Effects of long-term feeding of high-protein or high-fat diets on the response to exercise in the rat

Summary The aim of this work was to find by which mechanisms an increased availability of plasma free fatty acids (FFA) reduced carbohydrate utilization during exercise. Rats were fed high-protein medium-chain triglycerides (MCT), high-protein long-chain triglycerides (LCT), carbohydrate (CHO) or high-protein low-fat (HP) diets for 5 weeks, and liver and muscle glycogen, gluconeogenesis and FFA oxidation were studied in rested and trained runner rats. In the rested state the hepatic glycogen store was decreased by fat and protein feeding, whereas soleus muscle glycogen concentration was only affected by high-protein diets. The percentage decrease in liver and muscle glycogen stores, after running, was similar in fat-fed, high-protein and CHO-fed rats. The fact that plasma glucose did not drastically change during exercise could be explained by a stimulation of hepatic gluconeogenesis: the activity of phosphoenolpyruvate carboxykinase (PEPCK) and liver phosphoenolpyruvate (PEP) concentration increased as well as cyclic adenosine monophosphate (AMPc) while liver fructose 2,6-bisphosphate decreased and plasma FFA rose. In contrast, the stimulation of gluconeogenesis in rested HP-, MCT- and LCT-fed rats appears to be independent of cyclic AMP.     

Interaction of exercise and diet on GLUT-4 protein and gene expression in Type I and Type II rat skeletal muscle

We determined the interaction of exercise and diet on glucose transporter (GLUT-4) protein and mRNA expression in type I (soleus) and type II [extensor digitorum longus (EDL)] skeletal muscle. Forty-eight Sprague Dawley rats were randomly assigned to one of two dietary conditions: high-fat (FAT, n=24) or high-carbohydrate (CHO, n=24). Animals in each dietary condition were allocated to one of two groups: control (NT, n=8) or a group that performed 8 weeks of treadmill running (4 sessions week-1 of 1000 m @ 28 m min-1, RUN, n=16). Eight trained rats were killed after their final exercise bout for determination of GLUT-4 protein and mRNA expression: the remainder were killed 48 h after their last session for measurement of muscle glycogen and triacylglycerol concentration. GLUT-4 protein expression in NT rats was similar in both muscles after 8 weeks of either diet. However, there was a main effect of training such that GLUT-4 protein was increased in the soleus of rats fed with either diet (P < 0.05) and in the EDL in animals fed with CHO (P < 0.05). There was a significant diet-training interaction on GLUT-4 mRNA, such that expression was increased in both the soleus (100% upward arrowP < 0.05) and EDL (142% upward arrowP < 0.01) in CHO-fed animals. Trained rats fed with FAT decreased mRNA expression in the EDL ( downward arrow 45%, P < 0.05) but not the soleus ( downward arrow 14%, NS). We conclude that exercise training in CHO-fed rats increased both GLUT-4 protein and mRNA expression in type I and type II skeletal muscle. Despite lower GLUT-4 mRNA in muscles from fat-fed animals, exercise-induced increases in GLUT-4 protein were largely preserved, suggesting that control of GLUT-4 protein and gene expression are modified independently by exercise and diet.     

Glycogen depletion and resynthesis in the rat after downhill running

Summary to study the effect of downhill running on glycogen metabolism, 94 rats were exercised by running for 3 h on the level or down an 18° incline. Muscle and liver glycogen concentrations were measured before exercise and 0, 48 and 52 h postexercise. Rats were not fed during the first 48 h of recovery but ingested a glucose solution 48 h postexercise. Downhill running depleted glycogen in the soleus muscle and liver significantly more than level running (P<0.01). The amount of glycogen resynthesized in the soleus muscle and liver in fasting or nonfasting rats was not altered significantly by downhill running (P>0.05). On every day of recovery the rats were injected with dexamethasone, which induced similar increases in glycogen concentration in the soleus muscle and liver after the 52nd h of the postexercise period in the case of downhill and level running. The glycogen depletion and repletion results indicated that, under our experimental conditions, downhill running in the rat, a known model of eccentric exercise, affected muscle glycogen metabolism differently from eccentric cycling in humans.     

Effect of a high-carbohydrate diet intake on muscle glycogen repletion after exercise in rats previously fed a high-fat diet

Summary The effect of a high-carbohydrate (C) diet intake on muscle glycogen repletion during the early period of recovery from exercise was studied in rats previously fed a high-fat (F) diet. In experiment 1, 3 weekold male and in experiment 2, 3 week-old female rats were used. Rats were fed either the F or the C diet for 2–10 weeks ad libitum and then were meal-fed regularly twice a day for 25 days in experiment 1, or for 5 weeks in experiment 2. During the period of regular feeding, half of the rats in both dietary groups continued to eat as before (F-F and C-C) but the other half of the rats were switched to the counterpart diets (F-C and C-F) in experiment 1. In experiment 2, half of the F-F group were switched to the C diet (F-C) for 3, 7, and 14 days after the period of regular feeding. Pre-exercise glycogen content in soleus, red gastrocnemius, and heart muscles and liver was higher in rats fed the C diet (C-C and F-C) than in rats fed the F diet (F-F and C-F) in experiment 1. Glycogen repletion in red muscle 2 h after the ingestion of a glucose and citrate (3.0 and 0.5 g, respectively, per kg body mass) drink was also higher in the former than in the latter. There was a positive relationship in skeletal muscles between pre-exercise glycogen content and the rate of glycogen repletion. Compared with the rats maintained on the F diet (F-F), the rats switched to the C diet (F-C) for 3 and 7 days showed faster glycogen repletion in soleus and/or red gastrocnemius muscles in experiment 2. These results indicated that the poor capacity of restoration of skeletal muscle glycogen in rats previously fed the F diet was improved by the short-term dietary switch to the C diet.     

Effect of different types of high carbohydrate diets on glycogen metabolism in liver and skeletal muscle of endurance-trained rats

Male Wistar rats were fed ad libitum four different diets containing fructose, sucrose, maltodextrins or starch as the source of carbohydrate (CH). One group was subjected to moderate physical training on a motor-driven treadmill for 10 weeks (trained rats). A second group received no training and acted as a control (sedentary rats). Glycogen metabolism was studied in the liver and skeletal muscle of these animals. In the sedentary rats, liver glycogen concentrations increased by 60%–90% with the administration of simple CH diets compared with complex CH diets, whereas skeletal muscle glycogen stores were not significantly affected by the diet. Physical training induced a marked decrease in the glycogen content in liver (20%–30% of the sedentary rats) and skeletal muscle (50%–80% of the sedentary rats) in animals fed simple (but not complex) CH diets. In liver this was accompanied by a two-fold increase of triacylglycerol concentrations. Compared with simple CH diets, complex CH feeding increased by 50%–150% glycogen synthase (GS) activity in liver, whereas only a slight increase in GS activity was observed in skeletal muscle. In all the animal groups, a direct relationship existed between tissue glucose 6-phosphate concentration and glycogen content (r = 0.9911 in liver, r = 0.7177 in skeletal muscle). In contrast, no relationship was evident between glycogen concentrations and either glycogen phosphorylase activity or adenosine 5-monophosphate tissue concentration. The results from this study thus suggest that for trained rats diets containing complex CH (compared with diets containing simple CH) improve the glycogenic capacity of liver and skeletal muscle, thus enabling the adequate regeneration of glycogen stores in these two tissues.     

Training decreases muscle glycogen turnover during exercise

The present study was undertaken to determine the effects of endurance training on glycogen kinetics during exercise. A new model describing glycogen kinetics was applied to quantitate the rates of synthesis and degradation of glycogen. Trained and untrained rats were infused with a 25% glucose solution with 6-³H-glucose and U-¹⁴C-lactate at 1.5 and 0.5?μCi?·?min^?1 (where 1 Ci?=?3.7?×?10¹⁰ Bq), respectively, during rest (30?min) and exercise (60?min). Blood samples were taken at 10-min intervals starting just prior to isotopic infusion, until the cessation of exercise. Tissues harvested after the cessation of exercise were muscle (soleus, deep, and superficial vastus lateralis, gastrocnemius), liver, and heart. Tissue glycogen was quantitated and analyzed for incorporation of ³H and ¹⁴C via liquid scintillation counting. There were no net decreases in muscle glycogen concentration from trained rats, whereas muscle glycogen concentration decreased to as much as 64% (P?P?相似文献   

The effects of a high carbohydrate diet on postprandial energy expenditure during exercise in rats

Summary Whether or not a high intake of carbohydrate increases postprandial energy expenditure during exercise was studied in rats. The rats were meal-fed regularly twice a day (0800–0900 hours and 1800–1900 hours) on either a high carbohydrate (CHO) (carbohydrate/fat/protein = 70/5/25, % of energy) or high fat (FAT) (35/40/25) diet for 12 days. On the final day of the experiment, all of the rats in each dietary group were fed an evening meal containing equal amounts of energy (420 kJ · kg^–1 body mass). After the meal, they were divided into three subgroups: pre-exercise control (PC), exercise (EX), and resting control (RC). The PC-CHO and PC-FAT groups were sacrificed at 2030 hours. The EX-CHO and EX-FAT groups were given a period of 3-h swimming, and then sacrificed at 2330 hours. The RC-CHO and RC-FAT groups rested after the meal and were sacrificed at 2330 hours. Total energy expenditure during the period 1.5 h from the commencement of exercise was higher in EX-CHO than in EX-FAT. The respiratory exchange ratio was also higher in EX-CHO than in EX-FAT, suggesting enhanced carbohydrate oxidation in the former. Compared with both PC-FAT and RC-FAT, the liver glycogen content of EX-FAT rats was significantly decreased by exercise. On the other hand, the liver glycogen content of both EX-CHO and RC-CHO was higher than that of PC-CHO rats. The glycogen content of soleus muscle of EX-FAT was slightly decreased during exercise, however, that of EX-CHO increased significantly. Thus postprandial energy expenditure during exercise was higher in the rats fed the CHO diet than in those fed the FAT diet, which could have been related to the increase of both liver and muscle glycogen storage during exercise in the former.     

The effect of exercise training on glycogen,glycogen synthase and phosphorylase in muscle and liver

Summary It is thought that exercise training in both man and the rat results in a protective effect against the depletion of carbohydrate stores during exercise (glycogen-sparing). However there has been no comprehensive study of the effects of training on glycogen anabolic and catabolic enzymes with liver or muscle. The aim of this study was to examine whether changes in these enzymes occur and whether these changes may provide an explanation for the glycogen-sparing which results from exercise training.Male rats were trained by a treadmill running program at three different workloads. In addition, there were three control groups: free eating (SF), food restricted (SR), and one SF with a single bout of exercise prior to sacrifice.Exercise training was associated with a 60–150% increase in glycogen synthase and phosphorylase and a 50–70% increase in glycogen content in soleus, an intermediate muscle, but not in extensor digitorum longus (EDL), a white muscle nor in liver. The increase in glycogen synthase and phosphorylase in intermediate muscle was proportional to the degree of training and there was a significant correlation between glycogen content, glycogen synthase, and phosphorylase activity in intermediate muscle. Cytochrome c oxidase activity, an indicator of respiratory capacity, increased 50% in gastrocnemius of trained rats and was significantly correlated with glycogen synthase and phosphorylase in soleus.These results indicate a significant effect of exercise training on glycogen anabolic and catabolic enzymes in intermediate muscle, with no significant effects in white muscle or liver. The changes do not provide an explanation for glycogen-sparing, but are consistent with improved capacity of intermediate muscle for rapid glycogen mobilisation and repletion.     

Metabolic effects of glucose,medium chain triglyceride and long chain triglyceride feeding before prolonged exercise in rats

Summary The purpose of this study was to test the hypothesis that oral ingestion of lipids could increase endurance by slowing the rate of glycogen depletion. Trained rats were killed after a 2 h run on a rodent treadmill, following an intragastric infusion of water, glucose, medium chain triglycerides (MCT) or long chain triglycerides (LCT). Glucose and triglycerides were administered in equicaloric concentrations (50 kJ).The results show that oral ingestion of lipids (MCT or LCT) did not reduce glycogen depletion in liver, heart or skeletal muscle after exercise whereas the fat diet increased muscle and heart glycogen stores in resting conditions. In contrast, glucose feeding induced a significant sparing effect on endogenous carbohydrate utilization and reduced physical exercise lipolysis. These data indicated, firstly, that enhanced lipid availability induced by a single lipid meal before exercise was not able to modify the glycogen depletion occuring after exercise and, secondly, that the glucose/fatty acid cycle was not effective in these conditions. The comparison between lipids indicated that the effect on glycogen use of MCT did not differ from that of LCT, and did not seem to be of any particular importance during physical exercise.     

Effect of short-term exercise training on muscle glycogen in resting conditions in rats fed a high fat diet

Summary It has been reported that exercise training increases muscle glycogen storage in rats fed a high carbohydrate (CHO) diet in resting conditions. The purpose of this study was to examine whether a 3-week swimming training programme would increase muscle glycogen stores in rats fed a high-fat (FAT) diet in resting conditions. Rats were fed either the FAT or CHO diet for 7 days ad libitum, and then were fed regularly twice a day (between 0800 and 0830 hours and 1800 and 1830 hours) for 32 days. During this period of regular feeding, half of the rats in both dietary groups had swimming training for 3 weeks and the other half were sedentary. The rats were not exercised for 48 h before sacrifice. All rats were killed 2 h after their final meal (2030 hours). The glycogen contents in red gastrocnemius muscle, heart and liver were significantly higher in sedentary rats fed the CHO diet than in those fed the FAT diet. Exercise training clearly increased glycogen content in soleus, red gastrocnemius and heart muscle in rats fed the CHO diet. In rats fed the FAT diet, however, training did not increase glycogen content in these muscles or the heart. Exercise training resulted in an 87% increase of total glycogen synthase activity in the gastrocnemius muscle of rats fed the CHO diet. However, this was not observed in rats fed the FAT diet. The total glycogen phosphorylase activity in the gastrocnemius muscle of the rats of both dietary groups was increased approximately twofold by training. These results suggested that muscle glycogen was enhanced in rats fed the CHO diet and that the glycogen content of the muscle of rats fed the FAT diet was not increased by exercise training.     

Effects of short-term dietary change from high fat to high carbohydrate diets on the storage and utilization of glycogen and triacylglycerol in untrained rats

The effects of short-term diet change from high fat (F) to high carbohydrate (C) (or vice versa) on the storage and utilization of glycogen and triacylglycerol (TG) in muscle and liver were studied in untrained rats. Rats were fed on an F or C diet for 28 days. For an additional 3 days, half of the rats in both F and C groups were fed the same diets as before (F-F and C-C) and the other half of the rats were switched to the counterpart diets (F-C and C-F). On the final day of the experiment, half of the rats in each diet group were exercised by swimming for 1.5 h and the other half were rested. Short-term diet change from F to C diets increased, but the change from C to F diets decreased, glycogen stores of soleus and plantaris muscles and liver, resulting in no difference in glycogen stores between F-C and C-C, and between F-F and C-F. The dietary change also had an affect on TG stores of red gastrocnemius muscle and liver - however, muscle TG stores were still higher in F-C than in C-C and C-F, and there were no differences in liver TG stores between F-C and C-F. Exercise decreased muscle glycogen contents markedly in F-C and C-C, whereas, it decreased muscle TG concentrations in F-F and C-F. Liver glycogen depletion was lower in F-C than in other groups. Lipolytic activities of epididymal adipose tissue at rest and postexercise were no differences between F-F and F-C, and were higher in F-C than in C-C and C-F. -adrenergic receptor binding was determined with [¹²⁵I] iodocyanopindolol, and maximal numbers of -adrenergic receptor of plasma membrane from perirenal adipose tissue were approximately 170%–200% higher in F-C than in other groups at rest and postexercise. These results suggested that short-term C diet fed rats adapted to F diet enhanced not only glycogen stores of muscle and liver but also did not decrease lipolytic activity of adipose tissue with increased -adrenergic receptor density, resulting in the preservation of energy reserves (glycogen and TG) of muscle at rest, and liver glycogen sparing during exercise.     

Fatty acids and exercise affect glucose transport but not tumour growth in F-344 rats.

This study examined the effect of diet and exercise on tumour growth, and the effect of dietary fatty acids on glucose uptake. Male Fischer 344 rats were divided into 4 dietary groups and fed for 2 weeks. The diets were 5% (wt/wt) safflower oil, 10% safflower oil, 5% docosahexaenoic acid(DHA)-rich, and 10% DHA-rich. On Day 14 the animals were injected with rat fibrosarcoma tumour cells. After 3 days of tumour growth the animals in each diet group were divided into exercise and nonexercise groups. Exercise was achieved by voluntary wheel running. Dietary intake, body weight, tumour growth, and distance run were determined daily. Two weeks later the animals were euthanized and the following tissues were dissected out: tumour, liver, heart, epididymal fat pads, gastrocnemius, epitrochlearis, and soleus muscles. Glucose transport experiments were performed on the epitrochlearis and soleus muscles whereas phospholipid analysis was completed on the gastrocnemius muscle. We observed no effect of either diet or exercise on tumour growth. The glucose transport data demonstrates that short-term voluntary running can cause increased insulin-sensitive transport and that DHA may inhibit transport. DHA-containing diets were associated with increased oxidation products TBARM. In conclusion, exercise benefits on glucose disposal are maintained in tumour-bearing animals but are influenced by fat content and composition. High DHA diets may also increase oxidative damage in muscle through enhanced TBARM production.     

Muscle glycogen repletion and pre-exercise glycogen content: effect of carbohydrate loading in rats previously fed a high fat diet

We have recently reported that rates of muscle glycogen repletion during the early period of recovery were increased by carbohydrate (CHO) loading in rats previously fed a high fat diet. However, the reason for this remained unanswered. The purpose of this study was to examine whether an increase of glycogen utilization due to an elevated pre-exercise glycogen store would enhance rates of glycogen repletion in muscle. Despite an equal degree of glycogen depletion, the rates of glycogen repletion of soleus, red and white gastrocnemius muscles by postexercise administration of glucose (3.0 g · kg^–1 body mass) and citrate (0.5 g · kg^–1 body mass) were faster in the CHO loaded (3 days) rats than in the nonloaded rats, as a result of elevated pre-exercise glycogen content and consequently the greater glycogen utilization. The higher rate of muscle glycogen repletion may in part be explained by increased postexercise glycogen synthase activity.     

Interaction of exercise and diet on GLUT‐4 protein and gene expression in Type I and Type II rat skeletal muscle

We determined the interaction of exercise and diet on glucose transporter (GLUT‐4) protein and mRNA expression in type I (soleus) and type II [extensor digitorum longus (EDL)] skeletal muscle. Forty‐eight Sprague Dawley rats were randomly assigned to one of two dietary conditions: high‐fat (FAT, n=24) or high‐carbohydrate (CHO, n=24). Animals in each dietary condition were allocated to one of two groups: control (NT, n=8) or a group that performed 8 weeks of treadmill running (4 sessions week^–1 of 1000 m @ 28 m min^–1, RUN, n=16). Eight trained rats were killed after their final exercise bout for determination of GLUT‐4 protein and mRNA expression: the remainder were killed 48 h after their last session for measurement of muscle glycogen and triacylglycerol concentration. GLUT‐4 protein expression in NT rats was similar in both muscles after 8 weeks of either diet. However, there was a main effect of training such that GLUT‐4 protein was increased in the soleus of rats fed with either diet (P < 0.05) and in the EDL in animals fed with CHO (P < 0.05). There was a significant diet–training interaction on GLUT‐4 mRNA, such that expression was increased in both the soleus (100% ↑P < 0.05) and EDL (142% ↑P < 0.01) in CHO‐fed animals. Trained rats fed with FAT decreased mRNA expression in the EDL (↓ 45%, P < 0.05) but not the soleus (↓ 14%, NS). We conclude that exercise training in CHO‐fed rats increased both GLUT‐4 protein and mRNA expression in type I and type II skeletal muscle. Despite lower GLUT‐4 mRNA in muscles from fat‐fed animals, exercise‐induced increases in GLUT‐4 protein were largely preserved, suggesting that control of GLUT‐4 protein and gene expression are modified independently by exercise and diet.     

Changes in glycemia induced by exercise in rats: contribution of hepatic glycogenolysis and gluconeogenesis

The participation of hepatic glycogenolysis and gluconeogenesis to the glycemic changes promoted by exercise was investigated. For this purpose, we employed swimming rats (2.5% body weight extra load attached to the tail, at 24 degrees C) using a favorable condition to measure hepatic glycogenolysis (fed rats) and a favorable condition to measure hepatic gluconeogenesis (fasted rats). This experimental approach permits us to compare the contribution of hepatic glycogenolysis and gluconeogenesis to glucose changes for a specific schedule of exercise. The animals were investigated at rest, after 5 minutes of swimming and after swimming to exhaustion. Our results show that hepatic glycogen has a crucial role to determine hyperglycemia during exercise. In contrast, hypoglycemia developed during exercise when glycogen was depleted. However, the ability of the liver to produce glucose from L-lactate, glycerol and L-glutamine was increased during exercise. Taken together, these findings suggest that the hepatic capacity to produce glucose from gluconeogenic substrates (except for L-alanine) was increased when hepatic glycogen stores were depleted. Thus, the increased capacity to produce glucose shown by livers from exercising rats must to be an important metabolic adaptation to protect against severe hypoglycemia.     

Acute and chronic effects of strenuous exercise on glucose metabolism in isolated, incubated soleus muscle of exercise-trained rats

Male and female Wistar rats were exercise-trained for 6 or 11 weeks respectively, to examine the effects of acute exercise or exercise training per se on insulin-stimulated glucose utilization in soleus muscles isolated and incubated in vitro. The maximal activities of hexokinase and 2-oxoglutarate dehydrogenase were significantly elevated (by greater than 50%) in gastrocnemius muscle of exercise-trained male and female rats, indicating an adaptation to the training regime. No significant differences in any of the variables studied were observed between appropriately matched male and female rats. There were no significant differences in the sensitivity or responsiveness of the rates of lactate formation or glycogen synthesis in soleus muscles isolated from exercise-trained and sedentary animals at rest (exercise-trained animals were studied 40 h after the last exercise bout). On the other hand, acute exercise caused significant changes in soleus muscle glucose metabolism. Basal and insulin-stimulated rates of glycogen synthesis were significantly elevated in soleus muscles incubated from both sedentary and exercise-trained rats immediately after an exercise bout. In addition, the responsiveness of glucose utilization to insulin in soleus muscles from exercise-trained rats was significantly increased after acute exercise. The results indicate that significant changes in the control of glucose metabolism by insulin in soleus muscle occur as a result of an acute exercise bout, while no adaptive changes in insulin sensitivity occur in soleus muscle after exercise training.     

Alterations in glucose transporter proteins in alcoholic liver disease in the rat.

We used the intragastric feeding rat model for alcoholic liver disease to investigate alterations in glucose transporter isoforms GLUT 1 and GLUT 2 in response to different dietary fats and ethanol. Six groups of rats (three rats/group) were fed ethanol or dextrose with either saturated fat, corn oil, or fish (menhaden) oil. All control animals were pair fed the same diets as ethanol-fed rats except that ethanol was isocalorically replaced by dextrose. In all animals, the following were assessed: pathological changes in the liver, immunohistochemical and Western blot analysis of GLUT 1 and GLUT 2 isoforms, and glycogen distribution. The most severe pathological changes were seen in fish oil/ethanol fed rats, moderate changes were seen in the corn oil/ethanol group and no changes were observed in the dextrose-fed or saturated fat/ethanol groups. In the groups of rats showing pathological liver injury (corn oil/ethanol and fish oil/ethanol), the depletion in liver glycogen was accompanied by decreased GLUT 2 expression and increased GLUT 1 expression. A decrease in glycogen and GLUT 2 expression was also seen in the fish oil/dextrose-fed rats. We hypothesize that the shift in glucose transporters from GLUT 2 to GLUT 1 probably reflects a compensatory response to attenuated gluconeogenic activity and to meet the increased intracellular demand for glucose. This demand for glucose in the presence of depleted glycogen may serve to provide a source for ATP synthesis in the centrilobular zone where hypoxia occurs secondary to ethanol metabolism.     

Physical activity and liver cholesterol

Summary The effects of various types and intensities of exercise on hepatic cholesterol and fecal sterol levels of rats fed a high fat diet was investigated. The results demonstrate that liver cholesterol of such animals trained either by running or swimming are lower than non-trained controls.The concentration of cholesterol in the livers of rats 4 weeks after the termination of training had increased significantly and approached the level of the control animals. Detrained animals also had higher liver cholesterol levels than trained animals. These results suggest that high fat diets produce a progressive accumulation of cholesterol in the livers of rats which can be retarded by exercise. The excretion of sterol in the fecals was significantly higher in the trained animals. The elevated excretion of cholesterol may be one of the methods by which exercise retards the accumulation of cholesterol in the livers of rats fed high fat diets.This investigation supported by Research Grant HE 08262 from the National Heart Institute, National Institutes of Health, U.S. Public Health Service.     

Effect of creatine supplementation on creatine and glycogen content in rat skeletal muscle

The effects of high dose creatine feeding (5 g kg(-1) BW day(-1), 5 days) on creatine content, glucose transport, and glycogen accumulation in white gastrocnemius, red gastrocnemius and soleus muscles of the rat was investigated. Isolated rat hindquarters of creatine fed and control rats were perfused with a standard medium containing either insulin alone (0, 100 or 20 000 microU mL(-1)) or in combination with creatine (2 or 10 mmol L(-1)). Furthermore, plasma insulin concentration was measured in normal rats during creatine feeding, as well as in anaesthetized rats during intravenous creatine infusion. Five days of creatine feeding increased (P < 0.05) total creatine content in soleus (+ 20%) but not in red gastrocnemius (+15%, n.s.) and white gastrocnemius (+ 10%, n.s.). In parallel, glycogen content was markedly elevated (P < 0.05) in soleus (+ 40%), less (P < 0.05) in red gastrocnemius (+ 15%), and not in white gastrocnemius (+ 10%, n.s.). Glucose transport rate, muscle GLUT-4 content, glycogen synthase activity in perfused muscles and glycogen synthesis rate were not significantly altered by creatine feeding in either muscle type. Furthermore, high dose creatine feeding raised (P < 0.05) plasma creatine concentration fivefold but did not alter circulating insulin level. It is concluded that short-term high dose creatine feeding enhances creatine disposal and glycogen storage in rat skeletal muscle. However, the creatine and glycogen response to creatine supplementation is markedly greater in oxidative than in glycolytic muscles.     

Effects of dietary manipulations on blood glucose and hormonal responses following supramaximal exercise

Summary The effects of supramaximal exercise on blood glucose, insulin, and catecholamine responses were examined in 7 healthy male physical education students (mean±SD: age=21±1.2 years; =54±6 ml · kg^–1 · min^–1) in response to the following three dietary conditions: 1) a normal mixed diet (N); 2) a 24-h low carbohydrate (CHO) diet intended to reduce liver glycogen content (D₁); and 3) a 24-h low CHO diet preceded by a leg muscle CHO overloading protocol intended to reduce hepatic glycogen content with increased muscle glycogen store (D₂). Exercise was performed on a bicycle ergometer at an exercise intensity of 130% for 90 s. Irrespective of the dietary manipulation, supramaximal exercise was associated with a similar significant (p<0.01) increase in the exercise and recovery plasma glucose values. The increase in blood glucose levels was accompanied by a similar increase in insulin concentrations in all three groups despite lower resting insulin levels in conditions D₁ and D₂. Lactate concentrations were higher during the early phase of the recovery period in the D₂ as compared to the N condition. At cessation of exercise, epinephrine and norepinephrine were greatly elevated in all three conditions. These results indicate that the increase in plasma glucose and insulin associated with very high intensity exercise, persists in spite of dietary manipulations intended to reduce liver glycogen content or increase muscle glycogen store. These data suggest that the blood glucose increase following supramaximal exercise is most likely related to hepatic glycogenolysis in spite of a substantial decrease in liver glycogen content.