
271 (Endocrinol. Metab. 34):
E821-E826,
1996
14.
Green, A. L.,
E. J. Simpson,
J. J. Littlewood,
I. A. Macdonald,
and
P. L. Greenhaff.
Carbohydrate ingestion augments creatine retention during creatine feeding in humans.
Acta Physiol. Scand.
158:
195-202,
1996[Medline].
15.
Greenhaff, P. L.,
K. Bodin,
K. Söderlund,
and
E. Hultman.
The effect of oral creatine supplementation on skeletal muscle phosphocreatine resynthesis.
Am. J. Physiol.
266 (Endocrinol. Metab. 29):
E725-E730,
1994
16.
Greenhaff, P. L.,
A. Casey,
A. H. Short,
R. Harris,
K. Söderlund,
and
E. Hultman.
The influence of oral creatine supplementation on muscle torque during repeated bouts of maximal voluntary exercise in man.
Clin. Sci. (Colch.)
84:
565-571,
1993[Medline].
17.
Harris, R. C.,
E. Hultman,
and
L.-O. Norjesö.
Glycogen, glycolytic intermediates and high-energy phosphates determined in biopsy samples of the musculus quadriceps of man at rest. Methods and variance of values.
Scand. J. Clin. Lab. Invest.
33:
109-120,
1974[Medline].
18.
Harris, R. C.,
K. Söderlund,
and
E. Hultman.
Elevation of creatine in resting and exercised muscle of normal subjects by creatine supplementation.
Clin. Sci. (Colch.)
83:
367-374,
1992[Medline].
19.
Haughland, R. B.,
and
D. T. Chang.
Insulin effects on creatine transport in skeletal muscle.
Proc. Soc. Exp. Biol. Med.
148:
1-4,
1975[Abstract].
20.
Hultman, E.,
K. Söderlund,
J. Timmons,
G. Cederblad,
and
P. L. Greenhaff.
Muscle creatine loading in man.
J. Appl. Physiol.
81:
232-237,
1996
21.
Hundal, S. H.,
A. Marette,
Y. Mitsumoto,
T. Ramlal,
R. Blostein,
and
A. Klip.
Insulin induces translocation of the 2 and
1 subunits of the Na+/K+-ATPase from intracellular compartments to the plasma membrane in mammalian skeletal muscle.
J. Biol. Chem.
267:
5040-5043,
1992
22.
Katz, A.,
K. Sahlin,
and
J. Henriksson.
Muscle ATP turnover rate during isometric contractions in humans.
J. Appl. Physiol.
60:
1839-1842,
1986
23.
Koszalka, T. R.,
and
C. L. Andrew.
Effect of insulin on creatinuria and hypercreatinemia induced by creatine loading.
Proc. Soc. Exp. Biol. Med.
135:
905-910,
1970.
24.
Koszalka, T. R.,
and
C. L. Andrew.
Effect of insulin on the uptake of creatine-1-14C by skeletal muscle in normal and X-irradiated rats.
Proc. Soc. Exp. Biol. Med.
139:
1265-1271,
1972.
25.
Loike, J. D.,
M. Somes,
and
S. C. Silverstein.
Creatine uptake, metabolism, and afflux in human monocytes and macrophages.
Am. J. Physiol.
251 (Cell Physiol. 20):
C128-C135,
1986
26.
Loike, J. D.,
D. L. Zalutsky,
E. Kaback,
A. F. Miranda,
and
S. C. Silverstein.
Extracellular creatine regulates creatine transport in rat and human muscle cells.
Proc. Natl. Acad. Sci. USA
85:
807-811,
1988[Abstract].
27.
Lourdes Guerrero-Ontiveros, M. L.,
and
T. Wallimann.
Effects of creatine supplementation in vivo: down-regulation of the expression of creatine transporter isoforms in rat skeletal muscle.
Mol. Cell. Biochem.
184:
427-437,
1998[Medline].
28.
Marette, A.,
J. Krischer,
L. Lavoie,
C. Ackerley,
J.-L. Carpentier,
and
A. Klip.
Insulin increases the Na+-K+-ATPase 2-subunit in the surface of rat skeletal muscle: morphological evidence.
Am. J. Physiol.
265 (Cell Physiol. 34):
C1716-C1722,
1993
29.
Nash, S. R.,
B. Giros,
S. F. Kingsmore,
J. M. Rochelle,
S. T. Suter,
P. Gregor,
M. F. Seldin,
and
M. G. Caron.
Cloning, pharmacological characterisation and genomic localisation of the human creatine transporter.
Receptors Channels
2:
165-174,
1994[Medline].
30.
Odland, L. M.,
J. D. MacDougall,
M. A. Tarnopolsky,
A. Elorriaga,
and
A. Borgmann.
Effect of oral creatine supplementation on muscle [PCr] and short-term maximum power output.
Med. Sci. Sports Exerc.
29:
216-219,
1997[Medline].
31.
Odoom, J. E.,
G. J. Kemp,
and
G. K. Radda.
The regulation of total creatine content in a myoblast cell line.
Mol. Cell. Biochem.
158:
179-188,
1996[Medline].
32.
Redondo, D. R.,
E. A. Dowling,
B. L. Graham,
A. L. Almada,
and
M. H. Williams.
The effect of oral creatine monohydrate supplementation on running velocity.
Int. J. Sport Nutr.
6:
213-221,
1996[Medline].
33.
Söderlund, K.,
and
E. Hultman.
Effect of delayed freezing on content of phosphagens in human skeletal muscle biopsy samples.
J. Appl. Physiol.
61:
832-835,
1986
34.
Sora, I.,
J. Richman,
G. Santor,
H. Wei,
Y. Wang,
T. Vanderah,
R. Horvath,
M. Nguyen,
S. Waite,
W. R. Roeske,
and
H. I. Yamamura.
The cloning and expression of a human creatine transporter.
Biochem. Biophys. Res. Commun.
204:
419-427,
1994[Medline].
35.
Wallimann, T.,
M. Wyss,
D. Brdiczka,
K. Nicolay,
and
H. M. Eppenberger.
Intracellular compartmentation, structure and function of creatine kinase isoenzymes in tissues with high and fluctuating energy demands: the phosphocreatine circuit for cellular energy homeostasis.
Biochem. J.
281:
21-40,
1992[Medline].
36.
Witney, R. J.
The measurement of volume changes in human limbs.
J. Physiol. (Lond.)
121:
1-27,
1953[Medline].