Enhancement of hybrid-fiber types in rat soleus muscle after clenbuterol administration during hindlimb unloading

Icaritin requires Phosphatidylinositol 3 kinase (PI3K)/Akt signaling to counteract skeletal muscle atrophy following mechanical unloading

Counteracting muscle atrophy induced by mechanical unloading/inactivity is of great clinical need and challenge. A therapeutic agent that could counteract muscle atrophy following mechanical unloading in safety is desired. This study showed that natural product Icaritin (ICT) could increase the phosphorylation level of Phosphatidylinositol 3 kinase (PI3K) at p110 catalytic subunit and promote PI3K/Akt signaling markers in C2C12 cells. This study further showed that the high dose ICT treatment could significantly attenuate the decreases in the phosphorylation level of PI3K at p110 catalytic subunit and its downstream markers related to protein synthesis, and inhibit the increases in protein degradation markers at mRNA and protein levels in rat soleus muscle following 28-day hindlimb unloading. In addition, the decreases in soleus muscle mass, muscle fiber cross-sectional area, twitch force, specific force, contraction time and half relaxation time could be significantly attenuated by the high dose ICT treatment. The low dose ICT treatment could moderately attenuate the above changes induced by unloading. Wortmannin, a specific inhibitor of PI3K at p110 catalytic subunit, could abolish the above effects of ICT in vitro and in vivo, indicating that PI3K/Akt signaling could be required by ICT to counteract skeletal muscle atrophy following mechanical unloading.

Effects of co-administration of clenbuterol and testosterone propionate on skeletal muscle in paraplegic mice

Spinal cord injury (SCI) is generally associated with a rapid and significant decrease in muscle mass and corresponding changes in skeletal muscle properties. Although beta(2)-adrenergic and androgen receptor agonists are anabolic substances clearly shown to prevent or reverse muscle wasting in some pathological conditions, their effects in SCI patients remain largely unknown. Here we studied the effects of clenbuterol and testosterone propionate administered separately or in combination on skeletal muscle properties and adipose tissue in adult CD1 mice spinal-cord-transected (Tx) at the low-thoracic level (i.e., induced complete paraplegia). Administered shortly post-Tx, these substances were found to differentially reduce loss in body weight, muscle mass, and muscle fiber cross-sectional area (CSA) values. Although all three treatments induced significant effects, testosterone-treated animals were generally less protected against Tx-related changes. However, none of the treatments prevented fat tissue loss or muscle fiber type conversion and functional loss generally found in Tx animals. These results provide evidence suggesting that clenbuterol alone or combined with testosterone may constitute better clinically-relevant treatments than testosterone alone to decrease muscle atrophy (mass and fiber CSA) in SCI subjects.

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.

Rat hindlimb unloading down-regulates insulin like growth factor-1 signaling and AMP-activated protein kinase, and leads to severe atrophy of the soleus muscle

Inactivity is known to induce muscle atrophy, which is associated with insulin and insulin-like growth factor-1 (IGF-1) resistance, but the associated mechanisms remain poorly defined. The hindlimb unloading model has been used to reduce muscle activity. The objective of this study was to show the effect of hindlimb unloading on IGF-1 signaling and AMP-activated protein kinase (AMPK) activity in rat soleus and extensor digitorum longus (EDL) muscles. Twelve 7-week-old male Sprague–Dawley rats were assigned to 2 treatments: (i) rats without hindlimb unloading (Con) and (ii) rats with hindlimb unloading (Unload). After 2 weeks of treatment, the soleus and EDL muscles were dissected and used for biochemical analyses. Hindlimb unloading induced severe muscle atrophy in soleus muscle (0.122 ± 0.007 g for Con vs. 0.031 ± 0.004 g for Unload, p < 0.01), but only slight atrophy in EDL muscle. The phosphorylation of AMPK (p < 0.05) and its downstream substrate, acetyl-CoA carboxylase (ACC) (p < 0.01) were reduced in soleus muscle due to unloading. The concentration of insulin receptor substrate-1 (IRS-1) and phosphorylation of IRS-1 at Ser636–639and Ser789were also reduced. Downstream IGF-1 signaling was downregulated in Unload rats. A reduction in IGF-1 concentration in unloaded soleus muscle was also observed. A slight reduction in AMPK activity and IGF-1 signaling were observed in EDL muscle. Since AMPK controls the sensitivity of IGF-1 signaling through phosphorylation at Ser789, the reduction in AMPK activity is expected to reduce the response of downstream IGF-1 signaling to IGF-1; this, in combination with reduced IGF-1 concentration, might be responsible for the severe muscle atrophy observed in unloaded soleus muscle.