Skeletal muscle type comparison of pyruvate dehydrogenase phosphatase activity and isoform expression: effects of obesity and endurance training

Pyruvate dehydrogenase (PDH) plays an important role in regulating carbohydrate metabolism in skeletal muscle. PDH is activated by PDH phosphatase (PDP) and deactivated by PDH kinase (PDK). Obesity has a large negative impact on skeletal muscle carbohydrate metabolism, whereas endurance training has been shown to improve regulatory control of skeletal muscle carbohydrate metabolism, more so when coupled with obesity. A majority of this literature has focused on PDK, with little information available on PDP. To determine the relative role of PDP in regulating skeletal muscle PDH activity with obesity and endurance training, obese and lean Zucker rats remained sedentary or were endurance trained (1 h/day, 5 days/wk) for a period of 8 wk. Soleus, red gastrocnemius, (RG), and white gastrocnemius (WG) muscles were sampled after the training period. The main findings were 1) obesity resulted in a 46% decrease in PDP activity expressed per milligram extracted mitochondrial protein only in RG, while PDP isoform content was unchanged; 2) 8 wk of endurance training led to a significant 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; 3) 8 wk of endurance training led to a trending 1.4-2.2-fold increase in PDP activity of all muscle examined from obese rats, and the concomitant increase in PDP1 protein was only seen in soleus and RG; and 4) PDP2 protein content was not affected by obesity or training. These results suggest that decreased PDP activity in oxidative skeletal muscles may play a role in the impairment of carbohydrate metabolism in obese rats, which is reversible with endurance training.     

The effects of amino acid supplementation on muscular performance during resistance training overreaching

The purpose of this study was to examine the effects of amino acid supplementation on muscular strength, power, and high-intensity endurance during short-term resistance training overreaching. Seventeen resistance-trained men were randomly assigned to either an amino acid (AA) or placebo (P) group and underwent 4 weeks of total-body resistance training consisting of two 2-week phases of overreaching (phase 1: 3 x 8-12 repetitions maximum [RM], 8 exercises; phase 2: 5 x 3-5 RM, 5 exercises). Muscle strength, power, and high-intensity endurance were determined before (T1) and at the end of each training week (T2-T5). One repetition maximum squat and bench press decreased at T2 in P (5.2 and 3.4 kg, respectively) but not in AA, and significant increases in 1 RM squat and bench press were observed at T3-T5 in both groups. A decrease in the ballistic bench press peak power was observed at T3 in P but not AA. The fatigue index during the 20-repetition jump squat assessment did not change in the P group at T3 and T5 (fatigue index = 18.6 and 18.3%, respectively) whereas a trend for reduction was observed in the AA group (p = 0.06) at T3 (12.8%) but not T5 (15.2%; p = 0.12). These results indicate that the initial impact of high-volume resistance training overreaching reduces muscle strength and power, and it appears that these reductions are attenuated with amino acid supplementation. In addition, an initial high-volume, moderate-intensity phase of overreaching followed by a higher intensity, moderate-volume phase appears to be very effective for enhancing muscle strength in resistance-trained men.     

Role of AMPKalpha2 in basal, training-, and AICAR-induced GLUT4, hexokinase II, and mitochondrial protein expression in mouse muscle

We investigated the role of AMPKalpha2in basal, exercise training-, and AICAR-induced protein expression of GLUT4, hexokinase II (HKII), mitochondrial markers, and AMPK subunits. This was conducted in red (RG) and white gastrocnemius (WG) muscle from wild-type (WT) and alpha2-knockout (KO) mice after 28 days of activity wheel running or daily AICAR injection. Additional experiments were conducted to measure acute activation of AMPK by exercise and AICAR. At basal, mitochondrial markers were reduced by approximately 20% in alpha2-KO muscles compared with WT. In both muscle types, AMPKalpha2 activity was increased in response to both stimuli, whereas AMPKalpha1 activity was increased only in response to exercise. Furthermore, AMPK signaling was estimated to be 60-70% lower in alpha2-KO compared with WT muscles. In WG, AICAR treatment increased HKII, GLUT4, cytochrome c, COX-1, and CS, and the alpha2-KO abolished the AICAR-induced increases, whereas no AICAR responses were observed in RG. Exercise training increased GLUT4, HKII, COX-1, CS, and HAD protein in WG, but the alpha2-KO did not affect training-induced increases. Furthermore, AMPKalpha1, -alpha2, -beta1, -beta2, and -gamma3 subunits were reduced in RG, but not in WG, by 30-60% in response to exercise training. In conclusion, the alpha2-KO was associated with an approximately 20% reduction in mitochondrial markers in both muscle types and abolished AICAR-induced increases in protein expression in WG. However, the alpha2-KO did not reduce training-induced increases in HKII, GLUT4, COX-1, HAD, or CS protein in WG, suggesting that AMPKalpha2 may not be essential for metabolic adaptations of skeletal muscles to exercise training.     

(Over)training effects on quantitative electromyography and muscle enzyme activities in standardbred horses

Too intensive training may lead to overreaching or overtraining. To study whether quantitative needle electromyography (QEMG) is more sensitive to detect training (mal)adaptation than muscle enzyme activities, 12 standardbred geldings trained for 32 wk in age-, breed-, and sex-matched fixed pairs. After a habituation and normal training (NT) phase (phases 1 and 2, 4 and 18 wk, respectively), with increasing intensity and duration and frequency of training sessions, an intensified training (IT) group (phase 3, 6 wk) and a control group (which continued training as in the last week of phase 2) were formed. Thereafter, all horses entered a reduced training phase (phase 4, 4 wk). One hour before a standardized exercise test (SET; treadmill), QEMG analysis and biochemical enzyme activity were performed in muscle or in biopsies from vastus lateralis and pectoralis descendens muscle in order to identify causes of changes in exercise performance and eventual (mal)adaptation in skeletal muscle. NT resulted in a significant adaptation of QEMG parameters, whereas in muscle biopsies hexokinase activity was significantly decreased. Compared with NT controls, IT induced a stronger adaptation (e.g., higher amplitude, shorter duration, and fewer turns) in QEMG variables resembling potentially synchronization of individual motor unit fiber action potentials. Despite a 19% decrease in performance of the SET after IT, enzyme activities of 3-hydroxyacyl dehydrogenase and citrate synthase displayed similar increases in control and IT animals. We conclude that 1) QEMG analysis is a more sensitive tool to monitor training adaptation than muscle enzyme activities but does not discriminate between overreaching and normal training adaptations at this training level and 2) the decreased performance as noted in this study after IT originates most likely from a central (brain) rather than peripheral level.     

Interval sprint training enhances endothelial function and eNOS content in some arteries that perfuse white gastrocnemius muscle.

The purpose of this study was to test the hypothesis that interval sprint training (IST) selectively increases endothelium-dependent dilation (EDD) and endothelial nitric oxide synthase and/or superoxide dismutase-1 protein content in arteries and/or arterioles that perfuse the white portion of rat gastrocnemius muscle (WG). Male Sprague-Dawley rats completed 10 wk of IST (n = 62) or remained sedentary (Sed) (n = 63). IST rats performed six 2.5-min exercise bouts, with 4.5 min of rest between bouts (60 m/min, 15% incline), 5 days/wk. EDD was assessed from acetylcholine (ACh)-induced increases in muscle blood flow measured in situ and by ACh-induced dilation of arteries and arterioles [first to third order (1A-3A)] that perfuse red gastrocnemius muscle (RG) and WG. Artery protein content was determined with immunoblot analysis. ACh-induced increases in blood flow were enhanced in WG of IST rats. eNOS content was increased in conduit arteries, gastrocnemius feed artery, and fourth-order arterioles from WG and fifth-order arterioles of RG but not in 2As from RG. EDD was examined in 2As and 3As from a subset of IST and Sed rats. Arterioles were canulated with micropipettes, and intraluminal pressure was set at 60 cmH2O. Results indicate that passive diameter (measured in 0 calcium PSS) of WG 2As was similar in IST and Sed, whereas diameter of WG 3As was greater in IST (96 +/- 8 microm) than Sed (73 +/- 4 microm). WG 2As and 3As of IST rats exhibited greater spontaneous tone, but sensitivity to stretch, phenylephrine, and sodium nitroprusside was similar to Sed arterioles. ACh-induced dilation was enhanced by IST in WG 2As but not in RG 2As or WG 3As. We conclude that IST induces vascular adaptations nonuniformly among arteries that perfuse WG muscle.     

Differential apoptosis-related protein expression, mitochondrial properties, proteolytic enzyme activity, and DNA fragmentation between skeletal muscles

Increased skeletal muscle apoptosis has been associated with a number of conditions including aging, disuse, and cardiovascular disease. Skeletal muscle is a complex tissue comprised of several fiber types with unique properties. To date, no report has specifically examined apoptotic differences across muscles or fiber types. Therefore, we measured several apoptotic indices in healthy rat red (RG) and white gastrocnemius (WG) muscle, as well as examined the expression of several key proteins across fiber types in a mixed muscle (mixed gastrocnemius). The protein content of apoptosis-inducing factor (AIF), apoptosis repressor with caspase recruitment domain (ARC), Bax, Bcl-2, cytochrome c, heat shock protein 70 (Hsp70), and second mitochondria-derived activator of caspases (Smac) were significantly (P < 0.05) higher in RG vs. WG muscle. Cytosolic AIF, cytochrome c, and Smac as well as nuclear AIF were also significantly (P < 0.05) higher in RG compared with WG muscle. In addition, ARC protein expression was related to muscle fiber type and found to be highest (P < 0.001) in type I fibers. Similarly, AIF protein expression was differentially expressed across fibers; however, AIF was correlated to oxidative potential (P < 0.001). Caspase-3, -8, and -9 activity, calpain activity, and DNA fragmentation (a hallmark of apoptosis) were also significantly higher (P < 0.05) in RG compared with WG muscle. Furthermore, total muscle reactive oxygen species generation, as well as Ca(2+)-induced permeability transition pore opening and loss of membrane potential in isolated mitochondria were greater in RG muscle. Collectively, these data suggest that a number of apoptosis-related indices differ between muscles and fiber types. Given these findings, muscle and fiber-type differences in apoptotic protein expression, signaling, and susceptibility should be considered when studying cell death processes in skeletal muscle.     

Inverse relationship between PGC-1alpha protein expression and triacylglycerol accumulation in rodent skeletal muscle.

PGC-1alpha is a key regulator of tissue metabolism, including skeletal muscle. Because it has been shown that PGC-1alpha alters the capacity for lipid metabolism, it is possible that PGC-1alpha expression is regulated by the intramuscular lipid milieu. Therefore, we have examined the relationship between PGC-1alpha protein expression and the intramuscular fatty acid accumulation in hindlimb muscles of animals in which the capacity for fatty acid accumulation in muscle is increased (Zucker obese rat) or reduced [FAT/CD36 null (KO) mice]. Rates of palmitate incorporation into triacylglycerols were determined in perfused red (RG) and white gastrocnemius (WG) muscles of lean and obese Zucker rats and in perfused RG and WG muscles of FAT/CD36 KO and wild-type (WT) mice. In obese Zucker rats, the rate of palmitate incorporation into triacylglycerol depots in RG and WG muscles were 28 and 24% greater than in lean rats (P < 0.05). In FAT/CD36 KO mice, the rates of palmitate incorporation into triacylglycerol depots were lower in RG (-50%) and WG muscle (-24%) compared with the respective muscles in WT mice (P < 0.05). In the obese animals, PGC-1alpha protein content was reduced in both RG (-13%) and WG muscles (-15%) (P < 0.05). In FAT/CD36 KO mice, PGC-1alpha protein content was upregulated in both RG (+32%, P < 0.05) and WG muscles (+50%, P < 0.05). In conclusion, from studies in these two animal models, it appears that PGC-1alpha protein expression is inversely related to components of intramuscular lipid metabolism, because 1) PGC-1alpha protein expression is downregulated when triacylglycerol synthesis rates, an index of intramuscular lipid metabolism, are increased, and 2) PGC-1alpha protein expression is upregulated when triacylglycerol synthesis rates are reduced. Therefore, we speculate that the intramuscular lipid sensing may be involved in regulating the protein expression of PGC-1alpha in skeletal muscle.     

Diaphragm arterioles are less responsive to alpha1- adrenergic constriction than gastrocnemius arterioles.

The sympathetic nervous system has greater influence on vascular resistance in low-oxidative, fast-twitch skeletal muscle than in high-oxidative skeletal muscle (17). The purpose of this study was to test the hypothesis that arterioles isolated from low-oxidative, fast-twitch skeletal muscle [the white portion of gastrocnemius (WG)] possess greater responsiveness to adrenergic constriction than arterioles isolated from high-oxidative skeletal muscle [red portion of the gastrocnemius muscle (RG) and diaphragm (Dia)]. Second-order arterioles (2As) were isolated from WG, RG, and Dia of rats and reactivity examined in vitro. Results reveal that Dia 2As constrict less to norepinephrine (NE) (10(-9) to 10 (-4) M) than 2As from RG and WG, which exhibited similar NE-induced constrictions. This difference was not endothelium dependent, because responses of denuded 2As were similar to those of intact arterioles. The blunted NE-induced constrictor response of Dia 2As appears to be the result of differences in alpha1-receptor effects because 1) arterioles from Dia also responded less to selective alpha1-receptor stimulation with phenylephrine than RG and WG arterioles; 2) arterioles from Dia, RG, and WG dilated similarly to isoproterenol (10(-9) to 10(-4) M) and did not respond to selective alpha2-receptor stimulation with UK-14304; and 3) endothelin-1 produced similar constriction in 2As from Dia, RG, and WG. We conclude that differences in oxidative capacity and/or fiber type composition of muscle tissue do not explain different NE responsiveness of Dia 2As compared with 2As from gastrocnemius muscle. Differences in alpha1-adrenergic constrictor responsiveness among arterioles in skeletal muscle may contribute to nonuniform muscle blood flow responses observed during exercise and serve to maintain blood flow to Dia during exercise-induced increases in sympathetic nerve activity.     

Glycogenesis in muscle and liver during exercise

We hypothesized that glycogenesis increases in muscle during exercise before significant glycogen depletion occurs. Therefore, rats ran for 15 or 90 min at speeds of 8-22 m/min. D-[5-3H]glucose (10 microCi/100 g body wt) was administered 10 min before the end of exercise. Hindlimb muscles [soleus (SOL), plantaris (PL), extensor digitorum longus (EDL), and red (RG) and white gastrocnemius (WG)] and a portion of liver were analyzed for glycogen concentrations and rates of glycogen synthesis (i.e., D-[3H]glucose incorporated into glycogen). At rest, marked differences were observed among muscles in their rates of glucose incorporation into glycogen: i.e., SOL = 24.3 +/- 3.1, RG = 5.4 +/- 1.9, PL = 2.8 +/- 1.1, EDL = 0.54 +/- 0.10, WG = 0.12 +/- 0.02 (SE) dpm.micrograms glycogen-1.10 min-1 (P less than 0.05 between respective muscles). Compared with the glucose incorporation into glycogen at rest, increments in the PL (272%), RG (189%), WG (400%), EDL (274%), and liver (175%) were observed after 90 min of exercise (P less than 0.05, all data). In contrast, a decrease in glucose incorporation into glycogen (-62%) occurred in the SOL at min 15 (P less than 0.05), but this returned to the rates observed at rest after 90 min of exercise. This measure for rates of net glycogen synthesis (dpm.microgram glycogen-1.10 min-1) was weakly related to the ambient glycogen levels in most muscles; the exception was the SOL (r = -0.79; P less than 0.05). There was up to a 50-fold difference in glycogen synthesis among muscles.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differential adenosine sensitivity of diaphragm and skeletal muscle arterioles.

The hyperemic response in exercising skeletal muscle is dependent on muscle fiber-type composition and fiber recruitment patterns, but the vascular control mechanisms producing exercise hyperemia in skeletal muscle remain poorly understood. The purpose of this study was to test the hypothesis that arterioles from white, low-oxidative skeletal muscle are less responsive to adenosine-induced dilation than are arterioles from diaphragm (Dia) and red, high-oxidative skeletal muscle. Second-order arterioles (2As) were isolated from the white portion of gastrocnemius muscle (WG; low-oxidative, fast-twitch muscle tissue) and two types of high-oxidative skeletal muscle [Dia and red portion of gastrocnemius muscle (RG)] of rats. Results reveal that 2As from all three types of muscle dilated in response to the endothelium-dependent dilator acetylcholine (WG: 48 +/- 3%, Dia: 51 +/- 3%, RG: 74 +/- 3%). In contrast, adenosine dilated only 2As from WG (48 +/- 4%) and Dia (46 +/- 5%) but not those from RG (5 +/- 5%). Thus adenosine-induced dilator responses differed among 2As of these different types of muscle tissue. However, the results do not support our hypothesis because 2As from Dia and WG dilated in response to adenosine, whereas 2As from RG did not. We conclude that the adenosine responsiveness of 2As from rat skeletal muscle cannot be predicted only by the fiber-type composition or oxidative capacity of the skeletal muscle tissue wherein the arteriole lies.     

Beta2-agonist administration increases sarcoplasmic reticulum Ca2+-ATPase activity in aged rat skeletal muscle

Aging is associated with a slowing of skeletal muscle contractile properties, including a decreased rate of relaxation. In rats, the age-related decrease in the maximal rate of relaxation is reversed after 4-wk administration with the beta2-adrenoceptor agonist (beta2-agonist) fenoterol. Given the critical role of the sarcoplasmic reticulum (SR) in regulating intracellular Ca2+ transients and ultimately the time course of muscle contraction and relaxation, we tested the hypothesis that the mechanisms of action of fenoterol are mediated by alterations in SR proteins. Sarcoendoplasmic reticulum Ca2+-ATPase (SERCA) kinetic properties were assessed in muscle homogenates and enriched SR membranes isolated from the red (RG) and white (WG) portions of the gastrocnemius muscle in adult (16 mo) and aged (28 mo) F344 rats that had been administered fenoterol for 4 wk (1.4 mg/kg/day ip, in saline) or vehicle only. Aging was associated with a 29% decrease in the maximal activity (Vmax) of SERCA in the RG but not in the WG muscles. Fenoterol treatment increased the Vmax of SERCA and SERCA1 protein levels in RG and WG. In the RG, fenoterol administration reversed an age-related selective nitration of the SERCA2a isoform. Our findings demonstrate that the mechanisms underlying age-related changes in contractile properties are fiber type dependent, whereas the effects of fenoterol administration are independent of age and fiber type.     

Differential effects of repetitive activity on sarcoplasmic reticulum responses in rat muscles of different oxidative potential

We investigated the hypothesis that muscles of different oxidative potential would display differences in sarcoplasmic reticulum (SR) Ca2+ handling responses to repetitive contractile activity and recovery. Repetitive activity was induced in two muscles of high oxidative potential, namely, soleus (SOL) and red gastrocnemius (RG), and in white gastrocnemius (WG), a muscle of low oxidative potential, by stimulation in adult male rats. Measurements of SR properties, performed in crude homogenates, were made on control and stimulated muscles at the start of recovery (R0) and at 25 min of recovery (R25). Maximal Ca2+-ATPase activity (Vmax, micromol x g protein(-1) x min(-1)) at R0 was lower in stimulated SOL (105 +/- 9 vs. 135 +/- 7) and RG (269 +/- 22 vs. 317 +/- 26) and higher (P < 0.05) in WG (795 +/- 32 vs. 708 +/- 34). At R25, Vmax remained lower (P < 0.05) in SOL and RG but recovered in WG. Ca2+ uptake, measured at 2,000 nM, was depressed (P < 0.05) in SOL and RG by 34 and 13%, respectively, in stimulated muscles at R0 and remained depressed (P < 0.05) at R25. In contrast, Ca2+ uptake was elevated (P < 0.05) in stimulated WG at R0 by 9% and remained elevated (P < 0.05) at R25. Ca2+ release, unaltered in SOL and RG at both R0 and R25, was increased (P < 0.05) in stimulated WG at both R0 and R25. We conclude that SR Ca2+-handling responses to repetitive contractile activity and recovery are related to the oxidative potential of muscle.     

Subsarcolemmal and intermyofibrillar mitochondria play distinct roles in regulating skeletal muscle fatty acid metabolism

Skeletal muscle contains two populations of mitochondria that appear to be differentially affected by disease and exercise training. It remains unclear how these mitochondrial subpopulations contribute to fiber type-related and/or training-induced changes in fatty acid oxidation and regulation of carnitine palmitoyltransferase-1 (CPT1), the enzyme that controls mitochondrial fatty acid uptake in skeletal muscle. To this end, we found that fatty acid oxidation rates were 8.9-fold higher in subsarcolemmal mitochondria (SS) and 5.3-fold higher in intermyofibrillar mitochondria (IMF) that were isolated from red gastrocnemius (RG) compared with white gastrocnemius (WG) muscle, respectively. Malonyl-CoA (10 µM), a potent inhibitor of CPT1, completely abolished fatty acid oxidation in SS and IMF mitochondria from WG, whereas oxidation rates in the corresponding fractions from RG were inhibited only 89% and 60%, respectively. Endurance training also elicited mitochondrial adaptations that resulted in enhanced fatty acid oxidation capacity. Ten weeks of treadmill running differentially increased palmitate oxidation rates 100% and 46% in SS and IMF mitochondria, respectively. In SS mitochondria, elevated fatty acid oxidation rates were accompanied by a 48% increase in citrate synthase activity but no change in CPT1 activity. Nonlinear regression analyses of mitochondrial fatty acid oxidation rates in the presence of 0–100 µM malonyl-CoA indicated that IC₅₀ values were neither dependent on mitochondrial subpopulation nor affected by exercise training. However, in IMF mitochondria, training reduced the Hill coefficient (P < 0.05), suggesting altered CPT1 kinetics. These results demonstrate that endurance exercise provokes subpopulation-specific changes in mitochondrial function that are characterized by enhanced fatty acid oxidation and modified CPT1-malonyl-CoA dynamics. endurance exercise training; CPT-1; fiber type; rat; mitochondrial subpopulations     

Coordinately regulated expression of FAT/CD36 and FACS1 in rat skeletal muscle

Protein-mediated fatty acid uptake and intracellular fatty acid activation are key steps in fatty acid metabolism in muscle.We have examined (a) the abundance of fatty acid translocase (FAT/CD36) mRNA (a fatty acid transporter) and long-chain acyl CoA synthetase (FACS1) mRNA in metabolically heterogeneous muscles (soleus (SOL), red (RG) and white gastrocnemius (WG)), and (b) whether FAT/CD36 and FACS1 mRNAs were coordinately upregulated in red (RTA) and white tibialis muscles (WTA) that had been chronically stimulated for varying periods of time (0.25, 1, 6 and 24 h/day) for 7 days. FAT/CD36 mRNA and FACS1 mRNA abundance were scaled with (a) the oxidative capacity of muscle (SOL > RG > WG) (p < 0.05), (b) the rates of fatty acid oxidation in red and white muscles, and (c) fatty acid uptake by sarcolemmal vesicles, derived from red and white muscles. In chronically stimulated muscles (RTA and WTA), FAT/CD36 mRNA and FACS1 mRNA were up-regulated in relation to the quantity of muscle contractile activity (p < 0.05). FAT/CD36 mRNA and FACS1 mRNA up-regulation was highly correlated (r = 0.98). The coordinated expression of FAT/CD36 and FACS is likely a functional adaptive response to facilitate a greater rate of fatty acid activation in response to a greater rate of fatty acid transport, either among different types of muscles or in muscles in which capacity for fatty acid metabolism has been enhanced.     

Evidence of a malonyl-CoA-insensitive carnitine palmitoyltransferase I activity in red skeletal muscle

Carnitine palmitoyltransferase I (CPT I), which is expressed as two distinct isoforms in liver (alpha) and muscle (beta), catalyzes the rate-limiting step in the transport of fatty acid into the mitochondria. Malonyl-CoA, a potent inhibitor of CPT I, is considered a key regulator of fatty acid oxidation in both tissues. Still unanswered is how muscle beta-oxidation proceeds despite malonyl-CoA concentrations that exceed the IC(50) for CPT Ibeta. We evaluated malonyl-CoA-suppressible [(14)C]palmitate oxidation and CPT I activity in homogenates of red (RG) and white (WG) gastrocnemius, soleus (SOL), and extensor digitorum longus (EDL) muscles. Adding 10 microM malonyl-CoA inhibited palmitate oxidation by 29, 39, 60, and 89% in RG, SOL, EDL, and WG, respectively. Thus malonyl-CoA resistance, which correlated strongly (0.678) with absolute oxidation rates (RG > SOL > EDL > WG), was greater in red than in white muscles. Similarly, malonyl-CoA-resistant palmitate oxidation and CPT I activity were greater in mitochondria from RG compared with WG. Ribonuclease protection assays were performed to evaluate whether our data might be explained by differential expression of CPT I splice variants. We detected the presence of two CPT Ibeta splice variants that were more abundant in red compared with white muscle, but the relative expression of the two mRNA species was unrelated to malonyl-CoA resistance. These results provide evidence of a malonyl-CoA-insensitive CPT I activity in red muscle, suggesting fiber type-specific expression of distinct CPT I isoforms and/or posttranslational modulations that have yet to be elucidated.     

Nonuniform effects of endurance exercise training on vasodilation in rat skeletal muscle.

Endurance exercise training (Ex) has been shown to increase maximal skeletal muscle blood flow. The purpose of this study was to test the hypothesis that increased endothelium-dependent vasodilation is associated with the Ex-induced increase in muscle blood flow. Furthermore, we hypothesized that enhanced endothelium-dependent dilation is confined to vessels in high-oxidative muscles that are recruited during Ex. To test these hypotheses, sedentary (Sed) and rats that underwent Ex (30 m/min x 10% grade, 60 min/day, 5 days/wk, 8-12 wk) were studied using three experimental approaches. Training effectiveness was evidenced by increased citrate synthase activity in soleus and vastus lateralis (red section) muscles (P < 0.05). Vasodilatory responses to the endothelium-dependent agent acetylcholine (ACh) in situ tended to be augmented by training in the red section of gastrocnemius muscle (RG; Sed: control, 0.69 +/- 0.12; ACh, 1.25 +/- 0.15; Ex: control, 0.86 +/- 0.17; ACh, 1.76 +/- 0.27 ml x min(-1) x 100 g(-1) x mmHg(-1); 0.05 < P < 0.10 for Ex vs. Sed during ACh). Responses to ACh in situ did not differ between Sed and Ex for either the soleus muscle or white section of gastrocnemius muscle (WG). Dilatory responses of second-order arterioles from the RG in vitro to flow (4-8 microl/min) and sodium nitroprusside (SNP; 10(-7) through 10(-4) M), but not ACh, were augmented in Ex (vs. Sed; P < 0.05). Dilatory responses to ACh, flow, and SNP of arterioles from soleus and WG muscles did not differ between Sed and Ex. Content of the endothelial isoform of nitric oxide synthase (eNOS) was increased in second-order, fourth-order, and fifth-order arterioles from the RG of Ex; eNOS content was similar between Sed and Ex in vessels from the soleus and WG muscles. These findings indicate that Ex induces endothelial adaptations in fast-twitch, oxidative, glycolytic skeletal muscle. These adaptations may contribute to enhanced skeletal muscle blood flow in endurance-trained individuals.     

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.