Some hormonal factors (hypophysectomy, castration and testosterone administration) modifying the course of "compensatory" muscle hypertrophy in the rat.

1. Maximum compensatory hypertrophy of the soleus and plantaris muscle in male rats is attained seven days after tenotomy of the gastrocnemius muscle (39% and 9% respectively). When tenotomy of the gastrocnemius was performed seven days ater hypophysectomy, hypertrophy in these two muscles was aproximately half that found in control animals. 2. After 81-day castration of young male rats the weight of the saleus and plantaris was reduced and hypertrophy following tenotomy of the gastrocneumius muscle did not develop. 3. Chronically castrated rats received testosterone two weeks prior to tenotomy of the gastrocnemius and a week during the muscle hypertrophy phase. Hypertrophy of the soleus in castrated rats which had received testosterone seven days after tenotomy of the gastrocnemius was 25% as compared with muscles of castrated animals. The corresponding value in the plantaris muscle was 10%. 4. These results indicate that even calf muscles of the rat, namely the soleus and plantaris muscles, are significantly affected by testosterone under these conditions, although it is not, as yet, clear whether its action is direct or indirect.     

Effects of castration and androgen treatment on androgen-receptor levels in rat skeletal muscles.

The effects of castration and dihydrotestosterone (DHT) treatment on levels of skeletal muscle androgen receptor (AR) were examined in three groups of adult male rats: 1) intact normal rats, 2) rats castrated at 16 wk of age, and 3) rats castrated at 16 wk of age and given DHT for 1 wk starting at week 17. All animals were killed at 18 wk of age. Castration caused a decrease (P < 0.05) in the weights of the levator ani and bulbocavernosus muscles. The administration of DHT to the castrated rats increased (P < 0.05) the weights of the levator ani and bulbocavernosus muscles. Castration caused a significant downregulation of AR levels in the bulbocavernosus (P < 0.05) but had no significant effect on AR levels in the levator ani muscle. DHT administration to the castrated group upregulated AR levels in the bulbocavernosus and levator ani muscles. The plantaris muscle did not significantly (P > 0.05) change for any of the treatments. These findings suggest that the effects of castration and androgen replacement differentially affect skeletal muscle mass and AR levels.     

Effect of anabolic steroids on skeletal muscle mass during hindlimb suspension

The efficacy of anabolic steroid treatment [0.3 or 0.9 mg nandrolone decanoate (Deca-Durabolin) per day] was examined in the context of sparing rodent fast-twitch plantaris and slow-twitch soleus muscle weight, sparing subcellular protein, and altering isomyosin expression in response to hindlimb suspension. Female rats were assigned to four groups (7 rats/group for 6 wk): 1) normal control (NC), 2) normal steroid (NS), 3) normal suspension (N-SUS), and 4) suspension steroid (SUS-S). Compared with control values for the plantaris and soleus muscles, suspension induced 1) smaller body and muscle weight (P less than 0.05), 2) losses in myofibril content (mg/muscle, P less than 0.05), and 3) shifts in the relative expression (expressed as %of total isomyosin) of isomyosins which favored lesser slow myosin and greater fast myosin isotypes (P less than 0.05). Steroid treatment of suspended animals (SUS-S vs. N-SUS) partially spared body and muscle weight (P less than 0.05) and spared plantaris but not soleus myofibril content (mg/muscle, P less than 0.05). However, steroid treatment did not modify the isomyosin pattern induced by suspension. In normal rats (NS vs. NC), steroid treatment enhanced body and plantaris muscle weight but not soleus weight (P less than 0.05) and did not alter isomyosin expression in either muscle type. Collectively these data suggest that in young female rats anabolic steroids 1) enhance the body weight and the weight of a fast-twitch ankle extensor in normal rats, 2) ameliorate the loss in body weight, fast-twitch muscle weight and protein content and slow-twitch muscle weight associated with hindlimb suspension.(ABSTRACT TRUNCATED AT 250 WORDS)     

Androgens are necessary for the establishment of secretory protein expression in the guinea pig seminal vesicle epithelium.

The guinea pig seminal vesicle epithelium (GPSVE) synthesizes and secretes milligram quantities of four related secretory proteins in an androgen-dependent manner. To investigate the role of androgens in the establishment of secretory protein synthesis during the development of the GPSVE, animals were castrated at Day 5, approximately 10 days before secretory protein accumulation begins in intact animals. Castration did not eliminate secretory protein mRNA from the SVE, but it did indefinitely postpone the developmentally programmed increase in secretory protein mRNA. Injection of neonatally castrated guinea pigs with either estradiol or dexamethasone did not alter levels of secretory protein mRNAs. However, treatment of castrated neonates with either testosterone propionate or dihydrotestosterone (DHT) led to specific increases in secretory protein mRNAs within 4 days. Although neonatally castrated animals accumulated and translated significant amounts of secretory protein mRNA, the newly synthesized secretory proteins failed to accumulate until exogenous androgens were provided. This observation suggests that androgens regulate both the accumulation of secretory protein mRNA and the accumulation of secretory proteins in the GPSVE.     

Aging and respiratory muscle metabolic plasticity: effects of endurance training.

We examined the oxidative and antioxidant enzyme activities in respiratory and locomotor muscles in response to endurance training in young and aging rats. Young adult (4-mo-old) and old (24-mo-old) female Fischer 344 rats were divided into four groups: 1) young trained (n = 12), 2) young untrained (n = 12), 3) old trained (n = 10), and 4) old untrained (n = 6). Both young and old endurance-trained animals performed the same training protocol during 10 wk of continuous treadmill exercise (60 min/day, 5 days/wk). Compared with young untrained animals, the young trained group had significantly elevated (P less than 0.05) activities of 3-hydroxyacyl-CoA dehydrogenase (HADH), glutathione peroxidase (GPX), and citrate synthase (CS) in both the costal diaphragm and the plantaris muscle. In contrast, training had no influence (P greater than 0.05) on the activity of lactate dehydrogenase within the costal diaphragm in young animals. In the aging animals, training did not alter (P greater than 0.05) activities of CS, HADH, GPX, or lactate dehydrogenase in the costal diaphragm but significantly (P less than 0.05) increased CS, HADH, and GPX activities in the plantaris muscle. Furthermore, training resulted in higher activities of CS and HADH in the intercostal muscles in the old trained than in the old untrained animals. Finally, activities of CS, HADH, and GPX were significantly (P less than 0.05) lower in the plantaris in the old untrained than in the young untrained animals; however, CS, HADH, and GPX activities were greater (P less than 0.05) in the costal diaphragm in the old sedentary than in the young untrained animals.(ABSTRACT TRUNCATED AT 250 WORDS)     

Proliferation of skeletal muscle satellite cells after castration and administration of testosterone propionate

Thirty-six neonatal pigs were randomly assigned to the following treatment groups: sham implanted gonadally intact males (B), sham-implanted castrated males (C), or castrated males implanted with testosterone propionate (C + TP). Four pigs from each group were sacrificed at 7, 14, or 21 days of age after a 6-hr continuous infusion of [3H]thymidine. Myofibers isolated from the triceps brachii were prepared for satellite cell enumeration by light microscope autoradiography. A developmental decline in labeled myofiber nuclei occurred in all groups, however, the greatest decline occurred in C (P less than 0.01). A treatment-by-age interaction was observed for percentage of labeled nuclei. Castration reduced total and labeled nuclei per millimeter myofiber (P less than 0.05), and C + TP had a higher percentage of labeled nuclei than C (2.8 vs 2.2%; P less than 0.05). Since triceps brachii muscles from 21 day B and C + TP were 120% (P less than 0.05) of C, the results indicate that postnatal growth of skeletal muscle is dependent on satellite cell mitotic activity and that testosterone enhances this activity in neonatal pigs.     

β-Hydroxy-β-methylbutyrate reduces myonuclear apoptosis during recovery from hind limb suspension-induced muscle fiber atrophy in aged rats

β-Hydroxy-β-methylbutyrate (HMB) is a leucine metabolite shown to reduce protein catabolism in disease states and promote skeletal muscle hypertrophy in response to loading exercise. In this study, we evaluated the efficacy of HMB to reduce muscle wasting and promote muscle recovery following disuse in aged animals. Fisher 344×Brown Norway rats, 34 mo of age, were randomly assigned to receive either Ca-HMB (340 mg/kg body wt) or the water vehicle by gavage (n = 32/group). The animals received either 14 days of hindlimb suspension (HS, n = 8/diet group) or 14 days of unloading followed by 14 days of reloading (R; n = 8/diet group). Nonsuspended control animals were compared with suspended animals after 14 days of HS (n = 8) or after R (n = 8). HMB treatment prevented the decline in maximal in vivo isometric force output after 2 wk of recovery from hindlimb unloading. The HMB-treated animals had significantly greater plantaris and soleus fiber cross-sectional area compared with the vehicle-treated animals. HMB decreased the amount of TUNEL-positive nuclei in reloaded plantaris muscles (5.1% vs. 1.6%, P < 0.05) and soleus muscles (3.9% vs. 1.8%, P < 0.05). Although HMB did not significantly alter Bcl-2 protein abundance compared with vehicle treatment, HMB decreased Bax protein abundance following R, by 40% and 14% (P < 0.05) in plantaris and soleus muscles, respectively. Cleaved caspase-3 was reduced by 12% and 9% (P < 0.05) in HMB-treated reloaded plantaris and soleus muscles, compared with vehicle-treated animals. HMB reduced cleaved caspase-9 by 14% and 30% (P < 0.05) in reloaded plantaris and soleus muscles, respectively, compared with vehicle-treated animals. Although, HMB was unable to prevent unloading-induced atrophy, it attenuated the decrease in fiber area in fast and slow muscles after HS and R. HMB's ability to protect against muscle loss may be due in part to putative inhibition of myonuclear apoptosis via regulation of mitochondrial-associated caspase signaling.     

The effect of indomethacin on the stimulation of protein synthesis by insulin in young post-absorptive rats.

Groups of young rats (100 g body wt.) were starved from 23:00 to 11:00 h. The animals were then infused intravenously with diluent or insulin at three different doses to achieve plasma insulin concentrations of 20, 50 and 150 microunits/ml. Before the start of the infusion, animals received a single intravenous injection of indomethacin (250 micrograms) or diluent. After 20 min of infusion, the rats were injected with a large amount of labelled phenylalanine and were killed 10 min later. Insulin produced a dose-dependent decrease in plasma glucose and a dose-dependent rise in protein synthesis in cardiac, gastrocnemius, plantaris and soleus muscles. Protein synthesis in the liver was unaffected by insulin. Indomethacin had no effect on plasma glucose concentrations, but blocked the insulin-induced rise in protein synthesis in cardiac, gastrocnemius and plantaris, but not in soleus muscle. The hormone also increased the plasma concentration of prostaglandin E2 and of prostaglandins F2 alpha and E2 in gastrocnemius and plantaris muscle. The results show close similarities to previous observations with isolated rabbit muscles in vitro and suggest that the involvement of arachidonic acid metabolism in the action of insulin on protein synthesis is of physiological significance.     

Skeletal muscle protein and amino acid metabolism in hereditary mouse muscular dystrophy. Accelerated protein turnover and increased alanine and glutamine formation and release

Interactins between skeletal muscle protein and amino acid metabolism were investigated using C57BL and 129ReJ mice with hereditary muscular dystrophy. On incubation, hind limb muscle preparations from dystrophic mice released large quantities of amino acids, particularly alanine and glutamine which were increased 70% and 40% compared to muscles from carrier or control mice. The increased alanine release did not result from altered alanine oxidation to CO2 or reincorporation into protein. Alanine and glutamine formation from added amino acids were equal with dystrophic and control muscles. Incorporation in vitro of leucine, alanine, and glutamate into proteins of dystrophic muscle was 3- to 7-fold greater than control muscle, and the incorporation in vivo of [3H]- or [14C]arginine into muscle proteins was greater in extent and earlier in time with dystrophic as compared to control muscle. Proteins were also labeled in vivo using [guanido-14C]arginine. On incubation of these muscles in vitro, a 100% greater loss of label from protein was observed with dystrophic as compared to control preparations, and the appearance of label in the media was correspondingly increased. Sodium dodecyl sulfate-gel electrophoresis of dystrophic skeletal muscle showed numerous protein bands to be reduced in density, but autoradiographic studies demonstrated that these same bands were more highly labeled in vitro by [35S]methionine in dystrophic than in control muscle. Although insulin stimulation of glucose uptake was markedly blunted in dystrophic muscle, insulin inhibited alanine and glutamine release equally from both control and dystrophic muscle. These data indicate that alanine and glutamine formation and release are increased in hereditary mouse muscular dystrophy. An accelerated degradation and an increased resynthesis of many muscle proteins were also observed in dystrophic compared to control animals. This increased proteolysis may account for the increased alanine and glutamine formation in dystrophic muscle.     

Differential response of rat skeletal muscle glycogen metabolism to testosterone and estradiol.

Although reports on sex steroids have implicated them as promoting protein synthesis and also providing extra strength to the skeletal muscle, it remains unclear whether sex steroids affect glycogen metabolism to provide energy for skeletal muscle functions, since glycogen metabolism is one of the pathways that provides energy for the skeletal muscle contraction and relaxation cycle. The purpose of the current study was to show that testosterone and estradiol act differentially on skeletal muscles from different regions, differentially with reference to glycogen metabolism. To study this hypothesis, healthy mature male Wistar rats (90-120 days of age, weighing about 180-200 g) were castrated (a bilateral orchidectomy was performed to test the significance of skeletal muscle glycogen metabolism in the absence of testosterone). One group of castrated rats was supplemented with testosterone (100 microg/100 g body weight, i.m., for 30 days from day 31 postcastration onwards). To test whether estradiol has any effect on male skeletal muscle glycogen metabolism 17beta-estradiol (5 microg/100 g body weight, i.m., for 30 days from day 31 postcastration onwards) was administered to orchidectomized rats. To test whether these sex steroids have any differential effect on skeletal muscles from different regions, skeletal muscles from the temporal region (temporalis), muscle of mastication (masseter), forearm muscle (triceps and biceps), thigh muscle (vastus lateralis and gracilis), and calf muscle (gastrocnemius and soleus) were considered. Castration enhanced blood glucose levels and decreased glycogen stores in skeletal muscle from head, jaw, forearm, thigh, and leg regions. This was accompanied by diminished activity of glycogen synthetase and enhanced activity of muscle phosphorylase. Following testosterone supplementation to castrated rats, a normal pattern of all these parameters was maintained. Estradiol administration to castrated rats did not bring about any significant alteration in any of the parameters. The data obtained suggest a stimulatory effect of testosterone on skeletal muscle glycogenesis and an inhibitory effect on glycogenolysis. Estradiol did not play any significant role in the skeletal muscle glycogen metabolism of male rats.     

Effect of different sources of dietary protein on muscle hypertrophy in functionally overloaded mice

Dietary protein intake is important for skeletal muscle protein synthesis. In this study, we investigated the differential effect of protein sources on hypertrophy of plantaris muscle induced by surgical ablation of gastrocnemius and soleus muscles. Six-week old mice were fed diets containing caseinate, whey, or soy as protein sources for 2 weeks. Plantaris muscle hypertrophy was induced by a unilateral ablation of synergistic muscles after a week. Food intake of soy protein-fed mice was higher than that of caseinate and whey-fed mice, resulting in higher body and fat weights. Plantaris muscle weight in sham-operated mice was not different across the groups. Overload-operated plantaris muscle weight and increased ratio of overloaded muscle to sham-operated muscle weights were higher in caseinate-fed mice than in whey- and soy protein-fed mice, suggesting caseinate as a promising protein source for muscle hypertrophy.     

Adaptive response of hypertrophied skeletal muscle to endurance training

The response of hypertrophied soleus and plantaris muscle of rats to endurance training was studied. Hypertrophy was produced by bilateral extirpation of the gastrocnemius muscle. A 13-wk training program of treadmill running initiated 30 days after removal of the gastrocnemius muscle accentuated (P less than 0.01) the hypertrophy. Succinate dehydrogenase activities of the enlarged muscles of sedentary rats were similar to those of normal animals, as were the increases associated with training. Phosphorylase and hexokinase activities were unaltered as a result of the experimental perturbations. Rates of glycogen depletion during exercise were lower (P less than 0.01) in the liver and soleus and plantaris muscles of endurance-trained animals. No difference existed in the rate of glycogen depletion of normal and hypertrophied muscle within the sedentary or trained groups. These data demonstrate that extensively hypertrophied muscle responds to training and exercise in a manner similar to that of normal muscle.     

Further regression of seminal vesicles of castrated guinea pig by administration of cyproterone acetate

It has been established that a low level of secretory activity persisted in seminal vesicles of guinea pigs long after castration and that this may be due to a higher extratesticular androgen level in this animal. A RIA study revealed that the normal serum testosterone concentration of the guinea pigs was comparable to that of the rats, but the basal serum testosterone level after castration was ten times higher than rats under a similar condition. It was also shown that cyproterone acetate did not significantly lower the basal serum testosterone concentration in the castrated guinea pigs. The higher basal serum testosterone level is believed to be responsible for the slow and incomplete regression of this gland in the guinea pigs. There was a significant reduction in wet weight of the seminal vesicles after the treatment of castrated guinea pigs with cyproterone acetate. Ultrastructural study showed that there were both qualitative and quantitative changes in the cytoplasmic organelles. The Golgi apparatus further reduced in size and in the number of associated vesicles and vacuoles. There was a marked decrease in the number and size of secretory granules and lysosomes and an increase in the degree of undulation of the basement membrane. Accumulation of lipid droplets and glycogen was commonly observed. All these morphological evidences showed that further regression of the castrated guinea pig seminal vesicles can be achieved by cyproterone acetate treatment.     

Glucocorticoid cytosol binding in exercise-induced sparing of muscle atrophy

Female rats were initially divided into a sedentary or an exercise group that was trained by treadmill running to a final work rate of 31 m/min, 100 min/day, for 13-18 wk. During the last 12 days of training each of these groups were further subdivided into groups that received daily subcutaneous injections of cortisol acetate (CA) (100 mg/kg body wt) or the vehicle (1% carboxymethyl cellulose). Exercise prevented approximately 40% of the gastrocnemius muscle weight loss due to CA treatment. Training did not influence glucocorticoid cytosol-receptor binding concentrations, using [3H]triamcinolone acetonide (TA) as the labeled glucocorticoid in any of the skeletal muscle types investigated. TA-receptor binding capacities were depleted by the multiple injections but were higher in the red fiber types of the CA-treated trained than those in the CA-treated sedentary animals. In a second series of experiments in which receptor depletion and repletion rates were studied using a single injection of cortisol, TA binding capacities 2 h after the cortisol injection were higher in slow-twitch red soleus muscles of trained as compared with sedentary rats (36.4 +/- 2.0 vs. 26.8 +/- 2.5 fmol/mg protein). Similar patterns of TA binding were also observed at 2 h between trained and sedentary animals in the fast-twitch red muscle types, whereas no training related differences were observed in white muscle types. Total and free serum cortisol concentrations also returned to base-line values faster in the trained animals following the single injection protocol.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Effect of testosterone on muscle mass and muscle protein synthesis

We have studied the effect of a pharmacological dose of testosterone enanthate (3 mg.kg-1.wk-1 for 12 wk) on muscle mass and total-body potassium and on whole-body and muscle protein synthesis in normal male subjects. Muscle mass estimated by creatinine excretion increased in all nine subjects (20% mean increase, P less than 0.02); total body potassium mass estimated by 40K counting increased in all subjects (12% mean increase, P less than 0.0001). In four subjects, a primed continuous infusion protocol with L-[1-13C]leucine was used to determine whole-body leucine flux and oxidation. Whole-body protein synthesis was estimated from nonoxidative flux. Muscle protein synthesis rate was determined by measuring [13C]leucine incorporation into muscle samples obtained by needle biopsy. Testosterone increased muscle protein synthesis in all subjects (27% mean increase, P less than 0.05). Leucine oxidation decreased slightly (17% mean decrease, P less than 0.01), but whole-body protein synthesis did not change significantly. Muscle morphometry showed no significant increase in muscle fiber diameter. These studies suggest that testosterone increases muscle mass by increasing muscle protein synthesis.     

L-tryptophan binding to hepatic nuclei: Age and species differences

Summary The present experiments report differences in in vitro nuclear binding affinity for L-tryptophan 1) between livers of young (6 1/2 weeks old) and older (30 weeks old) NZBWF₁ mice, but not so in similar aged Swiss mice, and also, 2) in livers of hamsters compared to livers of guinea pigs. In vitro hepatic nuclear specific binding affinity after tube-feeding L-tryptophan (520mg/100g body weight) to mice 1 h before killing revealed less in young than in older NZWBF₁ mice, comparable to the above in vitro assay studies. In vitro nuclear binding affinity for L-tryptophan of livers of hamsters was significantly less than that of livers of guinea pigs or Swiss mice. In general, the degree of stimulatory effect on hepatic protein synthesis, as measured by in vitro [¹⁴C]leucine incorporation into protein using microsomes of animals tube-fed L-tryptophan 1 h before killing compared to that of animals tubefed water, correlated with the basal nuclear specific binding affinity to L-tryptophan of the animals (ages and species) used.This study was supported by U.S. Public Health Service Research Grant DK-45353 from the National Institute of Diabetes and Digestive and Kidney Diseases.     

In vivo MRI quantification of individual muscle and organ volumes for assessment of anabolic steroid growth effects

This study aimed to develop a quantitative and in vivo magnetic resonance imaging (MRI) approach to investigate the muscle growth effects of anabolic steroids. A protocol of MRI acquisition on a standard clinical 1.5 T scanner and quantitative image analysis was established and employed to measure the individual muscle and organ volumes in the intact and castrated guinea pigs undergoing a 16-week treatment protocol by two well-documented anabolic steroids, testosterone and nandrolone, via implanted silastic capsules. High correlations between the in vivo MRI and postmortem dissection measurements were observed for shoulder muscle complex (R=0.86), masseter (R=0.79), temporalis (R=0.95), neck muscle complex (R=0.58), prostate gland and seminal vesicles (R=0.98), and testis (R=0.96). Furthermore, the longitudinal MRI measurements yielded adequate sensitivity to detect the restoration of growth to or towards normal in castrated guinea pigs by replacing circulating steroid levels to physiological or slightly higher levels, as expected. These results demonstrated that quantitative MRI using a standard clinical scanner provides accurate and sensitive measurement of individual muscles and organs, and this in vivo MRI protocol in conjunction with the castrated guinea pig model constitutes an effective platform to investigate the longitudinal and cross-sectional growth effects of other potential anabolic steroids. The quantitative MRI protocol developed can also be readily adapted for human studies on most clinical MRI scanner to investigate the anabolic steroid growth effects, or monitor the changes in individual muscle and organ volume and geometry following injury, strength training, neuromuscular disorders, and pharmacological or surgical interventions.     

Limited resistance of hypertrophied skeletal muscle to glucocorticoids

Male hypophysectomized rats were initially assigned to a control or an overloaded group that underwent compensatory hypertrophy of plantaris muscles to steady-state levels following removal of synergistic musculature. Plantaris muscle mass of overloaded animals was higher than that of controls by 38% (391 +/- 8 vs 284 +/- 7 mg) and glucocorticoid cytosol specific binding concentrations, using [3H]triamcinolone acetonide (TA) as the labeled steroid, was also significantly higher in hypertrophied muscles (83.3 +/- 3.9 fmol . mg protein-1) than in control muscles 56.3 +/- 3.9 fmol . mg protein-1). Cortisone acetate (CA) was then administered daily subcutaneously in high, 100 mg; intermediate, 10 mg; or low, 1.0 mg . kg-1 body wt doses. Groups of rats were killed after 1/4, 2 days and 7 days. Absolute muscle mass losses after 7 days of CA treatment were approx 80 mg with high doses and 60 mg with intermediate doses in both hypertrophied and control muscles. The low CA dose did not produce atrophy. The absolute depletion of [3H]TA binding activity with CA treatment was always greater in hypertrophied muscles of high and intermediate dose treated than those of their respective controls, but TA binding capacities remained higher in hypertrophied muscles than in controls at almost all time points in all treatment groups. Unlike previous findings in which the simultaneous initiation of overload prevented glucocorticoid induced muscle wasting, no resistance to the effect of CA treatment was observed when treatment was begun after hypertrophy had occurred.     

Diminished overload-induced hypertrophy in aged fast-twitch skeletal muscle is associated with AMPK hyperphosphorylation.

Skeletal muscle mass declines with age, as does the potential for overload-induced fast-twitch skeletal muscle hypertrophy. Because 5'-AMP-activated protein kinase (AMPK) activity is thought to inhibit skeletal muscle protein synthesis and may therefore modulate muscle mass and hypertrophy, the purpose of this investigation was to examine AMPK phosphorylation status (a marker of AMPK activity) and its potential association with the attenuated overload-induced hypertrophy observed in aged skeletal muscle. One-week overload of fast-twitch plantaris and slow-twitch soleus muscles was achieved in young adult (8 mo; n = 7) and old (30 mo; n = 7) Fischer344 x Brown Norway male rats via unilateral gastrocnemius ablation. Significant (P < or = 0.05) age-related atrophy (as measured by total protein content) was noted in plantaris and soleus control (sham-operated) muscles. In fast-twitch plantaris muscles, percent hypertrophy with overload was significantly attenuated with age, whereas AMPK phosphorylation status as determined by Western blotting [phospho-AMPK (Thr172)/total AMPK] was significantly elevated with age (regardless of loading status). There was also a main effect of loading on AMPK phosphorylation status in plantaris muscles (overload > control). Moreover, a strong and significant negative correlation (r = -0.82) was observed between AMPK phosphorylation status and percent hypertrophy in the overloaded plantaris muscles of all animals. In contrast to the plantaris, overload-induced hypertrophy of the slow-twitch soleus muscle was similar between ages, and AMPK phosphorylation in this muscle was also unaffected by age or overload. These data support the possibility that an age-related elevation in AMPK phosphorylation may partly contribute to the attenuated hypertrophic response observed with age in overloaded fast-twitch plantaris muscle.