The effects of beta agonists on muscle cells in culture

Isoproterenol induces an increase in muscle fiber size by the proliferation of Pax7-positive cells and in a mTOR-independent mechanism

β-Adrenergic signaling regulates many physiological processes in skeletal muscles. A wealth of evidence has shown that β-agonists can increase skeletal muscle mass in vertebrates. Nevertheless, to date, the specific role of β-adrenergic receptors in different cell phenotypes (myoblasts, fibroblasts, and myotubes) and during the different steps of embryonic skeletal muscle differentiation has not been studied. Therefore, here we address this question through the analysis of embryonic chick primary cultures of skeletal muscle cells during the formation of multinucleated myotubes. We used isoproterenol (ISO), a β-adrenergic receptor agonist, to activate the β-adrenergic signaling and quantified several aspects of muscle differentiation. ISO induced an increase in myoblast proliferation, in the percentage of Pax7-positive myoblasts and in the size of skeletal muscle fibers, suggesting that ISO activates a hyperplasic and hypertrophic muscle response. Interestingly, treatment with ISO did not alter the number of fibroblast cells, suggesting that ISO effects are specific to muscle cells in the case of chick myogenic cell culture. We also show that rapamycin, an inhibitor of the mammalian target of rapamycin signaling pathway, did not prevent the effects of ISO on chick muscle fiber size. The collection of these results provides new insights into the role of β-adrenergic signaling during skeletal muscle proliferation and differentiation and specifically in the regulation of skeletal muscle hyperplasia and hypertrophy.

Effects of salmeterol on skeletal muscle cells: metabolic and proapoptotic features

PURPOSE

Salmeterol is a β2-adrenergic receptor agonist widely used for the treatment of asthma and chronic obstructive pulmonary disease. It has been shown that salmeterol is also used at supratherapeutic doses as performance-enhancing substance in sport practice. Although the abuse of β-agonists might determine some adverse effects, the molecular effects of salmeterol on skeletal muscle cells remain unclear.

METHODS

We evaluated the effects of salmeterol (0.1-10 μM) on both proliferative and differentiated rat L6C5 and mouse C2C12 skeletal muscle cell lines. The metabolic effects were evaluated by glyceraldehyde phosphate dehydrogenase, lactate dehydrogenase, citrate synthase, 3-OH acyl-CoA dehydrogenase, and alanine transglutaminase activities. Cytotoxic and apoptotic effects were analyzed by 3-(4,5-dimethylthiazol-1)-5-(3-carboxymeth-oxyphenyl)-2H-tetrazolium, trypan blue exclusion assay, terminal deoxynucleotidyl transferase dUTP nick end labeling assay, Western blot analysis, and immunofluorescence staining.

RESULTS

We showed that salmeterol reduced the growth rate of proliferating cells in a dose- and time-dependent manner (6-48 h). An increase in oxidative metabolism was found after 6 h in C2C12 and L6C5 myoblasts and in C2C12 myotubes with respect to control cells, while in L6C5 myotubes, anaerobic metabolism prevailed. Exposure of myoblasts and myotubes for 48 and 72 h at high salmeterol concentrations induced apoptosis by the activation of the intrinsic apoptotic pathway, as confirmed by the modulation of the apoptotic proteins Bcl-xL, caspase-9, and poly (ADP-ribose) polymerase and by the cytoplasmic release of Smac/DIABLO.

CONCLUSIONS

Altogether, our results demonstrate that short-term supratherapeutic salmeterol exposure increased oxidative metabolic pathways on skeletal muscle cells, whereas prolonged treatment inhibits cell growth and exerts either a cytostatic or a proapoptotic effect in a time- and dose-dependent way.

Cardiac cachexia: a systematic overview

Cardiac cachexia as a terminal stage of chronic heart failure carries a poor prognosis. The definition of this clinical syndrome has been a matter of debate in recent years. This review describes the ongoing discussion about this issue and the complex pathophysiology of cardiac cachexia and chronic heart failure with particular focus on immunological, metabolic, and hormonal aspects at the intracellular and extracellular level. These include regulators such as neuropeptide Y, leptin, melanocortins, ghrelin, growth hormone, and insulin. The regulation of feeding is discussed as are nutritional aspects in the treatment of the disease. The mechanisms of wasting in different body compartments are described. Moreover, we discuss several therapeutic approaches. These include appetite stimulants like megestrol acetate, medroxyprogesterone acetate, and cannabinoids. Other drug classes of interest comprise angiotensin-converting enzyme inhibitors, beta-blockers, anabolic steroids, beta-adrenergic agonists, anti-inflammatory substances, statins, thalidomide, proteasome inhibitors, and pentoxifylline.

[Effect of a beta-agonist and a beta-agonist/beta-antagonist combination on muscle growth, body composition and protein metabolism in rats]

In 3 experimental groups 9 female Wistar rats (initial live weight 150 g) were fed either the control diet, the control diet supplemented with 5 mg clenbuterol or the combination of 5 mg clenbuterol and 500 mg propranolol per kg diet over a 12-day period. The N-balance was estimated over days 6 to 10 followed by a 15N-tracer experiment for determining the influence of the feed additives on characteristics of protein metabolism on day 12. All differences in the means were concluded to be significant for P < 0.05. Live weight gain and feed efficiency were improved by clenbuterol. The animals treated with clenbuterol had 18%-24% higher muscle weights whereas the combined treatment increased the muscle weights by 10%-16% only. The good correlation between the increase of muscle weights and the total protein content indicates that clenbuterol does not change the relation between protein- and water accumulation. Histological-histochemical investigations showed that the higher muscle weights were achieved through muscle fibre hypertrophy. The number of muscle fibres remained constant. Concerning the distribution of the fibre types, clenbuterol increased the proportion of FTG-fibres (white, fast-twitch, glycolytic) on the expense of the FTO-fibres (fast-twitch, oxidative). While the number of nuclei per muscle fibre did not change, the nucleus-cytoplasm relation decreased by 24%. Compared to the animals fed the control diet, the N-balance in the clenbuterol-treated group was increased by 41%. Feeding the combination of clenbuterol and propranolol resulted in an increase of 24% only. Clenbuterol increased the N-content of the carcass by 6% and reduced the carcass fat content by 30%. In the group fed clenbuterol and propranolol, the N-content of the carcass only tended to be increased and the influence on carcass fat reduction was only 16%. The stimulated N-deposition in the carcass of clenbuterol-treated rats was obtained, since the calculated protein degradation rates were more reduced than protein synthesis rates. In-vitro investigations of the muscle protein synthesis and -protease activities support these results. The clenbuterol-induced increase in muscle protein was accompanied by an inhibition of the Ca-dependent protease activity and an increase of muscle DNA- and RNA-content. The additional application of propranolol reduced these effects of clenbuterol again. Since propranolol partly prevented the effects of clenbuterol on protein metabolism it is suggested that not only the lipolytic but also protein anabolic effects are caused by the beta-adrenergic action of clenbuterol.