1 School of Health Sciences and 2 Centre for Cellular
and Molecular Biology, Deakin University, Burwood 3125, Australia;
3 Department of Physiology, Monash University, Clayton 3168, Australia; and ...4 Institute of Cell Biology,
ETH-Hönggerberg, CH-8093 Zürich, Switzerland
The present study examined the gene
expression and cellular localization of the creatine transporter
(CreaT) protein in rat skeletal muscle. Soleus (SOL) and red (RG) and
white gastrocnemius (WG) muscles were analyzed for CreaT mRNA, CreaT
protein, and total creatine (TCr) content. Cellular location of the
CreaT protein was visualized with immunohistochemical analysis of
muscle cross sections. TCr was higher ( P 0.05) in WG
than in both RG and SOL, and was higher in RG than in SOL. Total CreaT
protein content was greater ( P 0.05) in SOL and RG
than in WG. Two bands (55 and 70 kDa) of the CreaT protein were found
in all muscle types. Both the 55-kDa (CreaT-55) and the 70-kDa
(CreaT-70) bands were present in greater ( P 0.05)
amounts in SOL and RG than in WG. SOL and RG had a greater amount
( P 0.05) of CreaT-55 than CreaT-70. Immunohistochemical analysis revealed that the CreaT was mainly associated with the sarcolemmal membrane in all muscle types. CreaT
mRNA expression per microgram of total RNA was similar across the three
muscle types. These data indicate that rat SOL and RG have an enhanced
potential to transport Cr compared with WG, despite a higher TCr in the latter.
phosphocreatine; metabolism; creatine supplementation.
The effect of different rates of fluid ingestion on heart rate, rectal temperature, plasma electrolytes, hormones and performance was examined during prolonged strenuous exercise conducted at 21 ...degree C. Seven well-trained males (24 plus or minus 1 yr; 68.6 plus or minus 2.9kg; Vo2 peak=4.69 plus or minus 0.17Lmin-1; mean plus or minus SEM) cycled for 2h at 69 plus or minus 1% Vo2 peak while receiving either no fluid replacement (NF), a volume of water estimated to prevent body weight loss (FR-100=2.32 plus or minus 0.10L2 h-1) or 50% ofthis volume (FR-50=1.16 plus or minus 0.05L2 h-1). The 2-h exercise bout was followed by a ride to exhaustion at a workload estimated to be 90% Vo2 peak. After 2h of exercise, NF was associated with a 3.2 plus or minus 0.1% weight loss, while FR-50 and FR-100 resulted in losses of 1.8 plus or minus 0.1 and 0.1 plus or minus 0.1%, respectively. Compared with FR-100, heart rate and rectal temperature were elevated (P<0.05) during the second hour of exercise in NF, with FR-50 intermediate. Reductions in plasma volume during exercise were greater in NF and FR-50, compared with FR-100 and plasma sodium concentration was elevated in NF, decreased slightly in FR-100, with FR-50 intermediate. Plasma renin activity, aldosterone and atrial natriuretic peptide increased to similar extents in the three trials. Plasma vasopressin remained unchanged for FR-100, increased for NF, with intermediate values for FR-50. Exercise time to exhaustion at 90% Vo2 peak was longer in FR-100 (328 plus or minus 93s) than NF (171 plus or minus 75s) with FR-50 (248 plus or minus 107s) not significantly different from either FR-100 or NF. In conclusion, the responses of heart rate, rectal temperature, plasma sodium, and vasopressin during, and performance following, prolonged cycling exercise conducted at 21 degree C are related to the amount of fluid ingested (i.e. the degree of dehydration).
This study compared the effects of carbohydrate ingestion throughout exercise with ingestion of an equal amount of carbohydrate late in exercise. Eight well-trained men cycled 2 h at 70 +/- 1% VO2 ...peak, followed immediately by a 15-min performance ride, while ingesting either a 7% carbohydrate-electrolyte solution (CHO-7), an artificially sweetened placebo (CON), or the placebo for the first 90 min then a 21% glucose solution (CHO-0/21). At the start of the performance ride, plasma glucose averaged 4.2 +/- 0.2, 5.2 +/- 0.1, and 5.7 +/- 0.2 mmol.l-1 in CON, CHO-7, and CHO-0/21, respectively (all different, P < 0.05). Plasma insulin levels were similar just prior to the performance ride in CHO-7 and CHO-0/21, with both higher than CON. A similar pattern was observed with respiratory exchange ratio (RER). Work performed during the performance ride was significantly greater in CHO-7 (268 +/- 8 kJ) compared with CON (242 +/- 9 kJ). Performance in CHO-0/21 (253 +/- 10 kJ), however, was not improved compared with CON, despite higher plasma glucose levels and plasma insulin levels similar to CHO-7. Seven of the eight subjects performed best in CHO-7. In conclusion, performance was improved, relative to the control trial, only when carbohydrate was ingested throughout exercise. Carbohydrate ingestion late in exercise did not improve performance despite increases in plasma glucose and insulin.
The intent of this study was to determine whether adding carbonation to either water or a low calorie sport drink would affect gastric emptying (GE). Fifteen subjects rode for 20 minutes on a cycle ...ergometer at 55% of max VO2. After 5 minutes of exercise, the subjects ingested 5.5 ml/kg body weight of a test solution: water (W), carbonated water (CW), and a low calorie sport drink in both a carbonated (C2C) and noncarbonated (2C) form. At the end of each ride, the stomach was emptied through gastric aspiration. The results indicate that carbonation has no effect on GE. However, the type of drink did have an effect on GE, as both 2C and C2C emptied from the stomach at a slower rate than either W or CW. Subjective ratings of gastrointestinal comfort were similar for both carbonated and noncarbonated forms, and at no time did the subjects report discomfort. The results were independent of the exercise challenge, as exercise intensity, heart rate, and ratings of perceived exertion did not differ between experimental trials. It is concluded that carbonation does not affect the GE characteristics of a drink taken during submaximal exercise, but the flavoring system of the low calorie beverage decreased the rate of GE by as much as 25% when compared to water.
1 Department of Physiology, Monash University, Clayton,
Victoria 3800; and 2 St. Vincent's Institute of Medical
Research, Fitzroy, Victoria 3065, Australia
The effect of prolonged
...moderate-intensity exercise on human skeletal muscle AMP-activated
protein kinase (AMPK) 1 and - 2 activity and acetyl-CoA carboxylase
(ACC ) and neuronal nitric oxide synthase (nNOSµ) phosphorylation
was investigated. Seven active healthy individuals cycled for 30 min at
a workload requiring 62.8 ± 1.3% of peak O 2
consumption ( O 2 peak ) with muscle biopsies obtained from the vastus lateralis at rest and at 5 and 30 min
of exercise. AMPK 1 activity was not altered by exercise; however,
AMPK 2 activity was significantly ( P < 0.05)
elevated after 5 min (~2-fold), and further elevated
( P < 0.05) after 30 min (~3-fold) of exercise.
ACC phosphorylation was increased ( P < 0.05) after
5 min (~18-fold compared with rest) and increased ( P < 0.05) further after 30 min of exercise (~36-fold compared with
rest). Increases in AMPK 2 activity were significantly correlated with both increases in ACC phosphorylation and reductions in muscle
glycogen content. Fat oxidation tended ( P = 0.058) to
increase progressively during exercise. Muscle creatine phosphate was
lower ( P < 0.05), and muscle creatine, calculated free
AMP, and free AMP-to-ATP ratio were higher ( P < 0.05)
at both 5 and 30 min of exercise compared with those at rest. At 30 min
of exercise, the values of these metabolites were not significantly
different from those at 5 min of exercise. Phosphorylation of nNOSµ
was variable, and despite the mean doubling with exercise,
statistically significance was not achieved ( P = 0.304). Western blots indicated that AMPK 2 was associated with both
nNOSµ and ACC consistent with them both being substrates of
AMPK 2 in vivo. In conclusion, AMPK 2 activity and ACC
phosphorylation increase progressively during moderate exercise at
~60% of O 2 peak in humans, with
these responses more closely coupled to muscle glycogen content than
muscle AMP/ATP ratio.
adenosine monophosphate-activated protein kinase; acetyl-coenzyme A
carboxylase- ; neuronal nitric oxide synthase; prolonged exercise; humans
*
T. J. Stephens and Z.-P. Chen contributed equally to
this study.
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