Adaptations in rat skeletal muscle following long-term resistance exercise training

The purpose of this investigation was to determine whether long-term, heavy resistance training would cause adaptations in rat skeletal muscle structure and function. Ten male Wistar rats (3 weeks old) were trained to climb a 40-cm vertical ladder (4 days/week) while carrying progressively heavier loads secured to their tails. After 26 weeks of training the rats were capable of lifting up to 800 g or 140% of their individual body mass for four sets of 12–15 repetitions per session. No difference in body mass was observed between the trained rats and age-matched sedentary control rats. Absolute and relative heart mass were greater in trained rats than control rats. When expressed relative to body mass, the mass of the extensor digitorum longus (EDL) and soleus muscles was greater in trained rats than control rats. No difference in absolute muscle mass or maximum force-producing capacity was evident in either the EDL or soleus muscles after training, although both muscles exhibited an increased resistance to fatigue. Individual fibre hypertrophy was evident in all four skeletal muscles investigated, i.e. EDL, soleus, plantaris and rectus femoris muscles of trained rats, but muscle fibre type proportions within each of the muscles tested remained unchanged. Despite an increased ability of the rats to lift progressively heavier loads, this heavy resistance training model did not induce gross muscle hypertrophy nor did it increase the force-producing capacity of the EDL or soleus muscles. Accepted: 17 September 1997     

Effect of anabolic steroids on mitochondria and sarcotubular system of skeletal muscle.

Soleus and extensor digitorum longus (EDL) mitochondria and sarcotubular system were examined in sedentary and trained (treadmill for 12 wk) male rats that were treated with fluoxymesterone or methandrostanolone (2 mg/kg, 5 days/wk, for 8 wk). Neither physical exercise nor anabolic/androgenic steroid administration resulted in a significant change in muscle wet weight. Treatment with the anabolizing androgens increased succinate dehydrogenase activity in fast-twitch muscle mitochondria; this effect was not enhanced by training and was not observed in soleus mitochondria. On the other hand, the content of the slow-twitch muscle in sarcotubular fraction was increased in sedentary rats by fluoxymesterone or methandrostanolone treatment, whereas no significant changes were found in EDL. The training program affected adenosinetriphosphatase (ATPase) activities in the sarcotubular fraction; Mg2(+)-ATPase was increased in both soleus and EDL, but Ca2(+)-ATPase was decreased only in soleus. However, in sedentary animals only the Mg2(+)-dependent activity of EDL was increased by anabolizing androgen treatment, and this change was not potentiated by additional training. The present data indicate that anabolic/androgenic steroids can affect mitochondrial and sarcotubular enzymes in skeletal muscle. The effects are muscle-type specific.     

Exercise training alleviates MCT1 and MCT4 reductions in heart and skeletal muscles of STZ-induced diabetic rats.

We compared the changes in monocarboxylate transporter 1 (MCT1) and 4 (MCT4) proteins in heart and skeletal muscles in sedentary control and streptozotocin (STZ)-induced diabetic rats (3 wk) and in trained (3 wk) control and STZ-induced diabetic animals. In nondiabetic animals, training increased MCT1 in the plantaris (+51%; P < 0.01) but not in the soleus (+9%) or the heart (+14%). MCT4 was increased in the plantaris (+48%; P < 0.01) but not in the soleus muscles of trained nondiabetic animals. In sedentary diabetic animals, MCT1 was reduced in the heart (-30%), and in the plantaris (-31%; P < 0.01) and soleus (-26%) muscles. MCT4 content was also reduced in sedentary diabetic animals in the plantaris (-52%; P < 0.01) and soleus (-25%) muscles. In contrast, in trained diabetic animals, MCT1 and MCT4 in heart and/or muscle were similar to those of sedentary, nondiabetic animals (P > 0.05) but were markedly greater than in the sedentary diabetic animals [MCT1: plantaris +63%, soleus +51%, heart +51% (P > 0.05); MCT4: plantaris +107%, soleus +17% (P > 0.05)]. These studies have shown that 1) with STZ-induced diabetes, MCT1 and MCT4 are reduced in skeletal muscle and/or the heart and 2) exercise training alleviated these diabetes-induced reductions.     

Skeletal muscle glucose uptake following overload-induced hypertrophy.

While endurance exercise training has been shown to enhance insulin action in skeletal muscle, the effects of high resistance strength training are less clear. The purpose of this study was to determine the rate of glucose uptake in skeletal muscle in which compensatory hypertrophy was induced by synergist muscle ablation. Basal and insulin mediated [3H] 2-deoxyglucose uptake were measured in soleus and EDL muscles using the perfused rat hindquarter preparation. Neither basal nor insulin mediated glucose uptake, when expressed per gram muscle, were enhanced in hypertrophied soleus muscles compared with control muscles, despite a twofold increase in mass (P less than 0.01). In the EDL, muscle mass increased 60% with synergist ablation (P less than 0.01), however insulin mediated glucose uptake was not different from that of control muscles. The basal rate of glucose uptake in hypertrophied EDL muscles was increased twofold over that of control muscles (P less than 0.05), possibly due to changes in neural input and/or loading. These results suggest that the stimulus for development of increased muscle mass is different from that for metabolic adaptations.     

Reduction of skeletal muscle atrophy by a proteasome inhibitor in a rat model of denervation

The ubiquitin-proteasome system is the primary proteolytic pathway implicated in skeletal muscle atrophy under catabolic conditions. Although several studies showed that proteasome inhibitors reduced proteolysis under catabolic conditions, few studies have demonstrated the ability of these inhibitors to preserve skeletal muscle mass and architecture in vivo. To explore this, we studied the effect of the proteasome inhibitor Velcade (also known as PS-341 and bortezomib) in denervated skeletal muscle in rats. Rats were given vehicle or Velcade (3 mg/kg po) daily for 7 days beginning immediately after induction of muscle atrophy by crushing the sciatic nerve. At the end of the study, the rats were euthanized and the soleus and extensor digitorum longus (EDL) muscles were harvested. In vehicle-treated rats, denervation caused a 33.5 +/- 2.8% and 16.2 +/- 2.7% decrease in the soleus and EDL muscle wet weights (% atrophy), respectively, compared to muscles from the contralateral (innervated) limb. Velcade significantly reduced denervation-induced atrophy to 17.1 +/- 3.3% in the soleus (P < 0.01), a 51.6% reduction in atrophy associated with denervation, with little effect on the EDL (9.8 +/- 3.2% atrophy). Histology showed a preservation of muscle mass and preservation of normal cellular architecture after Velcade treatment. Ubiquitin mRNA levels in denervated soleus muscle at the end of the study were significantly elevated 120 +/- 25% above sham control levels and were reduced to control levels by Velcade. In contrast, testosterone proprionate (3 mg/kg sc) did not alleviate denervation-induced skeletal muscle atrophy but did prevent castration-induced levator ani atrophy, while Velcade was without effect. These results show that proteasome inhibition attenuates denervation-induced muscle atrophy in vivo in soleus muscles. However, this mechanism may not be operative in all types of atrophy.     

Running during recovery from hindlimb suspension induces transient muscle injury

The objectives were to study morphological adaptations of soleus muscle to decreased loading induced by hindlimb suspension and the effect of run training during the subsequent recovery period. Adult female Wistar rats were kept for 28 days with hindlimbs suspended. For the next 28 days, rats were assigned to a cage-sedentary or daily running group. Compared with control soleus muscles, 28 days of hindlimb suspension reduced the mass and fiber cross-sectional area to 58 and 53% of control values, respectively, and decreased type I fibers from 92 +/- 2 to 81 +/- 2%. During recovery, clusters of damaged fibers were observed in the soleus muscle, and this observation was more pronounced in trained animals. Type IIc fibers appeared transiently during recovery, and their presence was exacerbated with training, as IIc fibers increased to approximately 20% of the total by day 14 of recovery and were no longer evident at day 28. Although muscle wet mass does not differ as a result of mode of recovery at day 14, training transiently decreased the overall fiber area compared with sedentary recovery at this point. By day 28 of recovery the morphological characteristics of soleus muscle in the trained group did not differ from control muscle, whereas in the sedentary group muscle mass and overall fiber cross-sectional area were approximately 14% less than control values.     

Prolonged treatment with the anabolic–androgenic steroid stanozolol increases antioxidant defences in rat skeletal muscle

Testosterone and its synthetic derivatives anabolic–androgenic steroids have been shown to increase skeletal muscle work capacity and fatigue resistance, but the molecular basis for these effects remains uncertain. Since muscle performance has been related to redox status of exercising muscles, this investigation was aimed at testing whether a treatment with suprapharmacological doses of the anabolic–androgenic steroid stanozolol, (2 mg/kg body weight, 5 days/week, for 8 weeks), either alone or in conjunction with treadmill training (12 weeks), enhanced antioxidant defences in rat muscles. Stanozolol treatment did not modify thiobarbituric acid reactive substances and glutathione content in soleus and extensor digitorum longus (EDL) homogenates. In soleus from sedentary rats, superoxide dismutase and glutathione reductase activities were increased by 25% (P < 0.05) and by 40% (P < 0.01) after stanozolol administration, whereas catalase and glutathione peroxidase activities were not modified. This response was similar to that induced by training alone. In EDL from sedentary rats, stanozolol increased only superoxide dismutase activity (20%, P < 0.05). In no case, the effects of steroid administration and training were additive. HSP72 levels were up-regulated in soleus (1.5-fold, P < 0.01) and EDL (threefold, P < 0.001) following training but remained unchanged after stanozolol treatment. Endurance capacity, assessed in a treadmill endurance test, was similar for treated and control rats. We conclude that stanozolol treatment increases antioxidant capacity in selected skeletal muscles from sedentary rats. However, the steroid was not effective in improving endurance capacity or enhancing the training effects on muscle antioxidant defence systems.     

The effect of creatine supplementation on mass and performance of rat skeletal muscle

This study investigated the effect of dietary creatine supplementation on hypertrophy and performance of rat skeletal muscle. Male Sprague-Dawley rats underwent either tibialis anterior ablation or partial ablation of the plantaris/gastrocnemius to induce compensatory hypertrophy of the extensor digitorum longus (EDL) or soleus respectively, or sham surgery. Creatine (300 mg/kg) was administered to one half of each group for 5 weeks, after which force production was measured. With the leg fixed at the knee and ankle, the distal tendon of the EDL or soleus was attached to a force transducer and the muscle was electrically stimulated via the sciatic nerve. Synergist ablation resulted in a significant increase in EDL mass and in soleus mass relative to control muscles. However, no effect of creatine supplementation on muscle mass or performance was found between control and either group of creatine-treated rats. Despite an apparent increase in muscle creatine content, creatine supplementation did not augment muscle hypertrophy or force production in rat EDL or soleus muscle, providing evidence that the potential benefits of creatine supplementation are not due to a direct effect on muscle but rather to an enhanced ability to train.     

Effects of beta 1- vs. beta 1- beta 2-blockade on training adaptations in rat skeletal muscle

The effect of selective vs. nonselective beta-blockade on fast-twitch [extensor digitorum longus (EDL)] and slow-twitch [soleus (SOL)] muscle enzyme activities following endurance training were characterized. Citrate synthase (CS), lactate dehydrogenase (LDH), and beta-hydroxyacyl-CoA dehydrogenase (HAD) activities were compared in SOL and EDL muscles of trained (T), metoprolol-trained (MT), propranolol-trained (PT), and sedentary (C) rats. Following 8 wk of treadmill running (1 h/day, 5 days/wk at approximately 30 m/min), LDH activity was depressed approximately 20% (P less than 0.05) in both SOL and EDL in only the PT rats, indicating inhibition of beta 2-mediated anaerobic glycolysis. EDL CS activity was similarly elevated in all three trained groups compared with sedentary controls. In SOL muscle, however, a drug attenuation effect was observed so that CS activity was increased only in the T (P less than 0.01) and MT (P less than 0.05) groups. HAD enzyme activity was increased somewhat (P less than 0.10) in SOL muscle in only the T group, but more so (P less than 0.05) in EDL in all three trained groups. The above findings suggest a training-induced selectivity effect not only with respect to beta 1-vs. beta 1-beta 2-blockers, but also with respect to muscle fiber type.     

First-pass hepatic uptake and utilization of glucose in the rat.

The concentration of NADH was determined a high-oxidative muscle (soleus) and a high-glycolytic muscle (extensor digitorum longus, EDL) from resting rats. The NADH content of freeze-clamped control muscles was 0.35 +/- 0.04 (mean +/- S.D.) and 0.31 +/- 0.04 mmol/kg dry wt. in EDL and soleus respectively, and increased to peak values of 0.58 +/- 0.05 (EDL) and 0.87 +/- 0.10 (soleus) after 10 min of NaCN treatment. The [lactate]/[pyruvate] ratio, which was not significantly changed in soleus and increased only slightly in EDL after NaCN incubation, shows that only minor changes occurred in the cytosolic NADH concentration. Provided that the major part of muscle NADH is located in the mitochondria it can be calculated that the mitochondrial NADH content in skeletal muscle at rest is about 36 (soleus) and 60% (EDL) of the anoxic value, respectively. These results are in contrast with previous studies with the surface-fluorescence technique, where mitochondrial NAD appeared to be almost completely reduced in resting skeletal muscle.     

Effects of high-intensity training on MCT1, MCT4, and NBC expressions in rat skeletal muscles: influence of chronic metabolic alkalosis

This study investigated the effects of high-intensity training, with or without induced metabolic alkalosis, on lactate transporter (MCT1 and MCT4) and sodium bicarbonate cotransporter (NBC) content in rat skeletal muscles. Male Wistar rats performed high-intensity training on a treadmill 5 times/wk for 5 wk, receiving either sodium bicarbonate (ALK-T) or a placebo (PLA-T) prior to each training session, and were compared with a group of control rats (CON). MCT1, MCT4, and NBC content was measured by Western blotting in soleus and extensor digitorum longus (EDL) skeletal muscles. Citrate synthase (CS) and phosphofructokinase (PFK) activities and muscle buffer capacity (betam) were also evaluated. Following training, CS and PFK activities were significantly higher in the soleus only (P < 0.05), whereas betam was significantly higher in both soleus and EDL (P < 0.05). MCT1 (PLA-T: 30%; ALK-T: 23%) and NBC contents (PLA-T: 85%; ALK-T: 60%) increased significantly only in the soleus following training (P < 0.01). MCT4 content in the soleus was significantly greater in ALK-T (115%) but not PLA-T compared with CON. There was no significant change in protein content in the EDL. Finally, NBC content was related only to MCT1 content in soleus (r = 0.50, P < 0.01). In conclusion, these results suggest that MCT1, MCT4, and NBC undergo fiber-specific adaptive changes in response to high-intensity training and that induced alkalosis has a positive effect on training-induced changes in MCT4 content. The correlation between MCT1 and NBC expression suggests that lactate transport may be facilitated by NBC in oxidative skeletal muscle, which may in turn favor better muscle pH regulation.     

Greater capillary-fiber interface per fiber mitochondrial volume in skeletal muscles of old rats.

The objective was to examine whether muscle structural capacity for O2 flux (i.e., capillary-to-fiber surface ratio) relative to fiber mitochondrial volume deteriorates with the muscle atrophy of aging in predominantly slow- (soleus, S) and fast-twitch (extensor digitorum longus, EDL) muscles of old (24 mo) and very old (35 mo) F344BN rats compared with adult (12 mo old). Wet muscle mass decreased 29% (196 +/- 4 to 139 +/- 5 mg) in S and 22% (192 +/- 3 to 150 +/- 3 mg) in EDL between 12 and 35 mo of age, without decline in body mass. Capillary density increased 65% (1,387 +/- 54 to 2,291 +/- 238 mm(-2)) in S and 130% (964 +/- 95 to 2,216 +/- 311 mm(-2)) in EDL, because of the muscle fiber atrophy, whereas capillary per fiber number remained unchanged. Altered capillary geometry, i.e., lesser contribution of tortuosity and branching to capillary length, was found in S at 35 compared with 12 and 24 mo, and not in EDL. Accounting for capillary geometry revealed 55% (1,776 +/- 78 to 2,750 +/- 271 mm(-2)) and 113% (1,194 +/- 112 to 2,540 +/- 343 mm(-2)) increases in capillary length-to-fiber volume ratio between 12 and 35 mo of age in S and EDL, respectively. Fiber mitochondrial volume density was unchanged over the same period, causing mitochondrial volume per micrometer fiber length to decrease in proportion to the fiber atrophy in both muscles. As a result of the smaller fiber mitochondrial volume in the face of the unchanged capillary-to-fiber number ratio, capillary-to-fiber surface ratio relative to fiber mitochondrial volume not only did not deteriorate, but in fact increased twofold in both muscles between 12 and 35 mo of age, independent of their different fiber type.     

Skeletal muscle adaptation to physical training and beta-adrenergic blockade in spontaneously hypertensive rats

The effects of training alone or in combination with long-term, non-selective, beta-adrenergic blockade on histochemical and biochemical properties of fast-twitch [extensor digitorum longus muscle (EDL)] and slow-twitch [soleus muscle (Sol)] muscle were analyzed in spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto strain rats (WKY). Fiber type distribution of Sol was drastically modified in SHR with fewer type I fibers and more type IIA fibers. No such histochemical alterations were observed in EDL. While prolonged swimming training remained ineffective in inducing both histochemical and biochemical improvement in WKY, SHR displayed a significant enhancement of capillarization and oxidative capacity in both Sol and EDL. However, in long-term beta-blocks rats training failed to improve significantly the oxidative capacity of SHR muscles, suggesting that beta-adrenoreceptor stimulation is necessary for a fully efficient adaptation of muscular metabolism to physical training.     

Experimental chronic low‐frequency resistance training produces skeletal muscle hypertrophy in the absence of muscle damage and metabolic stress markers

Volitional animal resistance training constitutes an important approach to modeling human resistance training. However, the lack of standardization protocol poses a frequent impediment to the production of skeletal muscle hypertrophy and the study of related physiological variables (i.e., cellular damage/inflammation or metabolic stress). Therefore, the purposes of the present study were: (1) to test whether a long‐term and low frequency experimental resistance training program is capable of producing absolute increases in muscle mass; (2) to examine whether cellular damage/inflammation or metabolic stress is involved in the process of hypertrophy. In order to test this hypothesis, animals were assigned to a sedentary control (C, n = 8) or a resistance trained group (RT, n = 7). Trained rats performed 2 exercise sessions per week (16 repetitions per day) during 12 weeks. Our results demonstrated that the resistance training strategy employed was capable of producing absolute mass gain in both soleus and plantaris muscles (12%, p < 0.05). Furthermore, muscle tumor necrosis factor (TNF‐α) protein expression (soleus muscle) was reduced by 24% (p < 0.01) in trained group when compared to sedentary one. Finally, serum creatine kinase (CK) activity and serum lactate concentrations were not affected in either group. Such information may have practical applications if reproduced in situations where skeletal muscle hypertrophy is desired but high mechanical stimuli of skeletal muscle and inflammation are not. Copyright © 2010 John Wiley & Sons, Ltd.     

Acute effect of fatty acids on metabolism and mitochondrial coupling in skeletal muscle

Acute effects of free fatty acids (FFA) were investigated on: (1) glucose oxidation, and UCP-2 and -3 mRNA and protein levels in 1 h incubated rat soleus and extensor digitorium longus (EDL) muscles, (2) mitochondrial membrane potential in cultured skeletal muscle cells, (3) respiratory activity and transmembrane electrical potential in mitochondria isolated from rat skeletal muscle, and (4) oxygen consumption by anesthetized rats. Long-chain FFA increased both basal and insulin-stimulated glucose oxidation in incubated rat soleus and EDL muscles and reduced mitochondrial membrane potential in C2C12 myotubes and rat skeletal muscle cells. Caprylic, palmitic, oleic, and linoleic acid increased O(2) consumption and decreased electrical membrane potential in isolated mitochondria from rat skeletal muscles. FFA did not alter UCP-2 and -3 mRNA and protein levels in rat soleus and EDL muscles. Palmitic acid increased oxygen consumption by anesthetized rats. These results suggest that long-chain FFA acutely lead to mitochondrial uncoupling in skeletal muscle.     

Acute effect of fatty acids on metabolism and mitochondrial coupling in skeletal muscle

Acute effects of free fatty acids (FFA) were investigated on: (1) glucose oxidation, and UCP-2 and -3 mRNA and protein levels in 1 h incubated rat soleus and extensor digitorium longus (EDL) muscles, (2) mitochondrial membrane potential in cultured skeletal muscle cells, (3) respiratory activity and transmembrane electrical potential in mitochondria isolated from rat skeletal muscle, and (4) oxygen consumption by anesthetized rats. Long-chain FFA increased both basal and insulin-stimulated glucose oxidation in incubated rat soleus and EDL muscles and reduced mitochondrial membrane potential in C2C12 myotubes and rat skeletal muscle cells. Caprylic, palmitic, oleic, and linoleic acid increased O₂ consumption and decreased electrical membrane potential in isolated mitochondria from rat skeletal muscles. FFA did not alter UCP-2 and -3 mRNA and protein levels in rat soleus and EDL muscles. Palmitic acid increased oxygen consumption by anesthetized rats. These results suggest that long-chain FFA acutely lead to mitochondrial uncoupling in skeletal muscle.     

Muscle glycogen repletion after exercise in trained normal and diabetic rats

We hypothesize that training results in a faster and greater repletion of glycogen in skeletal muscles of normal and diabetic rats. Normal male Sprague-Dawley rats (100-140 g) were divided into two groups--one to train by treadmill running for 10 wk and the other to remain sedentary. Forty-eight hours after the last training session the rats of both groups were exercised to exhaustion. One subgroup of each was fed oral glucose (3 g/kg) at exhaustion and killed 60 min later. The other was killed at exhaustion. The glycogen concentration of soleus, plantaris, and red and white gastrocnemius was determined in all rats. The trained group had higher glycogen levels after glucose feeding in all muscles (P less than 0.002) and repleted their muscle glycogen more rapidly (P less than 0.05). However, in diabetic rats (45 mg streptozotocin/kg body wt) the trained and sedentary rats have similar glycogen levels and glycogen repletion rates in all muscles. Compared with the normal trained rats, the diabetic trained rats had slower glycogen repletion rates (P less than 0.05).     

Regulation of skeletal muscle dihydropyridine receptor gene expression by biomechanical unloading.

Biomechanical unloading of the rat soleus by hindlimb unweighting is known to induce atrophy and a slow- to fast-twitch transition of skeletal muscle contractile properties, particularly in slow-twitch muscles such as the soleus. The purpose of this study was to determine whether the expression of the dihydropyridine (DHP) receptor gene is upregulated in unloaded slow-twitch soleus muscles. A rat DHP receptor cDNA was isolated by screening a random-primed cDNA lambda gt10 library from denervated rat skeletal muscle with oligonucleotide probes complementary to the coding region of the rabbit DHP receptor cDNA. Muscle mass and DHP receptor mRNA expression were assessed 1, 4, 7, 14, and 28 days after hindlimb unweighting in rats by tail suspension. Isometric twitch contraction times of soleus muscles were measured at 28 days of unweighting. Northern blot analysis showed that tissue distribution of DHP receptor mRNA was specific for skeletal muscle and expression was 200% greater in control fast-twitch extensor digitorum longus (EDL) than in control soleus muscles. A significant stimulation (80%) in receptor message of the soleus was induced as early as 24 h of unloading without changes in muscle mass. Unloading for 28 days induced marked atrophy (control = 133 +/- 3 vs. unweighted = 62.4 +/- 1.8 mg), and expression of the DHP receptor mRNA in the soleus was indistinguishable from levels normally expressed in EDL muscles. These changes in mRNA expression are in the same direction as the 37% reduction in time to peak tension and 28% decrease in half-relaxation time 28 days after unweighting. Our results suggest that muscle loading necessary for weight support modulates the expression of the DHP receptor gene in the soleus muscle.     

Response of XIAP, ARC, and FLIP apoptotic suppressors to 8 wk of treadmill running in rat heart and skeletal muscle.

Although it has been demonstrated that exercise training has an antiapoptotic effect on postmitotic myocytes, the mechanisms responsible for this effect are still largely unclear. Because the antiapoptotic effect of exercise training in postmitotic myocytes could be possibly mediated by the upregulation of apoptotic suppressors, this study examined the effect of endurance training on endogenous apoptotic suppressors including X-chromosome-linked inhibitor of apoptosis protein (XIAP), apoptosis repressor with caspases recruitment domain protein (ARC), and FADD-like inhibitor protein (FLIP) in skeletal and cardiac muscles. Eight adult Sprague-Dawley rats were trained 5 days weekly for 8 wk on treadmill, and eight sedentary rats served as controls. Soleus and ventricle muscles were dissected 2 days after the last training session. The mRNA content of XIAP, ARC, and FLIP was estimated by RT-PCR with ribosomal 18S RNA used as an internal control. The protein expression of XIAP, ARC, FLIP(S), and FLIP(alpha) was assessed by Western immunoblot. After training, mRNA content of ARC and FLIP was not different between the control and trained animals, whereas XIAP mRNA content was elevated by 22 and 14% in the trained soleus and cardiac muscles, respectively, relative to the control samples. No difference was found in the protein content of FLIP(S) and FLIP(alpha) between control and trained muscles, whereas XIAP and ARC protein content was increased by 18 and 38%, respectively, in the soleus muscle of trained animals. Furthermore, negative relationships were found between XIAP and apoptotic DNA fragmentation as well as ARC and caspase-3 activity. These findings are consistent with the hypothesis that the modulation of apoptotic suppressors is involved in training-induced attenuation of apoptosis in skeletal and cardiac muscles.