Effect of continuous and intermittent clenbuterol feeding on rat growth rate and muscle

Change of Computed Tomography-Based Body Composition after Adrenalectomy in Patients with Pheochromocytoma

Despite the potential biological importance of the sympathetic nervous system on fat and skeletal muscle metabolism in animal and in vitro studies, its relevance in humans remains undetermined. To clarify the influence of catecholamine excess on human body composition, we performed a retrospective longitudinal cohort study including 313 consecutive patients with histologically confirmed pheochromocytoma who underwent repeat abdominal computed tomography (CT) scans before and after adrenalectomy. Changes in CT-determined visceral fat area (VFA), subcutaneous fat area (SFA), skeletal muscle area (SMA), and skeletal muscle index (SMI) were measured at the level of the third lumbar vertebra. The mean age of all patients was 50.6 ± 13.6 years, and 171/313 (54.6%) were women. The median follow-up duration for repeat CTs was 25.0 months. VFA and SFA were 14.5% and 15.8% higher, respectively (both p < 0.001), after adrenalectomy, whereas SMA and SMI remained unchanged. Similarly, patients with visceral obesity significantly increased from 103 (32.9%) at baseline to 138 (44.1%) following surgery (p < 0.001); however, the prevalence of sarcopenia was unchanged. This study provides important clinical evidence that sympathetic hyperactivity can contribute to lipolysis in visceral and subcutaneous adipose tissues, but its impact on human skeletal muscle is unclear.

Ractopamine-induced fiber type-specific gene expression in porcine skeletal muscles is independent of growth

Feeding ractopamine (RAC), a β-adrenergic agonist (BAA), to pigs increases type IIB muscle fiber type-specific protein and mRNA expression. However, increases in the abundance of these fast-twitch fiber types occur with other forms of muscle hypertrophy and thus BAA-induced changes in myosin heavy chain (MyHC) composition may simply be associated with increased muscle growth known to occur in response to BAA feeding. The objective of this study was to determine whether RAC feeding could change the MyHC gene expression in the absence of maximal muscle growth. Pigs were fed either an adequate diet that supported maximal muscle hypertrophy or a low nutrient diet that limited muscle growth. RAC was included in diets at 0 or 20 mg/kg for 1, 2, or 4 wk. Backfat depth was less (P < 0.05) in pigs fed the low nutrient diet compared with the adequate diet but was not affected by RAC. Loin eye area was greater (P < 0.05) in pigs fed an adequate diet plus RAC at 1 wk but did not differ among remaining pigs. At 2 and 4 wk, however, pigs fed the adequate diet had greater loin eye areas (P < 0.05) than pigs fed the low nutrient diet regardless of RAC feeding. Gene expression of the MyHC isoforms, I, IIA, IIX, and IIB, as well as glycogen synthase, citrate synthase, β 1-adrenergic receptor (AR), and β 2-AR were determined in longissimus dorsi (LD) and red (RST) and white (WST) portions of the semitendinosus muscles. MyHC type I gene expression was not altered by RAC or diet. Feeding RAC decreased (P < 0.01) MyHC type IIA gene expression in all muscles, but to a greater extent in WST and LD. MyHC type IIX gene expression was lower (P < 0.05) in WST and LD muscles in response to RAC but was not altered in RST muscles. RAC increased (P < 0.05) MyHC type IIB gene expression in all muscles, but to a greater extent in RST. β 1-AR gene expression was unaffected by RAC or diet, whereas the expression of the β 2-AR gene was decreased (P < 0.001) by RAC. No significant RAC * diet interactions were observed in gene expression in this study, indicating that RAC altered MyHC and β 2-AR gene expression in porcine skeletal muscles independent of growth.

Change of skeletal muscle mass in patients with pheochromocytoma

The effects of catecholamine excess due to pheochromocytoma on body composition, including skeletal muscle mass, are unknown. Here, we investigated the effects of catecholamine metabolites on body composition in subjects with pheochromocytoma. After body compositions using bioelectrical impedance analysis, urinary metanephrine (UM), and urinary normetanephrine (UNM) were measured in 16 patients with pheochromocytoma and 224 patients with nonfunctioning adrenal incidentaloma (NFAI), we compared skeletal muscle mass and fat mass (FM) between the two groups. After adjustments for confounders, UM (β = - 0.171, P = 0.006) and UNM (β = - 0.249, P < 0.001) levels were correlated inversely with skeletal muscle mass index (SMI), but not FM or percentage FM (pFM), in all subjects. Patients with pheochromocytoma had lower ASM by 7.7% (P = 0.022) and SMI by 6.6% (P = 0.001) than patients with NFAI. Conversely, FM and pFM were not statistically different between the two groups. The odds ratio for low skeletal muscle mass in the presence of pheochromocytoma was 10.33 (95% confidence interval, 2.65-40.22). Our results indicate that patients with pheochromocytoma have a reduced skeletal muscle mass and suggest that catecholamine excess has adverse effects on skeletal muscle metabolism.

Beta2-Agonist Doping Control and Optical Isomer Challenges

The World Anti-Doping Agency (WADA) currently allows therapeutic use of the beta2-agonists salbutamol, formoterol and salmeterol when delivered via inhalation despite some evidence suggesting these anti-asthma drugs may be performance enhancing. Beta2-agonists are usually administered as 50:50 racemic mixtures of two enantiomers (non-superimposable mirror images), one of which demonstrates significant beta2-adrenoceptor-mediated bronchodilation while the other appears to have little or no pharmacological activity. For salbutamol and formoterol, urine thresholds have been adopted to limit supratherapeutic dosing and to discriminate between inhaled (permitted) and oral (prohibited) use. However, chiral switches have led to the availability of enantiopure (active enantiomer only) preparations of salbutamol and formoterol, which effectively doubles their urine thresholds and provides a means for athletes to take supratherapeutic doses for doping purposes. Given the availability of these enantiopure beta2-agonists, the analysis of these drugs using enantioselective assays should now become routine. For salmeterol, there is currently only a therapeutic dose threshold and adoption of a urinary threshold should be a high priority for doping control.

Role of phosphodiesterase 4 expression in the Epac1 signaling-dependent skeletal muscle hypertrophic action of clenbuterol

Clenbuterol (CB), a selective β2-adrenergic receptor (AR) agonist, induces muscle hypertrophy and counteracts muscle atrophy. However, it is paradoxically less effective in slow-twitch muscle than in fast-twitch muscle, though slow-twitch muscle has a greater density of β-AR We recently demonstrated that Epac1 (exchange protein activated by cyclic AMP [cAMP]1) plays a pivotal role in β2-AR-mediated masseter muscle hypertrophy through activation of the Akt and calmodulin kinase II (CaMKII)/histone deacetylase 4 (HDAC4) signaling pathways. Here, we investigated the role of Epac1 in the differential hypertrophic effect of CB using tibialis anterior muscle (TA; typical fast-twitch muscle) and soleus muscle (SOL; typical slow-twitch muscle) of wild-type (WT) and Epac1-null mice (Epac1KO). The TA mass to tibial length (TL) ratio was similar in WT and Epac1KO at baseline and was significantly increased after CB infusion in WT, but not in Epac1KO The SOL mass to TL ratio was also similar in WT and Epac1KO at baseline, but CB-induced hypertrophy was suppressed in both mice. In order to understand the mechanism involved, we measured the protein expression levels of β-AR signaling-related molecules, and found that phosphodiesterase 4 (PDE4) expression was 12-fold greater in SOL than in TA These results are consistent with the idea that increased PDE4-mediated cAMP hydrolysis occurs in SOL compared to TA, resulting in a reduced cAMP concentration that is insufficient to activate Epac1 and its downstream Akt and CaMKII/HDAC4 hypertrophic signaling pathways in SOL of WT This scenario can account for the differential effects of CB on fast- and slow-twitch muscles.

Protective Effects of Clenbuterol against Dexamethasone-Induced Masseter Muscle Atrophy and Myosin Heavy Chain Transition

Background

Glucocorticoid has a direct catabolic effect on skeletal muscle, leading to muscle atrophy, but no effective pharmacotherapy is available. We reported that clenbuterol (CB) induced masseter muscle hypertrophy and slow-to-fast myosin heavy chain (MHC) isoform transition through direct muscle β2-adrenergic receptor stimulation. Thus, we hypothesized that CB would antagonize glucocorticoid (dexamethasone; DEX)-induced muscle atrophy and fast-to-slow MHC isoform transition.

Methodology

We examined the effect of CB on DEX-induced masseter muscle atrophy by measuring masseter muscle weight, fiber diameter, cross-sectional area, and myosin heavy chain (MHC) composition. To elucidate the mechanisms involved, we used immunoblotting to study the effects of CB on muscle hypertrophic signaling (insulin growth factor 1 (IGF1) expression, Akt/mammalian target of rapamycin (mTOR) pathway, and calcineurin pathway) and atrophic signaling (Akt/Forkhead box-O (FOXO) pathway and myostatin expression) in masseter muscle of rats treated with DEX and/or CB.

Results and Conclusion

Masseter muscle weight in the DEX-treated group was significantly lower than that in the Control group, as expected, but co-treatment with CB suppressed the DEX-induced masseter muscle atrophy, concomitantly with inhibition of fast-to-slow MHC isoforms transition. Activation of the Akt/mTOR pathway in masseter muscle of the DEX-treated group was significantly inhibited compared to that of the Control group, and CB suppressed this inhibition. DEX also suppressed expression of IGF1 (positive regulator of muscle growth), and CB attenuated this inhibition. Myostatin protein expression was unchanged. CB had no effect on activation of the Akt/FOXO pathway. These results indicate that CB antagonizes DEX-induced muscle atrophy and fast-to-slow MHC isoform transition via modulation of Akt/mTOR activity and IGF1 expression. CB might be a useful pharmacological agent for treatment of glucocorticoid-induced muscle atrophy.

Enantioselective disposition of (R/S)-albuterol in skeletal and cardiac muscle

Significant enhancement of skeletal muscle function has been observed with racemic albuterol (salbutamol). There is now general acceptance that the R-albuterol enantiomer elicits the pharmacological response, both in the lungs and extrapulmonary, while S-albuterol is pharmacologically inert. The objective of this study was to investigate the distribution of (R/S)-albuterol enantiomers into skeletal and cardiac muscle. Initially oral dosing was undertaken in neonatal mice administered a maximum tolerable dose of racemic albuterol. An in vivo infusion rat model was employed for the investigation of albuterol uptake into skeletal and cardiac muscle over 4 h. Tissue concentrations were determined using liquid chromatography-tandem mass spectrometry (LC-MS/MS). From the oral dosing model, mean (±SD) levels of racemic albuterol after 5 days were 915 (±293) ng/mL in plasma, 2574 (±196) ng/g in muscle, and 53 (±6.6) ng/g in brain with enantioselective partitioning (muscle:plasma ratio of 5.7 and 1.7 for R- and S-albuterol, respectively). In the infusion model, enantioselective disposition was observed in skeletal muscle (muscle:plasma ratio of 1.2-1.7 and 0.6-0.7 for R- and S-albuterol, respectively) and in cardiac muscle (4.1 and 0.5, respectively). In conclusion, there is greater partitioning of active (R)-albuterol than inactive (S)-albuterol into both skeletal and cardiac muscle compared to plasma. These findings have relevance for albuterol sports doping, cardiac effects, and therapeutic use in muscle wasting diseases. Furthermore, the greater muscle partitioning of the active R-albuterol, and the availability of pure R-albuterol formulations highlight shortcomings in doping control measures using non-enantioselective assays.

Sympathetic hyperactivity differentially affects skeletal muscle mass in developing heart failure: role of exercise training

Sympathetic hyperactivity (SH) is a hallmark of heart failure (HF), and several lines of evidence suggest that SH contributes to HF-induced skeletal myopathy. However, little is known about the influence of SH on skeletal muscle morphology and metabolism in a setting of developing HF, taking into consideration muscles with different fiber compositions. The contribution of SH on exercise tolerance and skeletal muscle morphology and biochemistry was investigated in 3- and 7-mo-old mice lacking both alpha(2A)- and alpha(2C)-adrenergic receptor subtypes (alpha(2A)/alpha(2C)ARKO mice) that present SH with evidence of HF by 7 mo. To verify whether exercise training (ET) would prevent skeletal muscle myopathy in advanced-stage HF, alpha(2A)/alpha(2C)ARKO mice were exercised from 5 to 7 mo of age. At 3 mo, alpha(2A)/alpha(2C)ARKO mice showed no signs of HF and preserved exercise tolerance and muscular norepinephrine with no changes in soleus morphology. In contrast, plantaris muscle of alpha(2A)/alpha(2C)ARKO mice displayed hypertrophy and fiber type shift (IIA --> IIX) paralleled by capillary rarefaction, increased hexokinase activity, and oxidative stress. At 7 mo, alpha(2A)/alpha(2C)ARKO mice displayed exercise intolerance and increased muscular norepinephrine, muscular atrophy, capillary rarefaction, and increased oxidative stress. ET reestablished alpha(2A)/alpha(2C)ARKO mouse exercise tolerance to 7-mo-old wild-type levels and prevented muscular atrophy and capillary rarefaction associated with reduced oxidative stress. Collectively, these data provide direct evidence that SH is a major factor contributing to skeletal muscle morphological changes in a setting of developing HF. ET prevented skeletal muscle myopathy in alpha(2A)/alpha(2C)ARKO mice, which highlights its importance as a therapeutic tool for HF.

Role of beta-adrenoceptor signaling in skeletal muscle: implications for muscle wasting and disease

The importance of beta-adrenergic signaling in the heart has been well documented, but it is only more recently that we have begun to understand the importance of this signaling pathway in skeletal muscle. There is considerable evidence regarding the stimulation of the beta-adrenergic system with beta-adrenoceptor agonists (beta-agonists). Although traditionally used for treating bronchospasm, it became apparent that some beta-agonists could increase skeletal muscle mass and decrease body fat. These so-called "repartitioning effects" proved desirable for the livestock industry trying to improve feed efficiency and meat quality. Studying beta-agonist effects on skeletal muscle has identified potential therapeutic applications for muscle wasting conditions such as sarcopenia, cancer cachexia, denervation, and neuromuscular diseases, aiming to attenuate (or potentially reverse) the muscle wasting and associated muscle weakness, and to enhance muscle growth and repair after injury. Some undesirable cardiovascular side effects of beta-agonists have so far limited their therapeutic potential. This review describes the physiological significance of beta-adrenergic signaling in skeletal muscle and examines the effects of beta-agonists on skeletal muscle structure and function. In addition, we examine the proposed beneficial effects of beta-agonist administration on skeletal muscle along with some of the less desirable cardiovascular effects. Understanding beta-adrenergic signaling in skeletal muscle is important for identifying new therapeutic targets and identifying novel approaches to attenuate the muscle wasting concomitant with many diseases.

Inhibited longitudinal growth of bones in young male rats by clenbuterol

PURPOSE

Clenbuterol is one of the beta-2 adrenergic receptor agonists with potent anabolic properties in muscles, yet the concomitant effects on muscle and bone in young animals remain to be resolved. Therefore, the purpose of this study was to determine the effects of clenbuterol administration on muscles and bones of young rats.

METHODS

Twelve male Sprague-Dawley rats (9-wk-old) were randomly assigned to either a control (CON, N = 6) or clenbuterol group (CLE, N = 6). Clenbuterol of 2 mg x kg body wt x d(-1) was administered subcutaneously for 4 wk. After treatment, the soleus (SOL), extensor digitorum longus (EDL), and ventricle (VENT) muscles and the femurs (FE) and tibiae (TI) bones were excised and analyzed. The bone mineral content (BMC), area, and bone mineral density (BMD) of FE and TI were measured by dual-energy x-ray absorptiometry (DXA). The longitudinal lengths of bones were measured with the Vernier calipers.

RESULTS

CLE showed smaller body weight than CON (P < 0.05) after the treatment. The muscle wet weights in CLE tended (P = 0.08) to be higher than CON in SOL (9%) and EDL (12%), but the ratio of muscle wet weight-to-body weight were higher (SOL: P < 0.05, EDL: P < 0.01) than CON. VENT of CLE showed increases in both the wet weight and the ratio (P < 0.01). FEs in CLE showed smaller values in BMC (P < 0.01), area (P < 0.01), and length (P < 0.05) than CON but not in BMD. TIs showed significant decreases (P < 0.01) in BMC, area, and length but not in BMD.

CONCLUSION

These results indicated that clenbuterol induced the muscular hypertrophy but inhibited the longitudinal growth of bones in young male rats, which may be a serious concern in any ergogenic use.

Magnetic resonance imaging of denervation-induced muscle atrophy: effects of clenbuterol in the rat

We show that magnetic resonance imaging (MRI) can be used to quantify the amount of muscle in the lower legs of adult rats and to noninvasively monitor the onset and progression of denervation-induced atrophy. Muscle cross-sectional areas determined from 2D gradient-echo MR images allow longitudinal quantification of the protective effects of a beta(2)-adrenergic agonist clenbuterol. We also show that the estimation of clenbuterol's efficacy is improved by computation of the muscle volume. Rapid animal throughput and the ability to accurately estimate efficacy make MRI an attractive technology for studying skeletal muscle atrophy and hypertrophy, allowing the evaluation of potential therapies in longitudinal studies.

Effects of beta(2)-agonist clenbuterol on biochemical and contractile properties of unloaded soleus fibers of rat

The effects of clenbuterol beta(2)-agonist administration were investigated in normal and atrophied [15-day hindlimb-unloaded (HU)] rat soleus muscles. We showed that clenbuterol had a specific effect on muscle tissue, since it reduces soleus atrophy induced by HU. The study of Ca(2+) activation properties of single skinned fibers revealed that clenbuterol partly prevented the decrease in maximal tension after HU, with a preferential effect on fast-twitch fibers. Clenbuterol improved the Ca(2+) sensitivity in slow- and fast-twitch fibers by shifting the tension-pCa relationship toward lower Ca(2+) concentrations, but this effect was more marked after HU than in normal conditions. Whole muscle electrophoresis indicated slow-to-fast transitions of the myosin heavy chain isoforms for unloaded and for clenbuterol-treated soleus. The coupling of the two latter conditions did not, however, increase these phenotypical transformations. Our findings indicated that clenbuterol had an anabolic action and a beta(2)-adrenergic effect on muscle fibers and appeared to counteract some effects of unloading disuse conditions.

Long-term effects of clenbuterol on diaphragm morphology and contractile properties in emphysematous hamsters

The aim of the present study was to investigate the effect of chronic long-term clenbuterol treatment (1 mg/kg subcutaneously twice a day for 12 wk) on diaphragm morphology and function in emphysematous (EH) and normal hamsters (NH). Clenbuterol increased body weight, diaphragm weight, and skeletal muscle weight in both EH and NH to a similar extent. In the diaphragm, clenbuterol significantly increased myosin heavy chain type I, IIa, and IIx muscle fiber cross-sectional areas by approximately 35-55% in both EH and NH. This response to clenbuterol treatment was not significantly different between EH and NH diaphragm. In EH, twitch force (Pt), maximal tetanic force, and force-frequency curve were significantly reduced compared with NH. In EH, clenbuterol increased Pt by approximately 10%, restoring Pt to NH level. A similar improvement was observed in the force-frequency characteristics. Clenbuterol did not alter contractile properties in NH. In conclusion, long-term clenbuterol treatment resulted in an increased size of all diaphragm muscle fiber types in both NH and EH. Clenbuterol completely abolished the reduced force generation induced by emphysema.

Effects of beta 2-agonist administration and exercise on contractile activation of skeletal muscle fibers

Clenbuterol, a beta 2-adrenoceptor agonist, has therapeutic potential for the treatment of muscle-wasting diseases, yet its effects, especially at the single-fiber level, have not been fully characterized. Male C57BL/10 mice were allocated to three groups: Control-Treated mice were administered clenbuterol (2 mg.kg-1. day-1) via their drinking water for 15 wk; Trained-Treated mice underwent low-intensity training (unweighted swimming, 5 days/wk, 1 h/day) in addition to receiving clenbuterol; and Control mice were sedentary and untreated. Contractile characteristics were determined on membrane-permeabilized fibers from the extensor digitorum longus (EDL) and soleus muscles. Fast fibers from the EDL and soleus muscles of Treated mice exhibited decreases in Ca2+ sensitivity. Endurance exercise offset clenbuterol's effects, demonstrated by similar Ca2+ sensitivities in the Trained-Treated and Control groups. Long-term clenbuterol treatment did not affect the normalized maximal tension of fast or slow fibers but increased the proportion of fast fibers in the soleus muscle. Training increased the proportion of fibers with high and intermediate succinate dehydrogenase activity in the EDL and soleus muscles, respectively. If clenbuterol is to be used for treating muscle-wasting disorders, some form of low-intensity exercise might be encouraged such that potentially deleterious slow-to-fast fiber type transformations are minimized. Indeed, in the mouse, low-intensity exercise appears to prevent these effects.

Proteolysis results in altered leak channel kinetics and elevated free calcium in mdx muscle

Resting free calcium levels ([Ca2+]i) are elevated in Duchenne human myotubes and mdx mouse muscle and myotubes which lack the gene product dystrophin at the sarcolemma. Increased net muscle protein degradation has been directly related to this elevated [Ca2+]i. The [Ca2+]i rise may result from increased calcium influx via leak channels, which have increased opening probabilities (Po) in dystrophic cells. Dystrophin, therefore, might directly regulate leak channel activity. In intact mdx soleus muscles, protein degradation was reduced to normal levels by leupeptin, a thiol protease inhibitor. In muscle homogenates, leupeptin also abolished calcium-induced increases in protein degradation. When mouse myotubes were cultured in the continuous presence of leupeptin (50 microM), the elevation in mdx resting [Ca2+]i was prevented. Leak channel Po increased with age in mdx myotubes, whereas leupeptin-treated mdx leak channel opening probabilities were always lower or equal to the Po for untreated normal myotubes. These results indicate that increased leak channel activity in dystrophic muscle results in elevated [Ca2+]i levels, but also suggest that dystrophin does not directly regulate channel activity. Instead the results suggest that proteolysis may be responsible for the altered gating of calcium leak channels. The resultant increased channel Po in turn elevates [Ca2+]i, which further increases proteolytic activity in a positive feedback loop, leading to the eventual necrosis of the muscle fibers.

Postnatal exposure to the beta-adrenoceptor agonist clenbuterol has regionally selective direct and long-term effects on rat brain beta-adrenoceptors and monoamine metabolism

The effects of chronic postnatal beta 2-adrenoceptor activation on the maturation of the rat brain noradrenergic system have been studied. For that purpose, rat pups have been treated twice daily during the first 10 days of life with the beta 2-agonist clenbuterol-HCl (2.5 mg/kg s.c.), and the effects on the beta-adrenoceptor number and monoamine metabolism have been determined directly after the treatment and in adulthood. On postnatal day 10, 90 min after the last clenbuterol injection 4.5 micrograms/g of the drug was present in the brain. At the end of the treatment the beta-receptor binding had decreased in the cerebellum (35%), but not in the frontal cortex or mesolimbic system. Clenbuterol significantly increased the steady-state brain levels of noradrenaline (NA) in the striatum 90 min after the last injection, whereas the levels in the frontal cortex, meso-limbic system, medulla pons and cerebellum were unaffected. The NA metabolite 3-methoxy-4-hydroxyphenylglycol (MHPG), had significantly increased in the frontal cortex and striatum. The serotonergic (5-HT) and dopaminergic (DA) system were not altered. In general, no long-lasting effects on beta-adrenoceptor number and affinity or monoamine metabolism were measurable, except for the frontal cortex which showed a sustained increase of MHPG, a decrease of 5-HT and an increase of 5-HIAA/5-HT on PN 60. In conclusion, chronic postnatal activation of beta 2-adrenoceptors by clenbuterol treatment selectively causes changes in the setting of the neurochemical parameters investigated in the frontal cortex.

Muscle growth and exercise

This paper first reviews muscle growth and then considers the influence of exercise in growth. Knowledge about how muscle cells grow and some factors that may influence the growth pattern are discussed first since these effects must be considered before the influence of exercise becomes clear. Growth of muscle can occur in three ways: (1) by an increase in muscle cell numbers, (2) by an increase in muscle fiber diameter, and (3) by an increase in fiber length. All three of these mechanisms are involved in muscle growth. However, growth in cell numbers is limited to the prenatal and immediately postnatal period, with the animals and man being born with or soon reaching their full complement of muscle cells. Thus, growth occurs by either hypertrophy of the existing muscle fibers by adding additional myofibrils to increase the muscle mass or by adding new sarcomeres to the ends of the existing muscle fibers to increase their length. Both of these mechanisms occur during the growth process. Growth in the girth of the muscle fibers appears to take place by splitting of the myofibrils. This may be stimulated by development of stress creating an unequal pressure with splitting at the Z-band and development of additional SR and T-tubule systems. This adds to the diameter or girth of myofibers without any hyperplasia. The growth in length occurs at either end of the fibers and results in addition of new sarcomeres. In both cases, new myofibrillar protein must be synthesized and deposited in the muscle cells. It is suggested that adaptation by adding or removing sarcomeres is physiologically determined by the degree of force a muscle can generate that is in turn dependent on the degree of overlap of the thick and thin filaments. Thus, the amount of tension would control the number of in-series sarcomeres in a single muscle fiber. Nutrition is also known to play an important role in muscle and was discussed from the standpoint of the effects of nutritional adequacy and restriction. Although a nutritionally balanced and calorically adequate diet is required to achieve optimum muscle growth, it may be less efficient in terms of protein deposition than a moderately restricted diet. Muscle and bone deposition, however, can be limited on severely restricting the dietary intake. Although fat deposition is the first tissue to suffer on a severely restricted diet, muscle and bone follow next with the nervous system, brain and eyes being the last systems to be affected.(ABSTRACT TRUNCATED AT 400 WORDS)

Multiple actions of beta-adrenergic agonists on skeletal muscle and adipose tissue

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