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

Endurance exercise with reduced muscle glycogen content influences substrate utilization and attenuates acute mTORC1- and autophagic signaling in human type I and type II muscle fibers

BACKGROUND

Exercising with low muscle glycogen content can improve training adaptation, but the mechanisms underlying the muscular adaptation are still largely unknown. In this study, we measured substrate utilization and cell signaling in different muscle fiber types during exercise and investigated a possible link between these variables.

METHODS

Five subjects performed a single leg cycling exercise in the evening (day 1) with the purpose of reducing glycogen stores. The following morning (day 2), they performed two-legged cycling at ∼70% of VO2peak for 1 h. Muscle biopsies were taken from both legs pre- and post-exercise for enzymatic analyses of glycogen, metabolite concentrations using LC-MS/MS-based quantification, and protein signaling using Western blot in pools of type I or type II fibers.

RESULTS

Glycogen content was 60-65% lower for both fiber types (P < 0.01) in the leg that exercised on day 1 (low leg) compared to the other leg with normal level of glycogen (normal leg) before the cycling exercise on day 2. Glycogen utilization during exercise was significantly less in both fiber types in the low compared to the normal leg (P < 0.05). In the low leg, there was a 14- and 6-fold increase in long-chain fatty acids conjugated to carnitine in type I and type II fibers, respectively, post-exercise. This increase was 3-4 times larger than in the normal leg (P < 0.05). Post-exercise, mTORSer2448 phosphorylation was increased in both fiber types in the normal leg (P < 0.05) but remained unchanged in both fiber types in the low leg together with an increase in eEF2Thr56 phosphorylation in type I fibers (P < 0.01). Exercise induced a reduction in the autophagy marker LC3B-II in both fiber types and legs, but the post-exercise level was higher in both fiber types in the low leg (P < 0.05). Accordingly, the LC3B-II/I ratio decreased only in the normal leg (75% for type I and 87% for type II, P < 0.01).

CONCLUSIONS

Starting an endurance exercise session with low glycogen availability leads to profound changes in substrate utilization in both type I and type II fibers. This may reduce the mTORC1 signaling response, primarily in type I muscle fibers, and attenuate the normally observed reduction in autophagy.

Glucosinolate-rich broccoli sprouts protect against oxidative stress and improve adaptations to intense exercise training

Oxidative stress plays a vital role for the adaptive responses to physical training. However, excessive oxidative stress can precipitate cellular damage, necessitating protective mechanisms to mitigate this effect. Glucosinolates, found predominantly in cruciferous vegetables, can be converted into isothiocyanates, known for their antioxidative properties. These compounds activate crucial antioxidant defence pathways and support mitochondrial function and protein integrity under oxidative stress, in both Nrf2-dependent and independent manners. We here administered glucosinolate-rich broccoli sprouts (GRS), in a randomized double-blinded cross-over fashion to 9 healthy subjects in combination with daily intense exercise training for 7 days. We found that exercise in combination with GRS significantly decreased the levels of carbonylated proteins in skeletal muscle and the release of myeloperoxidase into blood. Moreover, it lowered lactate accumulation during submaximal exercise, and attenuated the severe nocturnal hypoglycaemic episodes seen during the placebo condition. Furthermore, GRS in combination with exercise improved physical performance, which was unchanged in the placebo condition.

Reduced glucose tolerance and insulin sensitivity after prolonged exercise in endurance athletes

AIM

The purpose of this study was to 1. investigate if glucose tolerance is affected after one acute bout of different types of exercise; 2. assess if potential differences between two exercise paradigms are related to changes in mitochondrial function; and 3. determine if endurance athletes differ from nonendurance-trained controls in their metabolic responses to the exercise paradigms.

METHODS

Nine endurance athletes (END) and eight healthy nonendurance-trained controls (CON) were studied. Oral glucose tolerance tests (OGTT) and mitochondrial function were assessed on three occasions: in the morning, 14 h after an overnight fast without prior exercise (RE), as well as after 3 h of prolonged continuous exercise at 65% of VO2 max (PE) or 5 × 4 min at ~95% of VO2 max (HIIT) on a cycle ergometer.

RESULTS

Glucose tolerance was markedly reduced in END after PE compared with RE. END also exhibited elevated fasting serum FFA and ketones levels, reduced insulin sensitivity and glucose oxidation, and increased fat oxidation during the OGTT. CON showed insignificant changes in glucose tolerance and the aforementioned measurements compared with RE. HIIT did not alter glucose tolerance in either group. Neither PE nor HIIT affected mitochondrial function in either group. END also exhibited increased activity of 3-hydroxyacyl-CoA dehydrogenase activity in muscle extracts vs. CON.

CONCLUSION

Prolonged exercise reduces glucose tolerance and increases insulin resistance in endurance athletes the following day. These findings are associated with an increased lipid load, a high capacity to oxidize lipids, and increased fat oxidation.

Effects of Diet, Lifestyle, Chrononutrition and Alternative Dietary Interventions on Postprandial Glycemia and Insulin Resistance

As years progress, we are found more often in a postprandial than a postabsorptive state. Chrononutrition is an integral part of metabolism, pancreatic function, and hormone secretion. Eating most calories and carbohydrates at lunch time and early afternoon, avoiding late evening dinner, and keeping consistent number of daily meals and relative times of eating occasions seem to play a pivotal role for postprandial glycemia and insulin sensitivity. Sequence of meals and nutrients also play a significant role, as foods of low density such as vegetables, salads, or soups consumed first, followed by protein and then by starchy foods lead to ameliorated glycemic and insulin responses. There are several dietary schemes available, such as intermittent fasting regimes, which may improve glycemic and insulin responses. Weight loss is important for the treatment of insulin resistance, and it can be achieved by many approaches, such as low-fat, low-carbohydrate, Mediterranean-style diets, etc. Lifestyle interventions with small weight loss (7-10%), 150 min of weekly moderate intensity exercise and behavioral therapy approach can be highly effective in preventing and treating type 2 diabetes. Similarly, decreasing carbohydrates in meals also improves significantly glycemic and insulin responses, but the extent of this reduction should be individualized, patient-centered, and monitored. Alternative foods or ingredients, such as vinegar, yogurt, whey protein, peanuts and tree nuts should also be considered in ameliorating postprandial hyperglycemia and insulin resistance. This review aims to describe the available evidence about the effects of diet, chrononutrition, alternative dietary interventions and exercise on postprandial glycemia and insulin resistance.

Changes in plasma concentration of kynurenine following intake of branched-chain amino acids are not caused by alterations in muscle kynurenine metabolism

Administration of branched-chain amino acids (BCAA) has been suggested to enhance mitochondrial biogenesis, including levels of PGC-1α, which may, in turn, alter kynurenine metabolism. Ten healthy subjects performed 60 min of dynamic one-leg exercise at ~70% of Wmax on two occasions. They were in random order supplied either a mixture of BCAA or flavored water (placebo) during the experiment. Blood samples were collected during exercise and recovery, and muscle biopsies were taken from both legs before, after and 90 and 180 min following exercise. Ingestion of BCAA doubled their concentration in both plasma and muscle while causing a 30-40% reduction (P<0.05 vs. placebo) in levels of aromatic amino acids in both resting and exercising muscle during 3-h recovery. The muscle concentration of kynurenine decreased by 25% (P<0.05) during recovery, similar in both resting and exercising leg and with both supplements, although plasma concentration of kynurenine during recovery was 10% lower (P<0.05) when BCAA were ingested. Ingestion of BCAA reduced the plasma concentration of kynurenic acid by 60% (P<0.01) during exercise and recovery, while the level remained unchanged with placebo. Exercise induced a 3-4-fold increase (P<0.05) in muscle content of PGC-1a1 mRNA after 90 min of recovery under both conditions, whereas levels of KAT4 mRNA and protein were unaffected by exercise or supplement. In conclusion, the reduction of plasma levels of kynurenine and kynurenic acid caused by BCAA were not associated with any changes in the level of muscle kynurenine, suggesting that kynurenine metabolism was altered in tissues other than muscle.

Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers

Exercise training positively affects metabolic health through increased mitochondrial oxidative capacity and improved glucose regulation and is the first line of treatment in several metabolic diseases. However, the upper limit of the amount of exercise associated with beneficial therapeutic effects has not been clearly identified. Here, we used a training model with a progressively increasing exercise load during an intervention over 4 weeks. We closely followed changes in glucose tolerance, mitochondrial function and dynamics, physical exercise capacity, and whole-body metabolism. Following the week with the highest exercise load, we found a striking reduction in intrinsic mitochondrial function that coincided with a disturbance in glucose tolerance and insulin secretion. We also assessed continuous blood glucose profiles in world-class endurance athletes and found that they had impaired glucose control compared with a matched control group.

Intake of branched-chain or essential amino acids attenuates the elevation in muscle levels of PGC-1α4 mRNA caused by resistance exercise

The transcriptional coactivator peroxisome proliferator-activated receptor-γ coactivator (PGC)-1α is recognized as the master regulator of mitochondrial biogenesis. However, recently a novel isoform, PGC-1α4, that specifically regulates muscle hypertrophy was discovered. Because stimulation of mechanistic target of rapamycin complex 1 (mTORC1) activity is tightly coupled to hypertrophy, we hypothesized that activation of this pathway would upregulate PGC-1α4. Eight male subjects performed heavy resistance exercise (10 × 8-12 repetitions at ∼75% of 1 repetition maximum in leg press) on four different occasions, ingesting in random order a solution containing essential amino acids (EAA), branched-chain amino acids (BCAA), leucine, or flavored water (placebo) during and after the exercise. Biopsies were taken from the vastus lateralis muscle before and immediately after exercise, as well as following 90 and 180 min of recovery. Signaling through mTORC1, as reflected in p70S6 kinase phosphorylation, was stimulated to a greater extent by the EAA and BCAA than the leucine or placebo supplements. Unexpectedly, intake of EAA or BCAA attenuated the stimulatory effect of exercise on PGC-1α4 expression by ∼50% (from a 10- to 5-fold increase with BCAA and EAA, P < 0.05) 3 h after exercise, whereas intake of leucine alone did not reduce this response. The 60% increase (P < 0.05) in the level of PGC-1α1 mRNA 90 min after exercise was uninfluenced by amino acid intake. Muscle glycogen levels were reduced and AMP-activated protein kinase α2 activity and phosphorylation of p38 mitogen-activated protein kinase enhanced to the same extent with all four supplements. In conclusion, induction of PGC-1α4 does not appear to regulate the nutritional (BCAA or EAA)-mediated activation of mTORC1 in human muscle.

Activation of mTORC1 by leucine is potentiated by branched-chain amino acids and even more so by essential amino acids following resistance exercise

Protein synthesis is stimulated by resistance exercise and intake of amino acids, in particular leucine. Moreover, activation of mammalian target of rapamycin complex 1 (mTORC1) signaling by leucine is potentiated by the presence of other essential amino acids (EAA). However, the contribution of the branched-chain amino acids (BCAA) to this effect is yet unknown. Here we compare the stimulatory role of leucine, BCAA, and EAA ingestion on anabolic signaling following exercise. Accordingly, eight trained volunteers completed four sessions of resistance exercise during which they ingested either placebo, leucine, BCAA, or EAA (including the BCAA) in random order. Muscle biopsies were taken at rest, immediately after exercise, and following 90 and 180 min of recovery. Following 90 min of recovery the activity of S6 kinase 1 (S6K1) was greater than at rest in all four trials (Placebo<Leucine<BCAA<EAA; P < 0.05 time × supplement), with a ninefold increase in the EAA trial. At this same time point, phosphorylation of eukaryotic translation initiation factor 4E (eIF4E)-binding protein 1 (4E-BP1) at Thr(37/46) was unaffected by supplementation, while that of Thr(46) alone exhibited a pattern similar to that of S6K1, being 18% higher with EAA than BCAA. However, after 180 min of recovery this difference between EAA and BCAA had disappeared, although with both these supplements the increases were still higher than with leucine (40%, P < 0.05) and placebo (100%, P < 0.05). In summary, EAA ingestion appears to stimulate translation initiation more effectively than the other supplements, although the results also suggest that this effect is primarily attributable to the BCAA.

Resistance exercise-induced S6K1 kinase activity is not inhibited in human skeletal muscle despite prior activation of AMPK by high-intensity interval cycling

Combining endurance and strength training in the same session has been reported to reduce the anabolic response to the latter form of exercise. The underlying mechanism, based primarily on results from rodent muscle, is proposed to involve AMPK-dependent inhibition of mTORC1 signaling. This hypothesis was tested in eight trained male subjects who in randomized order performed either resistance exercise only (R) or interval cycling followed by resistance exercise (ER). Biopsies taken from the vastus lateralis before and after endurance exercise and repeatedly after resistance exercise were assessed for glycogen content, kinase activity, protein phosphorylation, and gene expression. Mixed muscle fractional synthetic rate was measured at rest and during 3 h of recovery using the stable isotope technique. In ER, AMPK activity was elevated immediately after both endurance and resistance exercise (∼90%, P < 0.05) but was unchanged in R. Thr(389) phosphorylation of S6K1 was increased severalfold immediately after exercise (P < 0.05) in both trials and increased further throughout recovery. After 90 and 180 min recovery, S6K1 activity was elevated (∼55 and ∼110%, respectively, P < 0.05) and eukaryotic elongation factor 2 phosphorylation was reduced (∼55%, P < 0.05) with no difference between trials. In contrast, markers for protein catabolism were differently influenced by the two modes of exercise; ER induced a significant increase in gene and protein expression of MuRF1 (P < 0.05), which was not observed following R exercise only. In conclusion, cycling-induced elevation in AMPK activity does not inhibit mTOR complex 1 signaling after subsequent resistance exercise but may instead interfere with the hypertrophic response by influencing key components in protein breakdown.

Chronic stress and carbohydrate metabolism: persistent changes and slow return to normalcy in male albino rats

The present study tested the hypothesis that long-term repeated exposure to stressors results in irreversible changes in carbohydrate metabolism. Groups of adult male rats (five per group) were restrained for 1 h and 4 h later were forced to swim for 15 min everyday for 2, 4, or 24 weeks; five rats were autopsied after each interval. Groups of five rats exposed to stress for 2 or 4 weeks were maintained without further treatment (recovery groups) for up to 24 weeks. The fasting blood glucose concentration, measured at weekly intervals, was significantly higher in the stressed rats than in controls throughout the experiment, except in the 24th week, whereas that of the recovery groups was significantly higher than controls only up to the 8th week after the end of stress exposure and then reached normalcy. The blood concentrations of glucose, lactate, and pyruvate were significantly higher in the 2 and 4 weeks stress groups than in controls, whereas, except for lactate, in rats stressed for 24 weeks these values did not significantly differ from those in controls. These changes were accompanied by increased gluconeogenesis and glycogenolysis as shown by alterations in activities of hepatic carbohydrate metabolizing enzymes and unaltered blood insulin concentrations in rats stressed for 2, 4, and 24 weeks. Furthermore, the blood insulin levels did not significantly vary among controls and the 2, 4, and 24 weeks stress groups. The results reveal that though hyperglycemia induced by long-term stress exposure is reversible, it persists for a prolonged period, even after the termination of stress exposure, before reaching normalcy. Prevalence of hyperglycemia for a prolonged period through increased activities of hepatic enzymes in stressed rats exemplifies allostasis.

The effect of continuous and interval exercise on PGC-1α and PDK4 mRNA in type I and type II fibres of human skeletal muscle

AIM

Differences in fibre-type recruitment during exercise may induce a heterogenic response in fibre-type gene expression. We have investigated the effect of two different exercise protocols on the fibre-type-specific expression of master genes involved in oxidative metabolism [proliferator-activated receptor-γ coactivator-1α (PGC-1α) and Pyruvate dehydrogenase kinase 4 (PDK4)].

METHODS

Untrained subjects (n = 7) completed 90-min cycling either at a constant intensity [continuous exercise (CE): approximately 60% of VO(2max) ] or as interval exercise (IE: approximately 120/20% VO(2max) , duty cycle 12/18s). Muscle samples were taken before (pre) and 3 h after (post) exercise. Single fibres were isolated from freeze-dried muscle and characterized as type I or type II. The cDNA from two fibres of the same type was pooled and mRNA analysed with reverse transcription quantitative real-time PCR.

RESULTS

Continuous exercise and IE elicited a small increase in blood lactate (<2.5 mM) and moderate glycogen depletion (<40%) without difference between exercise modes. The mRNA of PGC-1α and PDK4 increased 5- to 8-fold in both fibre types after exercise, and the relative increase was negatively correlated with the basal level. However, the mRNA of PGC-1α and PDK4 was not different between type I and II fibres neither pre nor post, and there was no difference in the exercise-induced response between fibre types or exercise modes.

CONCLUSION

We conclude that the mRNA of PGC-1α and PDK4 increases markedly in both fibre types after prolonged exercise without difference between CE and IE. The similar response between fibre types may relate to that subjects were sedentary and that the metabolic stress was low.

Resistance exercise enhances the molecular signaling of mitochondrial biogenesis induced by endurance exercise in human skeletal muscle

Combining endurance and strength training (concurrent training) may change the adaptation compared with single mode training. However, the site of interaction and the mechanisms are unclear. We have investigated the hypothesis that molecular signaling of mitochondrial biogenesis after endurance exercise is impaired by resistance exercise. Ten healthy subjects performed either only endurance exercise (E; 1-h cycling at ∼65% of maximal oxygen uptake), or endurance exercise followed by resistance exercise (ER; 1-h cycling + 6 sets of leg press at 70–80% of 1 repetition maximum) in a randomized cross-over design. Muscle biopsies were obtained before and after exercise (1 and 3 h postcycling). The mRNA of genes related to mitochondrial biogenesis [(peroxisome proliferator-activated receptor-γ coactivator-1 (PGC-1)α, PGC-1-related coactivator (PRC)] related coactivator) and substrate regulation (pyruvate dehydrogenase kinase-4) increased after both E and ER, but the mRNA levels were about twofold higher after ER ( P < 0.01). Phosphorylation of proteins involved in the signaling cascade of protein synthesis [mammalian target of rapamycin (mTOR), ribosomal S6 kinase 1, and eukaryotic elongation factor 2] was altered after ER but not after E. Moreover, ER induced a larger increase in mRNA of genes associated with positive mTOR signaling (cMyc and Rheb). Phosphorylation of AMP-activated protein kinase, acetyl-CoA carboxylase, and Akt increased similarly at 1 h postcycling ( P < 0.01) after both types of exercise. Contrary to our hypothesis, the results demonstrate that ER, performed after E, amplifies the adaptive signaling response of mitochondrial biogenesis compared with single-mode endurance exercise. The mechanism may relate to a cross talk between signaling pathways mediated by mTOR. The results suggest that concurrent training may be beneficial for the adaptation of muscle oxidative capacity.

Insulin effects in muscle and adipose tissue

The major effects of insulin on muscle and adipose tissue are: (1) Carbohydrate metabolism: (a) it increases the rate of glucose transport across the cell membrane, (b) it increases the rate of glycolysis by increasing hexokinase and 6-phosphofructokinase activity, (c) it stimulates the rate of glycogen synthesis and decreases the rate of glycogen breakdown. (2) Lipid metabolism: (a) it decreases the rate of lipolysis in adipose tissue and hence lowers the plasma fatty acid level, (b) it stimulates fatty acid and triacylglycerol synthesis in tissues, (c) it increases the uptake of triglycerides from the blood into adipose tissue and muscle, (d) it decreases the rate of fatty acid oxidation in muscle and liver. (3) Protein metabolism: (a) it increases the rate of transport of some amino acids into tissues, (b) it increases the rate of protein synthesis in muscle, adipose tissue, liver, and other tissues, (c) it decreases the rate of protein degradation in muscle (and perhaps other tissues). These insulin effects serve to encourage the synthesis of carbohydrate, fat and protein, therefore, insulin can be considered to be an anabolic hormone.

Enhanced rates of muscle protein synthesis and elevated mTOR signalling following endurance exercise in human subjects

AIM

The major aim of this study was to determine the fractional rate of protein synthesis (FSR) during the early period of recovery after intensive aerobic exercise in the absence of nutritional supplementation.

METHODS

Sixteen male subjects performed one-legged cycling exercise for 1 h at approx. 65-70% of their one-legged maximal oxygen uptake. Using the stable isotope technique, the FSR in the vastus lateralis of both legs were determined during two periods, 0-90 min (n = 8) and 90-180 min (n = 8) after exercise. Biopsies were taken from both exercising and resting muscle before exercise, immediately after and following 90 or 180 min of recovery.

RESULTS

During the initial 90 min of recovery, FSR in the exercising muscle tended to be higher than in the resting muscle (1.57 ± 0.12 vs. 1.44 ± 0.07% 24 h(-1); P = 0.1) and was significantly higher during the period 90-180 min after exercise (1.74 ± 0.14 vs. 1.43 ± 0.12% 24 h(-1) ; P < 0.05). Exercise induced a 60% increase (P < 0.05) in phosphorylation of mTOR and a fivefold increase (P < 0.05) in Thr(389) phosphorylation of p70S6 kinase as well as a 30% reduction (P < 0.05) in phosphorylation of eEF2. Phosphorylation of AMP-activated protein kinase was enhanced by 40% (P < 0.05) after exercise, but no significant effect on phosphorylation of Akt, or eIF2Bε was observed immediately after exercise.

CONCLUSION

These findings indicate that during the first 3 h of recovery after intensive endurance exercise FSR gradually increases. Moreover, a stimulation of the mTOR-signalling pathway may be at least partially responsible for this elevated protein synthesis.

Similar expression of oxidative genes after interval and continuous exercise

PURPOSE

There is a debate whether interval or traditional endurance training is the most effective stimulus of mitochondrial biogenesis. Here, we compared the effects of acute interval exercise (IE) or continuous exercise (CE) on the muscle messenger RNA (mRNA) content for several genes involved in mitochondrial biogenesis and lipid metabolism.

METHODS

Nine sedentary subjects cycled for 90 min with two protocols: CE (at 67% VO2max) and IE (12 s at 120% and 18 s at 20% of VO2max). The duration of exercise and work performed with CE and IE was identical. Muscle biopsies were taken before and 3 h after exercise.

RESULTS

There were no significant differences between the two exercise protocols in the increases in VO2 and HR, the reduction in muscle glycogen (35%-40% with both protocols) or the changes in blood metabolites (lactate, glucose, and fatty acids). The mRNA content for major regulators of mitochondrial biogenesis [peroxisome proliferator-activated receptor (PPAR) gamma coactivator 1alpha (PGC-1alpha), PGC-1-related coactivator, PPARbeta/delta] and of lipid metabolism [pyruvate dehydrogenase kinase isozyme 4 (PDK4)] increased after exercise, but there was no significant difference between IE and CE. However, the mRNA content for several downstream targets of PGC-1alpha increased significantly only after CE, and mRNA content for nuclear respiratory factor 2 was significantly higher after CE (P < 0.025 vs IE).

CONCLUSIONS

The present findings demonstrate that, when the duration of exercise and work performed is the same, IE and CE influence the transcription of genes involved in oxidative metabolism in a similar manner.

Glycogen content regulates insulin- but not contraction-mediated glycogen synthase activation in the rat slow-twitch soleus muscles

AIM

The aim of this study was to investigate the effect of glycogen content on glycogen synthase (GS) activation and phosphorylation in the slow-twitch soleus muscles after contraction, during insulin stimulation and when these two stimuli were combined.

METHODS

Glycogen content was manipulated in vivo with 24 h fasting and fasting followed by 24 h refeeding. Soleus strips were electrically stimulated for 30 min in vitro, and GS activation and phosphorylation were investigated after an additional 30 min incubation with or without insulin.

RESULTS

Fasting reduced glycogen content in soleus muscle by 40% and refeeding enhanced by 40%, compared to rats with free access to chow. Insulin-stimulated GS fractional activity was inversely correlated with glycogen content (R = -0.95, P < 0.001, n = 24) and rate of glycogen synthesis was also inversely correlated with glycogen content (R = -0.70, P < 0.001, n = 36). After contraction, GS fractional activity was increased to similar levels in muscles with low, normal and high glycogen content; rate of glycogen synthesis after contraction was also similar. After contraction, insulin additively increased GS activation at all glycogen contents. Group means of GS fractional activity was inversely correlated with GS Ser(641) (R = -0.93, P < 0.001) and Ser(645,649,653,657) (R = -0.85, P < 0.001) phosphorylation, but not with Ser(7) phosphorylation.

CONCLUSION

Glycogen content regulates insulin- but not contraction-stimulated GS activation and glycogen synthesis in soleus muscles. Furthermore, phosphorylation of GS Ser(641) and Ser(645,649,653,657) seems to regulate GS activity in soleus.

Changes in signalling pathways regulating protein synthesis in human muscle in the recovery period after endurance exercise

AIM

Exercise induced alterations in the rate of muscle protein synthesis may be related to activity changes in signalling pathways involved in protein synthesis. The aim of the present study was to investigate whether such changes in enzyme phosphorylation occur after endurance exercise.

METHODS

Six male subjects performed ergometer cycling exercise for 1 h at 75% of the maximal oxygen uptake. Muscle biopsy samples from the vastus lateralis were taken before, immediately after, 30 min, 1 h, 2 h and 3 h after exercise for the determination of protein kinase B (PKB/Akt), mammalian target of rapamycin (mTOR), glycogen synthase 3 kinase (GSK-3), p70S6 kinase (p70(S6k)) and eukaryotic elongation factor 2 (eEF2) phosphorylation.

RESULTS

The phosphorylation of Akt was unchanged directly after exercise, but two- to fourfold increased 1 and 2 h after the exercise, whereas GSK-3alpha and beta phosphorylation were two- to fourfold elevated throughout most of the 3-h recovery period. Phosphorylation of mTOR was elevated threefold directly after, 30 min and 2 h after exercise and eEF2 phosphorylation was decreased by 35-75% from 30 min to 3 h-recovery. Exercise led to a five- to eightfold increase in Ser(424)/Thr(421) phosphorylation of p70(S6k) up to 30 min after exercise, but no change in Thr(389) phosphorylation.

CONCLUSIONS

The marked decrease in eEF2 phosphorylation suggests an activation of translation elongation and possibly protein synthesis in the recovery period after sustained endurance exercise. The lack of p70(S6k) activation suggests that translation initiation is activated via alternative pathways, possibly via the activation of eukaryotic initiating factor 2B.

Characterization of the role of gamma2 R531G mutation in AMP-activated protein kinase in cardiac hypertrophy and Wolff-Parkinson-White syndrome

AMP-activated protein kinase (AMPK) is the downstream component of a protein kinase cascade that plays a key role in the regulation of energy metabolism. In humans, mutations in the gamma2-subunit of AMPK cause cardiac hypertrophy associated with Wolff-Parkinson-White syndrome, characterized by ventricular preexcitation. The effect of these mutations on AMPK activity and in development of the disease is enigmatic. Here we report that transgenic mice with cardiac-specific expression of gamma2 harboring a mutation of arginine residue 531 to glycine (RG-TG) develop a striking cardiac phenotype by 4 wk of age, including hypertrophy, impaired contractile function, electrical conduction abnormalities, and marked glycogen accumulation. At this stage, AMPK activity isolated from hearts of RG-TG mice was almost completely abolished but could be restored after phosphorylation by an upstream AMPK kinase. At 1 wk of age, there was no detectable evidence of a cardiac phenotype, and AMPK activity in RG-TG hearts was similar to that in nontransgenic, control mice. We propose that mutations in gamma2 lead to suppression of total cardiac AMPK activity secondary to increased glycogen accumulation. The subsequent decrease in AMPK activity provides a mechanism that may explain the development of cardiac hypertrophy in this model.

Inhibition of glycogen breakdown by imino sugars in vitro and in vivo

The imino sugar N-butyldeoxynojirimycin (NB-DNJ) is a glucose analogue which inhibits the glycoprotein N-glycan processing enzymes alpha-glucosidases I and II and the ceramide glucosyltransferase that catalyses the first step of glycosphingolipid biosynthesis. This and other N-alkylated DNJ compounds have the potential to inhibit other glucosidase, including acid alpha-glucosidase and alpha-1,6-glucosidase, enzymes involved in glycogen breakdown. We have investigated the effect of NB-DNJ and N-nonyldeoxynojirimycin (NN-DNJ) on glycogen catabolism. Both NB-DNJ and NN-DNJ were potent inhibitors of acid alpha-glucosidase and alpha-1,6-glucosidase in vitro. NB-DNJ and NN-DNJ inhibited liver glycogen breakdown in vivo in fasting mice. Inhibition of glycogen catabolism occurred in the cytosol and lysosomes. The liver glycogen breakdown inhibition was only induced at high doses of NB-DNJ, whereas NN-DNJ caused glycogen accumulation at lower doses. The in vivo effect of NB-DNJ on liver glycogen was transient as there was no inhibition of breakdown after 90 days of treatment. The inhibition by NN-DNJ, was more pronounced, reached a plateau at 50 days and then remained unchanged. Increased glycogen was also observed in skeletal muscle in NB-DNJ- and NN-DNJ-treated mice. Since the effects on glycogen metabolism by NB-DNJ are transient and only occur at high concentrations, it is not predicted that glycogen breakdown will be impaired in patients receiving NB-DNJ therapy. NN-DNJ is the prototype of long alkyl chain derivatives of DNJ that are entering pre-clinical development as potential hepatitis B/hepatitis C (HBV/HCV) therapeutics. Depending on the dose of these compounds used, there is the potential for glycogen catabolism to be partially impaired in experimental animals and man.

Adaptive stress response of glutathione and uric acid metabolism in man following controlled exercise and diet

Ergometer cycling performance as well as acute exercise-induced changes in the metabolism of energy-intermediates and glutathione (GSH) were investigated in skeletal muscle (SM) of 15 healthy young male subjects (VO(2max) approximately 54.7 mL kg(-1) min(-1), age approximately 25 years), before and after 3 days of controlled 'ìoverload-training' in combination with either high (62% of energy intake) or low (26% of energy intake) dietary intake of carbohydrates. The intake of a carbohydrate-rich diet clearly reduced the depletion of SM glycogen following the short-term training period, paralleled with a positive effect on the endurance performance, but not on high-intensity work-performance. An 'delayed over-reaching effect', defined as impaired work-performance, was observed after 2.5 days of recovery from the short-term training period, irrespective of the carbohydrate content of the diet and basal glycogen level in SM. Taken together, the main and novel findings of present investigation are: (1) an acute decrease of reduced GSH content and altered thiol-redox homeostasis in SM induced by strenuous high-intensity exercise; (2) an adaptive elevation of basal GSH level following the short-term training period; (3) an adaptive decrease of basal GSH level following 2.5 days recovery from training; (4) evidence of a relationship between the SM fibre type, physical performance capacity and GSH turnover during acute bouts of exercise; and (5) no evident effect of the level of carbohydrate intake on metabolism of GSH or energy intermediates. Furthermore, the induction of acute oxidative stress in exercising human SM and the adaptive responses to training are suggested to provide a protective antioxidant phenotype to the exercising SM during periods with repeated intense intermittent training.

The lethal form of Cushing's in 7B2 null mice is caused by multiple metabolic and hormonal abnormalities

The neuroendocrine-specific protein 7B2, which serves as a molecular escort for proPC2 in the secretory pathway, promotes the production of enzymatically active PC2 and may have non-PC2 related endocrine roles. Mice null for 7B2 exhibit a lethal phenotype with a complex Cushing's-like pathology, which develops from intermediate lobe ACTH hypersecretion as a consequences of interruption of PC2-mediated peptide processing as well as undefined consequences of the loss of 7B2. In this study we investigated the endocrine and metabolic alterations of 7B2 null mice from pathological and biochemical points of view. Our results show that 7B2 nulls exhibit a multisystem disorder that includes severe pathoanatomical and histopathologic alterations of vital organs, including the heart and spleen but most notably the liver, in which massive steatosis and necrosis are observed. Metabolic derangements in glucose metabolism result in glycogen and fat deposition in liver under conditions of chronic hypoglycemia. Liver failure is also likely to contribute to abnormalities in blood coagulation and blood chemistry, such as lactic acidosis. A hypoglycemic crisis coupled with respiratory distress and intensive internal thrombosis most likely results in rapid deterioration and death of the 7B2 null.

Effect of exercise on concentrations of free amino acids in pools of type I and type II fibres in human muscle with reduced glycogen stores

A few animal studies have shown that some amino acid concentrations vary between different muscle fibre types. In the present study, amino acid concentrations were measured in separate pools of different fibre types in human skeletal muscle, with reduced glycogen stores, before and after sustained exercise. Five subjects exercised at a submaximal work rate for 60 min and then at a maximal rate for 20 min. Biopsy samples were taken from the vastus lateralis muscle before and after exercise; they were freeze-dried and individual fibres were dissected out. Fragments of these fibres were stained for myosin-adenosine triphosphatase (ATPase) and identified as type I or type II fibres. The concentrations of free amino acids were measured by high performance liquid chromatography (HPLC) in perchloric acid (PCA) extracts containing pools of either type of fibre. After exercise, glycogen was decreased in type I fibres (53%) and in four subjects also in type II fibres. The concentrations of most amino acids were similar in the two fibre types before exercise, but the glutamate, aspartate and arginine levels were 10% higher in type II than in type I fibres. After exercise, the glutamate concentration was decreased by 45% in both fibre types and the branched-chain amino acids (BCAA) were decreased in type II fibres (14%). Exercise caused an increase by 25-30% in tyrosine concentration in both type I and type II fibres. The results show that amino acids can be measured in pools of fibre fragments and suggest that amino acid metabolism play an important role in both type I and type II fibres during exercise.

Marathon running increases ERK1/2 and p38 MAP kinase signalling to downstream targets in human skeletal muscle

1. We tested the hypothesis that long-distance running activates parallel mitogen-activated protein kinase (MAPK) cascades that involve extracellular signal regulated kinase 1 and 2 (ERK1/2) and p38 MAPK and their downstream substrates. 2. Eleven men completed a 42.2 km marathon (mean race time 4 h 1 min; range 2 h 56 min to 4 h 33 min). Vastus lateralis muscle biopsies were obtained before and after the race. Glycogen content was measured spectrophotometrically. ERK1/2 and p38 MAPK phosphorylation was determined by immunoblot analysis using phosphospecific antibodies. Activation of the downstream targets of ERK1/2 and p38 MAPK, MAPK-activated protein kinase-1 (MAPKAP-K1; also called p90 ribosomal S6 kinase, p90rsk), MAPK-activated protein kinase-2 (MAPKAP-K2), mitogen- and stress-activated kinase 1 (MSK1) and mitogen- and stress-activated kinase 2 (MSK2) was determined using immune complex assays. 3. Muscle glycogen content was reduced by 40 +/- 6 % after the marathon. ERK1/2 phosphorylation increased 7.8-fold and p38 MAPK phosphorylation increased 4.4-fold post-exercise. Prolonged running did not alter ERK1/2 and p38 MAPK protein expression. The activity of p90rsk, a downstream target of ERK1/2, increased 2.8-fold after the marathon. The activity of MAPKAPK-K2, a downstream target of p38 MAPK, increased 3.1-fold post-exercise. MSK1 and MSK2 are downstream of both ERK1/2 and p38 MAPK. MSK1 activity increased 2.4-fold post-exercise. MSK2 activity was low, relative to MSK1, with little activation post-exercise. 4. In conclusion, prolonged distance running activates MAPK signalling cascades in skeletal muscle, including increased activity of downstream targets: p90rsk, MAPKAP-K2 and MSK. Activation of these downstream targets provides a potential mechanism by which exercise induces gene transcription in skeletal muscle.

BCAA intake affects protein metabolism in muscle after but not during exercise in humans

Branched-chain amino acids (BCAA) or a placebo was given to seven subjects during 1 h of ergometer cycle exercise and a 2-h recovery period. Intake of BCAA did not influence the rate of exchange of the aromatic amino acids, tyrosine and phenylalanine, in the legs during exercise or the increase in their concentration in muscle. The increase was approximately 30% in both conditions. On the other hand, in the recovery period after exercise, a faster decrease in the muscle concentration of aromatic amino acids was found in the BCAA experiment (46% compared with 25% in the placebo condition). There was also a tendency to a smaller release (an average of 32%) of these amino acids from the legs during the 2-h recovery. The results suggest that BCAA have a protein-sparing effect during the recovery after exercise, either that protein synthesis has been stimulated and/or protein degradation has decreased, but the data during exercise are too variable to make any conclusions about the effects during exercise. The effect in the recovery period does not seem to be mediated by insulin.

N-butyldeoxygalactonojirimycin: a more selective inhibitor of glycosphingolipid biosynthesis than N-butyldeoxynojirimycin, in vitro and in vivo

N-Butyldeoxynojirimycin (NB-DNJ) inhibits the ceramide glucosyltransferase which catalyses the first step in glycosphingolipid (GSL) biosynthesis. It has the potential to be used for the treatment of the GSL lysosomal storage diseases and is currently in clinical trials for the treatment of type 1 Gaucher's disease. However, NB-DNJ is also a potent inhibitor of other enzymes, including alpha-glucosidase I and II, which could potentially cause side effects in patients receiving life-long therapy. Wetherefore evaluated a potentially more selective GSL biosynthesis inhibitor, N-butyldeoxygalactonojirimycin (NB-DGJ), in vitro and in vivo. The distribution and degree of GSL depletion in the liver of mice treated with NB-DGJ or NB-DNJ were equivalent. Mice treated with NB-DGJ had normal body weights and lymphoid organ sizes, whereas NB-DNJ-treated mice showed weight loss and partial lymphoid organ shrinkage. NB-DNJ inhibited glycogen catabolism in the liver, whereas NB-DGJ did not. NB-DNJ was also a potent inhibitor of sucrase and maltase in vitro but not of lactase, while NB-DGJ inhibited lactase but not sucrase or maltase. NB-DGJ is therefore more selective than NB-DNJ, and deserves to be evaluated for human therapy.

The Greek contribution to diabetes research

Interest in diabetes mellitus research has escalated in Greece during the last decade. This may be attributed to the realization that diabetes is becoming a major problem for the Greek population, the effect of the St Vincent Declaration in passing specific government legislation, and the founding of the National Hellenic Center for the Prevention and Treatment of Diabetes and its Complications. Research areas include epidemiology, etiopathogenesis, glucose metabolism, complications, prevention and treatment of the disease.

Effect of muscle glycogen on glucose, lactate and amino acid metabolism during exercise and recovery in human subjects

1. Eight subjects performed two-legged exercise, one leg with low and the other with normal muscle glycogen content. The purpose was to study the effect of low initial muscle glycogen content on the metabolic response during 1 h of exercise and 2 h of recovery. This model allows direct comparison of net fluxes of substrates and metabolites over the exercising legs receiving the same arterial inflow. 2. Muscle glycogen breakdown during exercise was 60% lower in the leg with a reduced pre-exercise glycogen concentration and the rate of glucose uptake during exercise was 30% higher. 3. The amount of pyruvate that was oxidized during exercise was calculated to be approximately 450 mmol in the low-glycogen leg and 750 mmol in the normal-glycogen leg, which suggests more fat and amino acid oxidation in the low-glycogen leg. 4. During exercise, there was a significant release of amino acids not metabolized in the muscle, e. g. tyrosine and phenylalanine, only from the low-glycogen leg, suggesting an increased rate of net protein degradation in this leg. 5. The release of tyrosine and phenylalanine from the low-glycogen leg during the exercise period and the change in their muscle concentrations yield a net tyrosine and phenylalanine production rate of 1.4 and 1.5 mmol h-1, respectively. The net rate of protein degradation was then calculated to be 7-12 g h-1. 6. The results suggest that the observed differences in metabolism between the low-glycogen and the normal-glycogen leg are induced by the glycogen level per se, since the legs received the same arterial supply of hormones and substrates.

Fuel oxidation in skeletal muscle is increased by nitric oxide/cGMP--evidence for involvement of cGMP-dependent protein kinase

The cyclic guanosine-3',5'-monophosphate (cGMP) analogue, 8-bromo-cGMP (1 mM), increased glucose oxidation in isolated soleus muscle. The nitric oxide (NO) donor, sodium nitroprusside (SNP) (15 mM), increased glucose, pyruvate, palmitate and leucine oxidation. Removal of extracellular Ca2+ did not affect SNP-stimulated glucose oxidation (or other glucose utilization parameters), thus eliminating the influx of Ca2+ as a mechanism for the increases. The guanylate cyclase inhibitor, LY-83583 (10 microM), inhibited SNP-stimulated palmitate oxidation and activation of cGMP-dependent protein kinase (PKG). Activation of PKG might supersede any inhibitory effects of NO on respiration to stimulate metabolic fuel oxidation in skeletal muscle.

Evidence for altered sensitivity of the nitric oxide/cGMP signalling cascade in insulin-resistant skeletal muscle

Nitric oxide activates guanylate cyclase to form cGMP, comprising a signalling system that is believed to be a distinct mechanism for increasing glucose transport and metabolism in skeletal muscle. The effects of a selective cGMP phosphodiesterase inhibitor, zaprinast, on basal glucose utilization was investigated in incubated rat soleus muscle preparations isolated from both insulin-sensitive (lean Zucker; Fa/?) and insulin-resistant (obese Zucker; fa/fa) rats. Zaprinast at 27 microM significantly increased cGMP levels in incubated soleus muscle isolated from lean, but not obese, Zucker rats. Muscles were incubated with 14C-labelled glucose and various concentrations of zaprinast (3, 27 and 243 microM). Zaprinast (at 27 and 243 microM) significantly increased rates of net and 14C-labelled lactate release and of glycogen synthesis in lean Zucker rat soleus muscle; glucose oxidation was also increased by 27 microM zaprinast. In addition, regardless of concentration, the phosphodiesterase inhibitor failed to increase any aspect of 14C-labelled glucose utilization in soleus muscles isolated from obese Zucker rats. The maximal activity of nitric oxide synthase (NOS) was significantly decreased in insulin-resistant obese Zucker muscles. Thus the lack of effect of zaprinast in insulin-resistant skeletal muscle is consistent with decreased NOS activity. To test whether there is a defect in insulin-resistant skeletal muscle for endogenous activation of guanylate cyclase, soleus muscles were isolated from both insulin-sensitive and insulin-resistant Zucker rats and incubated with various concentrations of the NO donor sodium nitroprusside (SNP; 0.1, 1, 5 and 15 mM). SNP significantly increased rates of net and 14C-labelled lactate release, as well as glucose oxidation in muscles isolated from both insulin-sensitive and insulin-resistant rats. A decreased response to SNP was observed in the dose-dependent generation of cGMP within isolated soleus muscles from insulin-resistant rats. A possible link between impaired NO/cGMP signalling and abnormal glucose utilization by skeletal muscle is discussed.

Effect of branched-chain amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects

Five male endurance-trained subjects performed exhaustive exercise on a cycle ergometer at a work rate corresponding to 75% of their VO2max after reduction of their muscle glycogen stores. During exercise the subjects were given in random order a 6% carbohydrate solution continuing 7 g L-1 of branched-chain amino acids (BCAA), a 6% CHO solution and flavoured water. The physical performance was lowered in four of the five subjects when they were given flavoured water during exercise as compared with the two conditions when CHO was supplied. No difference in performance was found when the subjects were given CHO + BCAA or only CHO during exercise. When CHO + BCAA was supplied the plasma and muscle (vastus lateralis) concentrations of BCAA increased during exercise by 120 and 35%, respectively. In the other conditions there was no change or a slight decrease in the plasma concentrations of BCAA, but the muscle concentrations of BCAA were decreased after exercise. The plasma concentration of glutamine over the whole exercise period and 5 min after exercise was higher when CHO + BCAA were supplied during exercise compared with a supply of CHO alone or water. However, exercise caused no change in the muscle concentration of glutamine, whereas that of glutamate decreased in all three conditions. A supply of CHO + BCAA or CHO alone did not affect the exercise-induced increase in the plasma and muscle concentration of aromatic amino acids, indicating that neither BCAA nor CHO influenced the net protein degradation during exercise.

Limited capacity for glyconeogenesis from alanine by diaphragm muscle

The mammalian diaphragm (Dia) is a unique skeletal muscle because of its chronic contractile activity. Chronically active muscles have higher capacities for glycogen synthesis from lactate and glucose. In this study, the contribution of alanine to glycogen synthesis in the predominantly fast-twitch mouse Dia was determined. Chronically active fast- and slow-twitch muscles from dy2J/dy2J pseudomyotonic mice were also analyzed. Alanine aminotransferase (AlaAT) activity was significantly (P < 0.05) greater in Dia and chronically active gastrocnemius muscle (Gast) than control Gast. 14C-label incorporation into glycogen following in vivo injection of 14C-alanine, was significantly higher in Dia and chronically active Gast than control Gast. A direct incorporation of 14C from 14C-alanine into glycogen is also observed in vitro. The incorporation rate shows a linear concentration dependent relationship and a pH optimum of 6.3. Insulin had no effect on glycogen synthesis from alanine by Dia in vitro. The extensor digitorum longus muscle (EDL) and chronically active EDL and soleus muscle (Sol) had higher rates of glycogen synthesis from alanine than control Sol and Dia. The oxidation of alanine to CO2 was the primary route for alanine metabolism by the Dia. These results demonstrate that the Dia can synthesize glycogen from alanine, but only at low rates.