Skeletal muscle eEF2 and 4EBP1 phosphorylation during endurance exercise is dependent on intensity and muscle fiber type.

Protein synthesis in skeletal muscle is known to decrease during exercise, and it has been suggested that this may depend on the magnitude of the relative metabolic stress within the contracting muscle. To examine the mechanisms behind this, the effect of exercise intensity on skeletal muscle eukaryotic elongation factor 2 (eEF2) and eukaryotic initiation factor 4E binding protein 1 (4EBP1) phosphorylation, key components in the mRNA translation machinery, were examined together with AMP-activated protein kinase (AMPK) in healthy young men. Skeletal muscle eEF2 phosphorylation at Thr56 increased during exercise but was not influenced by exercise intensity, and was lower than rest 30 min after exercise. On the other hand, 4EBP1 phosphorylation at Thr37/46 decreased during exercise, and this decrease was greater at higher exercise intensities and was similar to rest 30 min after exercise. AMPK activity, as indexed by AMPK alpha-subunit phosphorylation at Thr172 and phosphorylation of the AMPK substrate ACCbeta at Ser221, was higher with higher exercise intensities, and these indices were higher than rest after high-intensity exercise only. Using immunohistochemistry, it was shown that the increase in skeletal muscle eEF2 Thr56 phosphorylation was restricted to type I myofibers. Taken together, these data suggest that the depression of skeletal muscle protein synthesis with endurance-type exercise may be regulated at both initiation (i.e., 4EBP1) and elongation (i.e., eEF2) steps, with eEF2 phosphorylation contributing at all exercise intensities but 4EBP1 dephosphorylation contributing to a greater extent at high vs. low exercise intensities.

Effect of gender on lipid-induced insulin resistance in obese subjects.

OBJECTIVE: In obese subjects, chronically elevated plasma concentrations of non-esterified fatty acids (NEFAs) exert a marked risk to contract insulin resistance and subsequently type 2 diabetes. When NEFA is acutely increased due to i.v. infusion of lipid, glucose disposal during a hyperinsulinemic-euglycemic clamp is reduced. This effect has been explained by a NEFA-induced decrease in skeletal muscle insulin sensitivity caused by accumulation of the lipid intermediates such as ceramide and diacylglycerol in the myocytes. However, neither the lipid-induced reduction of glucose disposal nor the intramyocellular lipid deposition has been compared directly in obese females and males. DESIGN: We studied eight obese females and eight obese males (body mass index (BMI): 32.6+/-1.4 and 32.8+/-0.8 respectively, non significant (NS)) matched for cardiorespiratory fitness relative to lean body mass (43.7+/-1.6 and 47.6+/-1.3 ml/kg min respectively, NS). METHODS: Each subject underwent two hyperinsulinemic-euglycemic clamps with infusion of lipid or saline respectively. Furthermore, the subjects exercised during the last half an hour of each clamp. RESULTS: The lipid-induced reduction in glucose disposal during the clamp was similar in females and males (46+/-10 and 60+/-4% respectively, NS). However, whole-body insulin sensitivity as well as non-oxidative glucose disposal was higher in obese females compared with obese males both during lipid and saline infusion (P<0.001 and P=0.01 respectively). Muscle ceramide, triacylglycerol (TAG), diacylglycerol (DAG), and glycogen content were similar between sexes and remained unchanged during the clamp and when exercise was superimposed. CONCLUSIONS: The lipid-induced inhibition of glucose disposal is similar in obese females and males. However, obese females are more insulin sensitive compared with obese males (both during saline and lipid infusion), which is not due to differences in the concentration of the muscle lipid intermediates such as ceramide and DAG.

Lymphatic recovery of exogenous oleic acid in rats on long chain or specific structured triacylglycerol diets.

Specific structured triacylglycerols, MLM (M = medium-chain fatty acid, L = long-chain fatty acid), rapidly deliver energy and long-chain fatty acids to the body and are used for longer periods in human enteral feeding. In the present study rats were fed diets of 10 wt% MLM or LLL (L = oleic acid [18:1 n-9], M = caprylic acid [8:01) for 2 wk. Then lymph was collected 24 h following administration of a single bolus of 13C-labeled MLM or LLL. The total lymphatic recovery of exogenous 18:1 n-9 24 h after administration of a single bolus of MLM or LLL was similar in rats on the LLL diet (43% and 45%, respectively). However, the recovery of exogenous 18:1 n-9 was higher after a single bolus of MLM compared with a bolus of LLL in rats on the MLM diet (40% and 24%, respectively, P = 0.009). The recovery of lymphatic 18:1 n-9 of the LLL bolus tended to depend on the diet triacylglycerol structure and composition (P= 0.07). This study demonstrated that with a diet containing specific structured triacylglycerol, the lymphatic recovery of 18:1 n-9 after a single bolus of fat was dependent on the triacylglycerol structure of the bolus. This indicates that the lymphatic recovery of long-chain fatty acids from a single meal depends on the overall long-chain fatty acid composition of the habitual diet. This could have implications for enteral feeding for longer periods.

The recovery of 13C-labeled oleic acid in rat lymph after administration of long chain triacylglycerols or specific structured triacylglycerols.

BACKGROUND: Consumption of specific structured triacylglycerols, MLM (M = medium chain fatty acid, L = long chain fatty acid), delivers fast energy and long chain fatty acids to the organism. AIM OF THE STUDY: The purpose of the present study was to compare lymphatic absorption of (13)C-labeled MLM and (13)C-labeled LLL in rats. Stable isotope labeling enables the separation of the endogenous and exogenous fatty acids. METHODS: Lymph was collected during 24 h following administration of MLM or LLL. Lymph fatty acid composition and (13)C-enrichment were determined and quantified by gas chromatography combustion isotope ratio mass spectrometry. RESULTS: The recovery of 18:1n-9 was higher after administration of LLL compared with MLM (58.1% +/- 7.4% and 29.1% +/- 3.9%, respectively, P < 0.001). This may be due to a higher chylomicron formation stimulated by a higher amount of long chain fatty acids in the intestine after LLL compared with MLM administration. This was confirmed by the tendencies of higher lymphatic transport of endogenous fatty acids. CONCLUSION: The study revealed a higher lymphatic recovery of the administered long chain fatty acids after LLL compared with MLM consumption.

Attenuated gastric distress but no benefit to performance with adaptation to octanoate-rich esterified oils in well-trained male cyclists.

We investigated the effects of modifying a normal dietary fatty acid composition and ingestion of high-fat exercise supplements on gastrointestinal distress, substrate oxidation, and endurance cycling performance. Nine well-trained male cyclists completed a randomized triple-crossover comprising a 2-wk diet high in octanoate-rich esterified oil (MCFA) or twice long-chain fatty acids (LCFA). Following the diets, participants performed 3-h of cycling at 50% of peak power followed by 10 maximal sprints while ingesting either 1) a carbohydrate (CHO)+MCFA-rich oil emulsion after the 2-wk MCFA-rich dietary condition (MC-MC, Intervention) and 2) after one of the LCFA-rich dietary conditions (LC-MC, Placebo) or 3) CHO only following a LCFA-rich diet (LC-CHO, Control). During the 3-h ride MCFA-adaptation decreased octanoic-acid oxidation by 24% (90% confidence interval: 14-34%). The CHO+MCFA-rich oil emulsion reduced endogenous fat oxidation by 61% (33-89%) and 110% (89-131%) in the MC-MC and LC-MC conditions, respectively, and MCFA-adaptation reduced endogenous-carbohydrate oxidation by 10% (-3-23%). MCFA-adaptation attenuated gastrointestinal distress and nausea during the sprints, but the effect of the oil emulsion was to lower sprint power by 10.9% (7.7-14.1%) in the LC-MC condition and by 7.1% (5.7-8.5%) in the MC-MC condition, relative to the LC-CHO control; every one unit increase in nausea decreased mean power by 6.0 W (3.2-8.8 W). We conclude that despite some attenuation of endogenous-carbohydrate oxidation and gastric distress following adaptation to a MCFA-rich diet, repeat sprint performance was substantially impaired in response to the ingestion of a CHO+MCFA-rich oil emulsion.

Sex differences in hormone-sensitive lipase expression, activity, and phosphorylation in skeletal muscle at rest and during exercise.

Women have been shown to use more intramuscular triacylglycerol (IMTG) during exercise than men. To investigate whether this could be due to sex-specific regulation of hormone-sensitive lipase (HSL) and to use sex comparison as a model to gain further insight into HSL regulation, nine women and eight men performed bicycle exercise (90 min, 60% Vo(2peak)), and skeletal muscle HSL expression, phosphorylation, and activity were determined. Supporting previous findings, basal IMTG content (P < 0.001) and net IMTG decrease during exercise (P < 0.01) were higher in women than in men and correlated significantly (r = 0.72, P = 0.001). Muscle HSL mRNA (80%, P = 0.11) and protein content (50%, P < 0.05) were higher in women than in men. HSL total activity increased during exercise (47%, P < 0.05) but did not differ between sexes. Accordingly, HSL specific activity (HSL activity per HSL protein content) increased during exercise (62%, P < 0.05) and was generally higher in men than in women (82%, P < 0.05). A similar pattern was observed for HSL Ser(659) phosphorylation, suggesting a role in regulation of HSL activity. Likewise, plasma epinephrine increased during exercise (P < 0.05) and was higher in men than in women during the end of the exercise bout (P < 0.05). We conclude that, although HSL expression and Ser(659) phosphorylation in skeletal muscle during exercise is sex specific, total muscle HSL activity measured in vitro was similar between sexes. The higher basal IMTG content in women compared with men is therefore the best candidate to explain the higher IMTG net hydrolysis during exercise in women.

Intramuscular triacylglycerol in energy metabolism during exercise in humans.

Intramuscular triacylglycerol (IMTG) represents an energy store that can be used during exercise, when it may contribute up to 20% of total energy turnover depending on diet, gender, and exercise type. It is important to consider how measurements of IMTG have been performed. Hormone-sensitive lipase is thought to regulate breakdown of IMTG during exercise.

Regulation of hormone-sensitive lipase activity and Ser563 and Ser565 phosphorylation in human skeletal muscle during exercise.

Hormone-sensitive lipase (HSL) catalyses the hydrolysis of myocellular triacylglycerol (MCTG), which is a potential energy source during exercise. Therefore, it is important to elucidate the regulation of HSL activity in human skeletal muscle during exercise. The main purpose of the present study was to investigate the role of 5'AMP-activated protein kinase (AMPK) in the regulation of muscle HSL activity and Ser565 phosphorylation (the presumed AMPK target site) in healthy, moderately trained men during 60 min bicycling (65%). Alpha2AMPK activity during exercise was manipulated by studying subjects with either low (LG) or high (HG) muscle glycogen content. HSL activity was distinguished from the activity of other neutral lipases by immunoinhibition of HSL using an anti-HSL antibody. During exercise a 62% higher (P < 0.01) alpha2AMPK activity in LG than in HG was paralleled by a similar difference (61%, P < 0.01) in HSL Ser565 phosphorylation but without any difference between trials in HSL activity or MCTG hydrolysis. HSL activity was increased (117%, P < 0.05) at 30 min of exercise but not at 60 min of exercise. In both trials, HSL phosphorylation on Ser563 (a presumed PKA target site) was not increased by exercise despite a fourfold increase (P < 0.001) in plasma adrenaline. ERK1/2 phosphorylation was increased by exercise in both trials (P < 0.001) and was higher in LG than in HG both at rest and during exercise (P = 0.06). In conclusion, the present study suggests that AMPK phosphorylates HSL on Ser565 in human skeletal muscle during exercise with reduced muscle glycogen. Apparently, HSL Ser565 phosphorylation by AMPK during exercise had no effect on HSL activity. Alternatively, other factors including ERK may have counterbalanced any effect of AMPK on HSL activity.

Differential effect of bicycling exercise intensity on activity and phosphorylation of atypical protein kinase C and extracellular signal-regulated protein kinase in skeletal muscle.

Atypical protein kinase C (aPKC) and extracellular signal-regulated kinase (ERK) are emerging as important signalling molecules in the regulation of metabolism and gene expression in skeletal muscle. Exercise is known to increase activity of aPKC and ERK in skeletal muscle but the effect of exercise intensity hereon has not been studied. Furthermore, the relationship between activity and phosphorylation of the two enzymes during exercise is unknown. Nine healthy young men exercised for 30 min on a bicycle ergometer on two occasions. One occasion consisted of three consecutive 10 min bouts of 35, 60 and 85% of peak pulmonary oxygen uptake V(O(2 peak)) and the second of one 30 min bout at 35% of V(O(2 peak)). Both trials also included 30 min recovery. Muscle biopsies were obtained from the vastus lateralis muscle before and after each exercise bout. Exercise increased muscle aPKC activity at 35% V(O(2 peak)), whereupon no further increase was observed at higher exercise intensities. Activation of aPKC was not accompanied by increased phosphorylation of aPKC Thr(410/403). ERK1/2 activity increased in a similar pattern to aPKC, reaching maximal activity at 35% V(O(2 peak)), whereas ERK1 Thr(202)/Tyr(204) and ERK2 Thr(183)/Tyr(185) phosphorylation increased with increasing exercise intensity. Thus, aPKC and ERK1/2 activity in muscle during exercise did not correspond to phosphorylation of sites on aPKC or ERK1/2, respectively, which are considered important for their activation. It is concluded that assessment of aPKC and ERK1/2 activity in muscle using phosphospecific antibodies did not reflect direct activity measurements on immunoprecipitated enzyme in vitro. Thus, estimation of enzyme activity during exercise by use of phosphospecific antibodies should not be performed uncritically. In addition, increase in muscle activity of aPKC or ERK1/2 during exercise is not closely related to energy demands of the muscle but may serve other regulatory or permissive functions in muscle.

Regulation of plasma long-chain fatty acid oxidation in relation to uptake in human skeletal muscle during exercise.

In the present study, we investigated possible sites of regulation of long-chain fatty acid (LCFA) oxidation in contracting human skeletal muscle. Leg plasma LCFA kinetics were determined in eight healthy men during bicycling (60 min, 65% peak oxygen uptake) with either high (H-FOX) or low (L-FOX) leg fat oxidation (H-FOX: 1,098 +/- 140; L-FOX: 494 +/- 84 micromol FA/min, P < 0.001), which was achieved by manipulating preexercise muscle glycogen (H-FOX: 197 +/- 21; L-FOX: 504 +/- 25 mmol/kg dry wt, P < 0.001). Several blood metabolites and hormones were kept nearly similar between trials by allocating a preexercise meal and infusing glucose intravenously during exercise. During exercise, leg plasma LCFA fractional extraction was identical between trials (H-FOX: 17.8 +/- 1.6; L-FOX: 18.2 +/- 1.8%, not significant), suggesting similar LCFA transport capacity in muscle. On the contrary, leg plasma LCFA oxidation was 99% higher in H-FOX than in L-FOX (421 +/- 47 vs. 212 +/- 37 micromol/min, P < 0.001). Probably due to the slightly higher (P < 0.01) plasma LCFA concentration in H-FOX than in L-FOX, leg plasma LCFA uptake was nonsignificantly (P = 0.17) higher (25%) in H-FOX than in L-FOX, yet the fraction of plasma LCFA uptake oxidized was 61% higher (P < 0.05) in H-FOX than in L-FOX. Accordingly, the muscle content of several lipid-binding proteins did not differ significantly between trials, although fatty acid translocase/CD36 and caveolin-1 were elevated (P < 0.05) by the high-intensity exercise and dietary manipulation allocated on the day before the experimental trial. The present data suggest that, in contracting human skeletal muscle with different fat oxidation rates achieved by manipulating preexercise glycogen content, transsarcolemmal transport is not limiting plasma LCFA oxidation. Rather, the latter seems to be limited by intracellular regulatory mechanisms.

Sarcolemmal FAT/CD36 in human skeletal muscle colocalizes with caveolin-3 and is more abundant in type 1 than in type 2 fibers.

FAT/CD36 is a transmembrane protein that is thought to facilitate cellular long-chain fatty acid uptake. However, surprisingly little is known about the localization of FAT/CD36 in human skeletal muscle. By confocal immunofluorescence microscopy, we demonstrate high FAT/CD36 expression in endothelial cells and weaker but significant FAT/CD36 expression in sarcolemma in human skeletal muscle. No apparent intracellular staining was observed in the muscle cells. There are indications in the literature that caveolae may be involved in the uptake of fatty acids, possibly as regulators of FAT/CD36 or other fatty acid transporters. We show that in sarcolemma, FAT/CD36 colocalizes with the muscle-specific caveolae marker protein caveolin-3, suggesting that caveolae may regulate cellular fatty acid uptake by FAT/CD36. Furthermore, we provide evidence that FAT/CD36 expression is significantly higher in type 1 compared with type 2 fibers, whereas caveolin-3 expression is significantly higher in type 2 fibers than in type 1 fibers.

Recoveries of rat lymph FA after administration of specific structured 13C-TAG.

The potential of the specific structured TAG MLM [where M = caprylic acid (8:0) and L = linoleic acid (18:2n-6)] is the simultaneous delivery of energy and EFA. Compared with long-chain TAG (LLL), they may be more rapidly hydrolyzed and absorbed. This study examined the lymphatic recoveries of intragastrically administered L*L*L*, M*M*M*, ML*M, and ML*L* (where * = 13C-labeled FA) in rats. Lymph lipids were separated into lipid classes and analyzed by GC combustion isotope ratio MS. The recoveries of lymph TAG 18:2n-6 8 h after administration of L*L*L*, ML*M, and ML*L* were 38.6, 48.4, and 49.1%, respectively, whereas after 24 h the recoveries were approximately 50% in all experimental groups. The exogenous contribution to lymph TAG 18:2n-6 was approximately 80 and 60% at maximum absorption of the specific structured TAG and L*L*L*, respectively, 3-6 h after administration. The tendency toward more rapid recovery of exogenous long-chain FA following administration of specific structured TAG compared with long-chain TAG was probably due to fast hydrolysis. The lymphatic recovery of 8:0 was 2.2% 24 h after administration of M*M*M*. This minor lymphatic recovery of exogenous 8:0 was probably due to low stimulation of chylomicron formation. These results demonstrate tendencies toward faster lymphatic recovery of long-chain FA after administration of specific structured TAG compared with long-chain TAG.

Minor amounts of plasma medium-chain fatty acids and no improved time trial performance after consuming lipids.

Medium-chain triacylglycerols (MCT) have a potential glycogen-saving effect during exercise due to rapid hydrolysis and oxidation. However, studies comparing intake of carbohydrates (CHO) plus 80-90 g MCT with intake of CHO alone have revealed different results. The present study tested performance after consumption of specific structured triacylglycerol, consisting of a mixture of medium-chain fatty acids and long-chain fatty acids, to prevent the adverse effects observed by MCT (pure medium-chain fatty acids) regarding gastrointestinal distress. Seven well-trained subjects cycled 3 h at 55% of maximum O2 uptake during which they ingested CHO or CHO plus specific structured triacylglycerols. Immediately after the constant-load cycling, the subjects performed a time trial of approximately 50-min duration. Breath and blood samples were obtained regularly during the experiment. Fatty acid composition of plasma triacylglycerols, fatty acids, and phospholipids was determined. Performance was similar after administration of CHO plus specific structured triacylglycerol [medium-, long-, and medium-chain fatty acid (MLM)] compared with CHO (50.0 +/- 1.8 and 50.8 +/- 3.6 min, respectively). No plasma 8:0 was detected in the plasma lipid classes, but the amount of phospholipid fatty acids was significantly higher after CHO+MLM compared with CHO intake. The lacking time trial improvement after intake of medium-chain fatty acids might be due to no available 8:0 in the systemic circulation. A higher level of plasma phospholipid fatty acids in the CHO+MLM compared with the CHO group was probably due to endogenous phospholipid release into chylomicrons.