Rapid upregulation of pyruvate dehydrogenase kinase activity in human skeletal muscle during prolonged exercise.

Prolonged moderate-intensity exercise is characterized by a progressive reduction in carbohydrate oxidation and concomitant increase in fat oxidation. Pyruvate dehydrogenase (PDH) controls the entry of pyruvate into oxidative pathways and is a rate-limiting enzyme for carbohydrate metabolism. PDH is controlled by the activities of a kinase (PDK, inhibitory) and phosphatase (stimulatory). To test the hypothesis that increased PDK activity was associated with decreased PDH activity and carbohydrate oxidation during an acute exercise bout, seven recreationally active men completed 4 h of cycle exercise at 55% peak oxygen consumption. Muscle samples were obtained before and at 10 min and 4 h of exercise for the measurement of PDH activity and the extraction of intact mitochondria for the measurements of PDK activity and PDK-2 and PDK-4 protein expression. Carbohydrate oxidation was reduced (P < 0.05) with exercise duration. Muscle glycogen content was lower (P < or = 0.05) at 4 h compared with rest and there was no change in muscle pyruvate content from 10 to 240 min during exercise (10 min: 0.28 +/- 0.05; 240 min: 0.35 +/- 0.09 mmol/kg dry muscle). PDH activity increased (P < 0.05) above resting values at 10 min (2.86 +/- 0.26 mmol.min(-1).kg wet muscle(-1)), but was lower than 10 min after 4 h (2.23 +/- 0.24 mmol.min(-1).kg wet muscle(-1)) of exercise. PDK-2 and PDK-4 protein expression was not different from rest at 10 min and 4 h of exercise. PDK activity at rest averaged 0.081 +/- 0.016 min(-1), was similar at 10 min, and increased (P < 0.05) to 0.189 +/- 0.013 min(-1) at 4 h. Although reduced glycolytic flux may have played a role in decreasing carbohydrate oxidation, the results suggest that increased PDK activity contributed to the reduction in PDH activity and carbohydrate oxidation late in prolonged exercise. The increased PDK activity was independent of changes in intra-mitochondrial effectors, and PDK-2 and PDK-4 protein content, suggesting that it was caused by a change in the specific activity of the existing kinases.     

Partial restoration of dietary fat induced metabolic adaptations to training by 7 days of carbohydrate diet.

We tested the hypothesis that a shift to carbohydrate diet after prolonged adaptation to fat diet would lead to decreased glucose uptake and impaired muscle glycogen breakdown during exercise compared with ingestion of a carbohydrate diet all along. We studied 13 untrained men; 7 consumed a high-fat (Fat-CHO; 62% fat, 21% carbohydrate) and 6 a high-carbohydrate diet (CHO; 20% fat, 65% carbohydrate) for 7 wk, and thereafter both groups consumed the carbohydrate diet for an eighth week. Training was performed throughout. After 8 wk, during 60 min of exercise (71 +/- 1% pretraining maximal oxygen uptake) average leg glucose uptake (1.00 +/- 0.07 vs. 1.55 +/- 0.21 mmol/min) was lower (P < 0.05) in Fat-CHO than in CHO. The rate of muscle glycogen breakdown was similar (4.4 +/- 0.5 vs. 4.2 +/- 0.7 mmol. min(-1). kg dry wt(-1)) despite a significantly higher preexercise glycogen concentration (872 +/- 59 vs. 688 +/- 43 mmol/kg dry wt) in Fat-CHO than in CHO. In conclusion, shift to carbohydrate diet after prolonged adaptation to fat diet and training causes increased resting muscle glycogen levels but impaired leg glucose uptake and similar muscle glycogen breakdown, despite higher resting levels, compared with when the carbohydrate diet is consumed throughout training.     

Physiological hyperinsulinemia impairs insulin-stimulated glycogen synthase activity and glycogen synthesis

Although chronic hyperinsulinemia has been shown to induce insulin resistance, the basic cellular mechanisms responsible for this phenomenon are unknown. The present study was performed 1) to determine the time-related effect of physiological hyperinsulinemia on glycogen synthase (GS) activity, hexokinase II (HKII) activity and mRNA content, and GLUT-4 protein in muscle from healthy subjects, and 2) to relate hyperinsulinemia-induced alterations in these parameters to changes in glucose metabolism in vivo. Twenty healthy subjects had a 240-min euglycemic insulin clamp study with muscle biopsies and then received a low-dose insulin infusion for 24 (n = 6) or 72 h (n = 14) (plasma insulin concentration = 121 +/- 9 or 143 +/- 25 pmol/l, respectively). During the baseline insulin clamp, GS fractional velocity (0.075 +/- 0.008 to 0.229 +/- 0.02, P < 0.01), HKII mRNA content (0.179 +/- 0.034 to 0.354 +/- 0.087, P < 0.05), and HKII activity (2.41 +/- 0.63 to 3.35 +/- 0.54 pmol x min(-1) x ng(-1), P < 0.05), as well as whole body glucose disposal and nonoxidative glucose disposal, increased. During the insulin clamp performed after 24 and 72 h of sustained physiological hyperinsulinemia, the ability of insulin to increase muscle GS fractional velocity, total body glucose disposal, and nonoxidative glucose disposal was impaired (all P < 0.01), whereas the effect of insulin on muscle HKII mRNA, HKII activity, GLUT-4 protein content, and whole body rates of glucose oxidation and glycolysis remained unchanged. Muscle glycogen concentration did not change [116 +/- 28 vs. 126 +/- 29 micromol/kg muscle, P = nonsignificant (NS)] and was not correlated with the change in nonoxidative glucose disposal (r = 0.074, P = NS). In summary, modest chronic hyperinsulinemia may contribute directly (independent of change in muscle glycogen concentration) to the development of insulin resistance by its impact on the GS pathway.     

Pyruvate overrides inhibition of PDH during exercise after a low-carbohydrate diet

The effects of carbohydrate deprivation on the regulation of pyruvate dehydrogenase (PDH) were studied at rest and during moderate-intensity exercise. An inhibitory effect of a chronic low-carbohydrate diet (LCD) on the active form of PDH (PDHa) mediated by a stable increase in PDH kinase (PDHK) activity has recently been reported (Peters SJ, Howlett RA, St. Amand TA, Heigenhauser GJF, and Spriet LL. Am J Physiol Endocrinol Metab 275: E980-E986, 1998.). In the present study, seven males cycled at 65% maximal O(2) uptake for 30 min after a 6-day LCD. Exercise was repeated 1 wk later after a mixed diet (MD). Muscle biopsies were sampled from the vastus lateralis at rest and at 2 and 30 min of exercise. At rest, PDHa activity (0.18 +/- 0.04 vs. 0.63 +/- 0.18 mmol x min(-1) x kg wet wt(-1)), muscle glycogen content (310.2 +/- 36.9 vs. 563.9 +/- 32.6 mmol/kg dry wt), and muscle lactate content (2.6 +/- 0.3 vs. 4.2 +/- 0.6 mmol/kg dry wt) were significantly lower after the LCD. Resting muscle acetyl-CoA (10.8 +/- 1.9 vs. 7.4 +/- 0.8 micromol/kg dry wt) and acetylcarnitine (5.3 +/- 1.4 vs. 1.6 +/- 0.3 mmol/kg dry wt) contents were significantly elevated after the LCD. During exercise, PDHa, glycogenolytic rate (LCD 5.8 +/- 0.4 vs. MD 6.9 +/- 0.2 mmol x min(-1) x kg dry wt(-1)), and muscle concentrations of acetylcarnitine, pyruvate, and lactate increased to the same extent in both conditions. The results of the present study suggest that inhibition of resting PDH by elevated PDHK activity after a LCD may be overridden by the availability of muscle pyruvate during exercise.     

Elevated n-3 fatty acids in a high-fat diet attenuate the increase in PDH kinase activity but not PDH activity in human skeletal muscle.

We tested the hypothesis that a high-fat diet (75% fat; 5% carbohydrates; 20% protein), for which 15% of the fat content was substituted with n-3 fatty acids, would not exhibit the diet-induced increase in pyruvate dehydrogenase kinase (PDK) activity, which is normally observed in human skeletal muscle. The fat content was the same in both the regular high-fat diet (HF) and in the n-3-substituted diet (N3). PDK activity increased after both high-fat diets, but the increase was attenuated after the N3 diet (0.051 +/- 0.007 and 0.218 +/- 0.047 min(-1) for pre- and post-HF, respectively; vs. 0.073 +/- 0.016 and 0.133 +/- 0.032 min(-1) for pre- and post-N3, respectively). However, the active form of pyruvate dehydrogenase (PDHa) activity decreased to a similar extent in both conditions (0.93 +/- 0.17 and 0.43 +/- 0.09 mmol/kg wet wt pre- and post-HF; vs. 0.87 +/- 0.19 and 0.39 +/- 0.05 mmol/kg wet wt pre- and post-N3, respectively). This suggested that the difference in PDK activity did not affect PDHa activation in the basal state, and it was regulated by intramitochondrial effectors, primarily muscle pyruvate concentration. Muscle glycogen content was consistent throughout the study, before and after both diet conditions, whereas muscle glucose-6-phosphate, glycerol-3-phosphate, lactate, and pyruvate were decreased after the high-fat diets. Plasma triglycerides decreased after both high-fat diets but decreased to a greater extent after the N3, whereas plasma free fatty acids increased after both diets, but to a lesser extent after the N3. In summary, PDK activity is decreased after a high-fat diet that is rich in n-3 fatty acids, although PDHa activity was unaltered. In addition, our data demonstrated that the hypolipidemic effect of n-3 fatty acids occurs earlier (3 days) than previously reported and is evident even when the diet has 75% of its total energy derived from fat.     

The effect of the new oral hypoglycemic agent A-4166 on glucose turnover in the high fat diet-induced and/or in the hereditary insulin resistance of rats.

A-4166, a phenylalanine derivative, is a hypoglycemic agent, which has been shown to improve blood glucose levels mainly due to the rapid and short term stimulation of insulin release. Nevertheless, a possible extrapancreatic action of A-4166 has not yet been investigated. Therefore, insulin action (euglycemic hyperinsulinemic 6.4 mU.kg-1.min-1 clamp plus 3H-2-deoxyglucose tracer administration) was studied after 3 weeks on either standard (BD) or high fat (HF) diet in normal control (C) or in hereditary insulin resistant (hHTg) rats which were given a single dose of A-4166 (10 mg per kg BW, i.v.) 60 min after clamp commencement. HF feeding reduced the glucose infusion rate (GIR) required to maintain euglycemia to about 50% of C (p < 0.001). In hHTg rats, HF did not further pronounce the pre-existing decrease of GIR of hHTg animals fed BD. A-4166 changed GIR neither in C, nor in the hHTg group. The estimated glucose disposal (Rd) (C-BD: 32.3 +/- 1.9 vs C-HF: 25.5 +/- 1.9 mg.kg-1.min-1, p < 0.001) and glucose metabolic index (Rg') in skeletal muscles (Q. femoris: C-BD: 25.6 +/- 1.5 vs C-HF: 12.3 +/- 1.1 mmol.100 g-1.min-1, p < 0.001) were reduced by HF in control rats but were not restored by a concomitant bolus of A-4166. Nevertheless, in hHTg rats fed the HF diet a single dose of A-4166 brought back their Rd (hHTg-HF: 23.5 +/- 1.3 vs hHTg-HF plus A-4166: 31.0 +/- 3.5 p < 0.03) and Rg' (Soleus muscle: hHTg-HF: 29.2 +/- 3.2 vs hHTg-HF plus A-4166: 41.3 +/- 4.0) to values of the control group on BD. In summary, a) a single bolus administration of A-4166 to the control or to the insulin resistant hHTg rats, fed either the BD or HF diets, did not abolish the reduction of GIR required to maintain euglycemia during hyperinsulinemic clamps; b) nevertheless, A-4166 caused a significant increase of the estimated plasma glucose disposal (Rd) and skeletal muscle glucose metabolic index (Rg') of hHTG rats fed the HF diet; c) we suggest that A-4166 may have an extrapancreatic action but this needs to be proven using a long-term administration plan of A-4166.     

Effects of hyperoxia on skeletal muscle carbohydrate metabolism during transient and steady-state exercise.

This study compared the effects of inspiring either a hyperoxic (60% O(2)) or normoxic gas (21% O(2)) while cycling at 70% peak O(2) uptake on 1) the ATP derived from substrate phosphorylation during the initial minute of exercise, as estimated from phosphocreatine degradation and lactate accumulation, and 2) the reliance on carbohydrate utilization and oxidation during steady-state cycling, as estimated from net muscle glycogen use and the activity of pyruvate dehydrogenase (PDH) in the active form (PDH(a)), respectively. We hypothesized that 60% O(2) would decrease substrate phosphorylation at the onset of exercise and that it would not affect steady-state exercise PDH activity, and therefore muscle carbohydrate oxidation would be unaltered. Ten active male subjects cycled for 15 min on two occasions while inspiring 21% or 60% O(2), balance N(2). Blood was obtained throughout and skeletal muscle biopsies were sampled at rest and 1 and 15 min of exercise in each trial. The ATP derived from substrate-level phosphorylation during the initial minute of exercise was unaffected by hyperoxia (21%: 52.2 +/- 11.1; 60%: 54.0 +/- 9.5 mmol ATP/kg dry wt). Net glycogen breakdown during 15 min of cycling was reduced during the 60% O(2) trial vs. 21% O(2) (192.7 +/- 25.3 vs. 138.6 +/- 16.8 mmol glycosyl units/kg dry wt). Hyperoxia had no effect on PDH(a), because it was similar to the 21% O(2) trial at rest and during exercise (21%: 2.20 +/- 0.26; 60%: 2.25 +/- 0.30 mmol.kg wet wt(-1).min(-1)). Blood lactate was lower (6.4 +/- 1.0 vs. 8.9 +/- 1.0 mM) at 15 min of exercise and net muscle lactate accumulation was reduced from 1 to 15 min of exercise in the 60% O(2) trial compared with 21% (8.6 +/- 5.1 vs. 27.3 +/- 5.8 mmol/kg dry wt). We concluded that O(2) availability did not limit oxidative phosphorylation in the initial minute of the normoxic trial, because substrate phosphorylation was unaffected by hyperoxia. Muscle glycogenolysis was reduced by hyperoxia during steady-state exercise, but carbohydrate oxidation (PDH(a)) was unaffected. This closer match between pyruvate production and oxidation during hyperoxia resulted in decreased muscle and blood lactate accumulation. The mechanism responsible for the decreased muscle glycogenolysis during hyperoxia in the present study is not clear.     

Muscle metabolism during prolonged exercise in humans: influence of carbohydrate availability.

Eight endurance-trained men cycled to volitional exhaustion at 69 +/- 1% peak oxygen uptake on two occasions to examine the effect of carbohydrate supplementation during exercise on muscle energy metabolism. Subjects ingested an 8% carbohydrate solution (CHO trial) or a sweet placebo (Con trial) in a double-blind, randomized order, with vastus lateralis muscle biopsies (n = 7) obtained before and immediately after exercise. No differences in oxygen uptake, heart rate, or respiratory exchange ratio during exercise were observed between the trials. Exercise time to exhaustion was increased by approximately 30% when carbohydrate was ingested [199 +/- 21 vs. 152 +/- 9 (SE) min, P < 0.05]. Plasma glucose and insulin levels during exercise were higher and plasma free fatty acids lower in the CHO trial. No differences between trials were observed in the decreases in muscle glycogen and phosphocreatine or the increases in muscle lactate due to exercise. Muscle ATP levels were not altered by exercise in either trial. There was a small but significant increase in muscle inosine monophosphate levels at the point of exhaustion in both trials, and despite the subjects in CHO trial cycling 47 min longer, their muscle inosine monophosphate level was significantly lower than in the Con trial (CHO: 0.16 +/- 0.08, Con: 0.23 +/- 0.09 mmol/kg dry muscle). These data suggest that carbohydrate ingestion may increase endurance capacity, at least in part, by improving muscle energy balance.     

PDH activation during in vitro muscle contractions in PDH kinase 2 knockout mice: effect of PDH kinase 1 compensation

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate oxidation in skeletal muscle. PDH is deactivated by a set of PDH kinases (PDK1, PDK2, PDK3, PDK4), with PDK2 and PDK4 being the most predominant isoforms in skeletal muscle. Although PDK2 is the most abundant isoform, few studies have examined its physiological role. The role of PDK2 on PDH activation (PDHa) at rest and during muscle stimulation at 10 and 40 Hz (eliciting low- and moderate-intensity muscle contractions, respectively) in isolated extensor digitorum longus muscles was studied in PDK2 knockout (PDK2KO) and wild-type (WT) mice (n = 5 per group). PDHa activity was unexpectedly 35 and 77% lower in PDK2KO than WT muscle (P = 0.043), while total PDK activity was nearly fourfold lower in PDK2KO muscle (P = 0.006). During 40-Hz contractions, initial force was lower in PDK2KO than WT muscle (P < 0.001) but fatigued similarly to ～75% of initial force by 3 min. There were no differences in initial force or rate of fatigue during 10-Hz contractions. PDK1 compensated for the lack of PDK2 and was 1.8-fold higher in PDK2KO than WT muscle (P = 0.019). This likely contributed to ensuring that resting PDHa activity was similar between the groups and accounts for the lower PDH activation during muscle contraction, as PDK1 is a very potent inhibitor of the PDH complex. Increased PDK1 expression appears to be regulated by hypoxia inducible factor-1α, which was 3.5-fold higher in PDK2KO muscle. It is clear that PDK2 activity is essential, even at rest, in regulation of carbohydrate oxidation and production of reducing equivalents for the electron transport chain. In addition, these results underscore the importance of the overall kinetics of the PDK isoform population, rather than total PDK activity, in determining transformation of the PDH complex and PDHa activity during muscle contraction.     

Effect of postexercise carbohydrate supplementation on glucose uptake-associated gene expression in the human skeletal muscle

We previously found that the exercise-induced elevation in GLUT4 mRNA of rat muscle can be rapidly down-regulated when glucose is given immediately following exercise. The purpose of this study was to determine the effect of postexercise carbohydrate diet on GLUT4 and hexokinase (HK) II mRNA levels in the human skeletal muscle. Eight untrained male subjects (age, 20.7+/-3.1 years) exercised for 60 min on a cycle ergometer at a 70-75% maximal oxygen consumption. The postexercise dietary treatment was performed in a crossover design. Immediately after the exercise, a diet with 70% carbohydrate content (1 g per kilogram of body weight; 356+/-19.8 kcal) was given to half of the subjects (eaten in 10 min) followed by a 3-h recovery, while the control subjects remained unfed for 3 h. Biopsies were performed on the deep portion of the vastus lateralis muscle of all subjects immediately after the exercise and 3 h after the carbohydrate ingestion. Blood glucose and serum insulin concentrations were measured every 30 min for 3 h. At the end of the 3-h recovery, blood glucose and serum insulin levels were not different from control levels, indicating that the oral carbohydrate was mostly disposed in the body within 3 h. In addition, GLUT4 and HK II mRNA levels were significantly lowered in the exercised human skeletal muscle in subjects receiving the carbohydrate diet. In conclusion, the present study demonstrates that GLUT4 mRNA and HK II mRNA in the exercised human skeletal muscle were significantly lowered by a high-carbohydrate diet.     

Leptin administration improves skeletal muscle insulin responsiveness in diet-induced insulin-resistant rats

In addition to suppressing appetite, leptin may also modulate insulin secretion and action. Leptin was administered here to insulin-resistant rats to determine its effects on secretagogue-stimulated insulin release, whole body glucose disposal, and insulin-stimulated skeletal muscle glucose uptake and transport. Male Wistar rats were fed either a normal (Con) or a high-fat (HF) diet for 3 or 6 mo. HF rats were then treated with either vehicle (HF), leptin (HF-Lep, 10 mg. kg(-1). day(-1) sc), or food restriction (HF-FR) for 12-15 days. Glucose tolerance and skeletal muscle glucose uptake and transport were significantly impaired in HF compared with Con. Whole body glucose tolerance and rates of insulin-stimulated skeletal muscle glucose uptake and transport in HF-Lep were similar to those of Con and greater than those of HF and HF-FR. The insulin secretory response to either glucose or tolbutamide (a pancreatic beta-cell secretagogue) was not significantly diminished in HF-Lep. Total and plasma membrane skeletal muscle GLUT-4 protein concentrations were similar in Con and HF-Lep and greater than those in HF and HF-FR. The findings suggest that chronic leptin administration reversed a high-fat diet-induced insulin-resistant state, without compromising insulin secretion.     

Glucose infusion attenuates endogenous glucose production and enhances glucose use of horses during exercise.

We examined the effects of increased glucose availability on glucose kinetics and substrate utilization in horses during exercise. Six conditioned horses ran on a treadmill for 90 min at 34 +/- 1% of maximum oxygen uptake. In one trial [glucose (Glu)], glucose was infused at a mean rate of 34.9 +/- 1.1 micromol. kg(-1). min(-1), whereas in the other trial [control (Con)] an equivalent volume of isotonic saline was infused. Plasma glucose increased during exercise in Glu (90 min: 8.3 +/- 1.7 mM) but was largely unchanged in Con (90 min: 5.1 +/- 0.4 mM). In Con, hepatic glucose production (HGP) increased during exercise, reaching a peak of 38.6 +/- 2.7 micromol. kg(-1). min(-1) after 90 min. Glucose infusion partially suppressed (P < 0.05) the rise in HGP (peak value 25.8 +/- 3.3 micromol. kg(-1). min(-1)). In Con, glucose rate of disappearance (R(d)) rose to a peak of 40.4 +/- 2.9 micromol. kg(-1). min(-1) after 90 min; in Glu, augmented glucose utilization was reflected by values for glucose R(d) that were twofold higher (P < 0.001) than in Con between 30 and 90 min. Total carbohydrate oxidation was higher (P < 0.05) in Glu (187.5 +/- 8.5 micromol. kg(-1). min(-1)) than in Con (159.2 +/- 7.3 micromol. kg(-1).min(-1)), but muscle glycogen utilization was similar between trials. We conclude that an increase in glucose availability in horses during low-intensity exercise 1) only partially suppresses HGP, 2) attenuates the decrease in carbohydrate oxidation during such exercise, but 3) does not affect muscle glycogen utilization.     

Effects of chronic N(omega)-nitro-L-arginine methyl ester administration on glucose tolerance and skeletal muscle glucose transport in the rat.

It has been suggested that nitric oxide (NO) is a key regulator of carbohydrate metabolism in skeletal muscle. The present study was undertaken to examine the effects of chronic in vivo competitive antagonism of NO synthase (NOS) by the administration of N(omega)-nitro-L-arginine methyl ester (L-NAME) in the drinking water (1 mg/ml) for 14 days on glucose tolerance and skeletal muscle glucose transport in rats. Oral glucose tolerance tests (OGTT) revealed an impaired glucose tolerance in the L-NAME-treated rats as reflected by the area under the glucose curve (4675 +/- 514 mg% x 120 min (control) vs 6653 +/- 571 mg% x 120 min (L-NAME treated); P < 0.03). While a large rise in plasma insulin concentration was present in the control rats (0.87 +/- 0.34 ng/ml, P < 0.001) during the first 15 min of the OGTT, rises in plasma insulin concentration were absent in the L-NAME-treated rats (0.18 +/- 0.13 ng/ml, P = NS). Intravenous glucose tolerance tests confirmed an impaired insulin secretion in the L-NAME-treated rats. In contrast, insulin-stimulated 2-deoxyglucose transport was enhanced (P < 0.03) by chronic NOS inhibition (5.29 +/- 0.83 nmol/g/min) compared to control rats (2.21 +/- 0.90 nmol/g/min). Plasma sodium concentrations were lower and plasma potassium concentrations were higher in the L-NAME-treated group, indicating an impaired electrolyte status. We conclude that chronic in vivo administration of a NOS inhibitor, while not impairing basal parameters of carbohydrate metabolism, may manifest different responses than acute exposure to the same agent in vitro.     

Enhanced leg exercise endurance with a high-carbohydrate diet and dihydroxyacetone and pyruvate

The effects of dietary supplementation of dihydroxyacetone and pyruvate (DHAP) on metabolic responses and endurance capacity during leg exercise were determined in eight untrained males (20-30 yr). During the 7 days before exercise, a high-carbohydrate diet was consumed (70% carbohydrate, 18% protein, 12% fat; 35 kcal/kg body weight). One hundred grams of either Polycose (placebo) or dihydroxyacetone and pyruvate (treatment, 3:1) were substituted for a portion of carbohydrate. Dietary conditions were randomized, and subjects consumed each diet separated by 7-14 days. After each diet, cycle ergometer exercise (70% of peak oxygen consumption) was performed to exhaustion. Biopsy of the vastus lateralis muscle was obtained before and after exercise. Blood samples were drawn through radial artery and femoral vein catheters at rest, after 30 min of exercise, and at exercise termination. Leg endurance was 66 +/- 4 and 79 +/- 2 min after placebo and DHAP, respectively (P less than 0.01). Muscle glycogen at rest and exhaustion did not differ between diets. Whole leg arteriovenous glucose difference was greater (P less than 0.05) for DHAP than for placebo at rest (0.36 +/- 0.05 vs. 0.19 +/- 0.07 mM) and after 30 min of exercise (1.06 +/- 0.14 vs. 0.65 +/- 0.10 mM) but did not differ at exhaustion. Plasma free fatty acids, glycerol, and beta-hydroxybutyrate were similar during rest and exercise for both diets. Estimated total glucose oxidation during exercise was 165 +/- 17 and 203 +/- 15 g after placebo and DHAP, respectively (P less than 0.05). It is concluded that feeding of DHAP for 7 days in conjunction with a high carbohydrate diet enhances leg exercise endurance capacity by increasing glucose extraction by muscle.     

Effect of a high protein diet on glucose tolerance in the rat model

The purpose of this study was to determine the effects of a high protein diet on glucose tolerance. Nine Sprague Dawley rats received a high protein (HP) diet (65% protein, 35% fat) and eight rats consumed a standard chow (SC) diet over eight weeks. Oral glucose tolerance tests (OGTT) were performed at the end of the third and the seventh week. The diet did not effect glucose tolerance in the first (SC=10357+/-294 mg/dl/120 min; HP=9846+/-300 mg/dl/120 min) or the second OGTT (SC=10134+/-395 mg/dl/120 min; HP=10721+/-438 mg/dl/120 min) as reflected by the area under the glucose concentration curve. Similarly, the area under the insulin concentration curve was not effected by the high protein diet during the first (SC=49.21+/-8.46 ng/ml/120 min; HP=41.75+/-10.54 ng/ml/120 min) or the second OGTT (SC=96.63+/-13.68 ng/ml/120 min; HP=92.77+/-17.44 ng/ml/120 min). The high protein diet group experienced a delayed glucose response for the first (SC=30 min at 112+/-7 mg/dl; HP=60 min at 101+/-5 mg/dl) and second OGTT (SC=15 min at 117+/-5 mg/dl; HP=60 min at 95+/-7 mg/dl). Body mass increased to the same extent in each diet group from the initial to final weighing (SC=159+/-2 g to 254+/-7 g; HP=157+/-2 g to 242+/-7 g). Despite a delay in peak glucose response, these findings suggest that glucose tolerance and body mass were neither adversely nor positively affected by a high protein diet.     

Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration

Five days of a high-fat diet while training, followed by 1 day of carbohydrate (CHO) restoration, increases rates of whole body fat oxidation and decreases CHO oxidation during aerobic cycling. The mechanisms responsible for these shifts in fuel oxidation are unknown but involve up- and downregulation of key regulatory enzymes in the pathways of skeletal muscle fat and CHO metabolism, respectively. This study measured muscle PDH and HSL activities before and after 20 min of cycling at 70% VO2peak and 1 min of sprinting at 150% peak power output (PPO). Estimations of muscle glycogenolysis were made during the initial minute of exercise at 70% VO2peak and during the 1-min sprint. Seven male cyclists undertook this exercise protocol on two occasions. For 5 days, subjects consumed in random order either a high-CHO (HCHO) diet (10.3 g x kg(-1) x day(-1) CHO, or approximately 70% of total energy intake) or an isoenergetic high-fat (FAT-adapt) diet (4.6 g x kg(-1) x day(-1) FAT, or 67% of total energy) while undertaking supervised aerobic endurance training. On day 6 for both treatments, subjects ingested an HCHO diet and rested before their experimental trials on day 7. This CHO restoration resulted in similar resting glycogen contents (FAT-adapt 873 +/- 121 vs. HCHO 868 +/- 120 micromol glucosyl units/g dry wt). However, the respiratory exchange ratio was lower during cycling at 70% VO2peak in the FAT-adapt trial, which resulted in an approximately 45% increase and an approximately 30% decrease in fat and CHO oxidation, respectively. PDH activity was lower at rest and throughout exercise at 70% VO2peak (1.69 +/- 0.25 vs. 2.39 +/- 0.19 mmol x kg wet wt(-1) x min(-1)) and the 1-min sprint in the FAT-adapt vs. the HCHO trial. Estimates of glycogenolysis during the 1st min of exercise at 70% VO2peak and the 1-min sprint were also lower after FAT-adapt (9.1 +/- 1.1 vs. 13.4 +/- 2.1 and 37.3 +/- 5.1 vs. 50.5 +/- 2.7 glucosyl units x kg dry wt(-1) x min(-1)). HSL activity was approximately 20% higher (P = 0.12) during exercise at 70% VO2peak after FAT-adapt. Results indicate that previously reported decreases in whole body CHO oxidation and increases in fat oxidation after the FAT-adapt protocol are a function of metabolic changes within skeletal muscle. The metabolic signals responsible for the shift in muscle substrate use during cycling at 70% VO2peak remain unclear, but lower accumulation of free ADP and AMP after the FAT-adapt trial may be responsible for the decreased glycogenolysis and PDH activation during sprinting.     

Effects of 7 wk of endurance training on human skeletal muscle metabolism during submaximal exercise.

This is the first study to examine the effects of endurance training on the activation state of glycogen phosphorylase (Phos) and pyruvate dehydrogenase (PDH) in human skeletal muscle during exercise. We hypothesized that 7 wk of endurance training (Tr) would result in a posttransformationally regulated decrease in flux through Phos and an attenuated activation of PDH during exercise due to alterations in key allosteric modulators of these important enzymes. Eight healthy men (22 +/- 1 yr) cycled to exhaustion at the same absolute workload (206 +/- 5 W; approximately 80% of initial maximal oxygen uptake) before and after Tr. Muscle biopsies (vastus lateralis) were obtained at rest and after 5 and 15 min of exercise. Fifteen minutes of exercise post-Tr resulted in an attenuated activation of PDH (pre-Tr: 3.75 +/- 0.48 vs. post-Tr: 2.65 +/- 0.38 mmol.min(-1).kg wet wt(-1)), possibly due in part to lower pyruvate content (pre-Tr: 0.94 +/- 0.14 vs. post-Tr: 0.46 +/- 0.03 mmol/kg dry wt). The decreased pyruvate availability during exercise post-Tr may be due to a decreased muscle glycogenolytic rate (pre-Tr: 13.22 +/- 1.01 vs. post-Tr: 7.36 +/- 1.26 mmol.min(-1).kg dry wt(-1)). Decreased glycogenolysis was likely mediated, in part, by posttransformational regulation of Phos, as evidenced by smaller net increases in calculated muscle free ADP (pre-Tr: 111 +/- 16 vs. post-Tr: 84 +/- 10 micromol/kg dry wt) and P(i) (pre-Tr: 57.1 +/- 7.9 vs. post-Tr: 28.6 +/- 5.6 mmol/kg dry wt). We have demonstrated for the first time that several signals act to coordinately regulate Phos and PDH, and thus carbohydrate metabolism, in human skeletal muscle after 7 wk of endurance training.     

Effect of epinephrine on glucose disposal during exercise in humans: role of muscle glycogen

This study examined the effect of epinephrine on glucose disposal during moderate exercise when glycogenolytic flux was limited by low preexercise skeletal muscle glycogen availability. Six male subjects cycled for 40 min at 59 +/- 1% peak pulmonary O2 uptake on two occasions, either without (CON) or with (EPI) epinephrine infusion starting after 20 min of exercise. On the day before each experimental trial, subjects completed fatiguing exercise and then maintained a low carbohydrate diet to lower muscle glycogen. Muscle samples were obtained after 20 and 40 min of exercise, and glucose kinetics were measured using [6,6-2H]glucose. Exercise increased plasma epinephrine above resting concentrations in both trials, and plasma epinephrine was higher (P < 0.05) during the final 20 min in EPI compared with CON. Muscle glycogen levels were low after 20 min of exercise (CON, 117 +/- 25; EPI, 122 +/- 20 mmol/kg dry matter), and net muscle glycogen breakdown and muscle glucose 6-phosphate levels during the subsequent 20 min of exercise were unaffected by epinephrine infusion. Plasma glucose increased with epinephrine infusion (i.e., 20-40 min), and this was due to a decrease in glucose disposal (R(d)) (40 min: CON, 33.8 +/- 3; EPI, 20.9 +/- 4.9 micromol. kg(-1). min(-1), P < 0.05), because the exercise-induced rise in glucose rate of appearance was similar in the trials. These results show that glucose R(d) during exercise is reduced by elevated plasma epinephrine, even when muscle glycogen availability and utilization are low. This suggests that the effect of epinephrine does not appear to be mediated by increased glucose 6-phosphate, secondary to enhanced muscle glycogenolysis, but may be linked to a direct effect of epinephrine on sarcolemmal glucose transport.     

Effect of carbohydrate ingestion on metabolism during running and cycling.

The effects of carbohydrate or water ingestion on metabolism were investigated in seven male subjects during two running and two cycling trials lasting 60 min at individual lactate threshold using indirect calorimetry, U-14C-labeled tracer-derived measures of the rates of oxidation of plasma glucose, and direct determination of mixed muscle glycogen content from the vastus lateralis before and after exercise. Subjects ingested 8 ml/kg body mass of either a 6.4% carbohydrate-electrolyte solution (CHO) or water 10 min before exercise and an additional 2 ml/kg body mass of the same fluid after 20 and 40 min of exercise. Plasma glucose oxidation was greater with CHO than with water during both running (65 +/- 20 vs. 42 +/- 16 g/h; P < 0.01) and cycling (57 +/- 16 vs. 35 +/- 12 g/h; P < 0.01). Accordingly, the contribution from plasma glucose oxidation to total carbohydrate oxidation was greater during both running (33 +/- 4 vs. 23 +/- 3%; P < 0.01) and cycling (36 +/- 5 vs. 22 +/- 3%; P < 0.01) with CHO ingestion. However, muscle glycogen utilization was not reduced by the ingestion of CHO compared with water during either running (112 +/- 32 vs. 141 +/- 34 mmol/kg dry mass) or cycling (227 +/- 36 vs. 216 +/- 39 mmol/kg dry mass). We conclude that, compared with water, 1) the ingestion of carbohydrate during running and cycling enhanced the contribution of plasma glucose oxidation to total carbohydrate oxidation but 2) did not attenuate mixed muscle glycogen utilization during 1 h of continuous submaximal exercise at individual lactate threshold.