Mechanical and signaling responses of unloaded rat soleus muscle to chronically elevated ß-myosin activity.

It has been reported that muscle functional unloading is accompanied by an increase in motoneuronal excitability despite the elimination of afferent input. Thus, we hypothesized that pharmacological potentiation of spontaneous contractile soleus muscle activity during hindlimb unloading could activate anabolic signaling pathways and prevent the loss of muscle mass and strength. To investigate these aspects and underlying molecular mechanisms, we used ß-myosin allosteric effector Omecamtiv Mekarbil (OM). We found that OM partially prevented the loss of isometric strength and intrinsic stiffness of the soleus muscle after two weeks of disuse. Notably, OM was able to attenuate the unloading-induced decrease in the rate of muscle protein synthesis (MPS). At the same time, the use of drug neither prevented the reduction in the markers of translational capacity (18S and 28S rRNA) nor activation of the ubiquitin-proteosomal system, which is evidenced by a decrease in the cross-sectional area of fast and slow muscle fibers. These results suggest that chemically-induced increase in low-intensity spontaneous contractions of the soleus muscle during functional unloading creates prerequisites for protein synthesis. At the same time, it should be assumed that the use of OM is advisable with pharmacological drugs that inhibit the expression of ubiquitin ligases.

Β-GPA administration activates slow oxidative muscle signaling pathways and protects soleus muscle against the increased fatigue under 7-days of rat hindlimb suspension.

Unloading of slow-twitch muscles results in increased muscle fatigue and the mechanisms of this effect are poorly studied. We aimed to analyze the role of high-energy phosphates accumulation during the first week of rat hindlimb suspension plays in a fiber-type phenotype shift towards fast-type fatigable muscle fibers. Male Wistar rats were divided into 3 groups (n = 8): C - vivarium control; 7HS - 7-day hindlimb suspension; 7HB - 7-day hindlimb suspension with intraperitoneal injection of beta-guanidine propionic acid (ß-GPA, 400 mg/kg b w). ß-GPA is a competitive inhibitor of creatine kinase and it reduces concentrations of ATP and phosphocreatine. In the 7HB group, ß-GPA treatment protected a slow-type signaling network in an unloaded soleus muscle, including MOTS-C, AMPK, PGC1 α and micro-RNA-499. These signaling effects resulted in a preserved soleus muscle fatigue resistance, slow-type muscle fibers percentage and mitochondrial DNA copy number under muscle unloading.

Early Deсline in Rat Soleus Passive Tension with Hindlimb Unloading: Inactivation of Cross-bridges or Activation of Calpains?

The study was aimed at testing the hypotheses about the role of cross-bridges and calpains in reduction of rat soleus passive tension under conditions of hindlimb unloading. For this purpose, we used an inhibitor of µ-calpain PD 150606 as well as a blocker of actomyosin interaction (blebbistatin). It was found for the first time that a decrease in passive tension of rat soleus after 3-day hindlimb unloading is associated with the activity of µ-calpain and does not depend on the processes of cross-bridges formation.

[SIGNALING PATHWAYS REGULATING PROTEIN SYNTHESIS IN RAT SOLEUS MUSCLE DURING EARLY STAGES OF HINDLIMB UNLOADING].

The impact of mechanical unloading (hindlimb suspension for 1-, 3- and 7 days) on the protein synthesis intensity and as well as intracellular signaling pathways controlling mRNA translation was investigated. The content of the key signaling molecules of the various anabolic signaling pathways was determined by Western-blotting. The rate of protein synthesis was assessed using in vivo SUnSET technique. Hindlimb suspension (HS) within 24 hours resulted in a significant (p < 0.05) decrease in p-4E-BP 1 content and increase in p-p70s6k content in soleus muscle. Follo- wing three days of HS the content of p-AKT and p-90RSK1 was decreased (p <0.05). After 7 days of HS phosphorylation level of AKT and GSK-3beta was significantly reduced (p < 0.05) compared to the control group. We also observed a significant decrease in the amount of 28S rRNA in soleus following 3 and 7 days of HS. The rate of protein synthesis after 3 and 7 days of HS was decreased (p < 0.05) in comparison with the basal level. Our data suggest that the decrease in the protein synthesis rate in rat soleus during early stages of simulated microgravity could be associated with the impaired ribosome biogenesis as well as reduced activity of the mTORC1-independent signaling pathways.