Plantar Mechanical Stimulation Maintains Slow Myosin Expression in Disused Rat Soleus Muscle via NO-Dependent Signaling

Abstract

It was observed that gravitational unloading during space missions and simulated microgravity in ground-based studies leads to both transformation of slow-twitch muscle fibers into fast-twitch fibers and to the elimination of support afferentation, leading to the “switching-off” of postural muscle motor units electrical activity. In recent years, plantar mechanical stimulation (PMS) has been found to maintain the neuromuscular activity of the hindlimb muscles. Nitric oxide (NO) was shown to be one of the mediators of muscle fiber activity, which can also promote slow-type myosin expression. We hypothesized that applying PMS during rat hindlimb unloading would lead to NO production upregulation and prevention of the unloading-induced slow-to-fast fiber-type shift in rat soleus muscles. To test this hypothesis, Wistar rats were hindlimb suspended and subjected to daily PMS, and one group of PMS-subjected animals was also treated with nitric oxide synthase inhibitor (L-NAME). We discovered that PMS led to sustained NO level in soleus muscles of the suspended animals, and NOS inhibitor administration blocked this effect, as well as the positive effects of PMS on myosin I and IIa mRNA transcription and slow-to-fast fiber-type ratio during rat hindlimb unloading. The results of the study indicate that NOS activity is necessary for the PMS-mediated prevention of slow-to-fast fiber-type shift and myosin I and IIa mRNA transcription decreases during rat hindlimb unloading.

The Role of GSK-3β in the Regulation of Protein Turnover, Myosin Phenotype, and Oxidative Capacity in Skeletal Muscle under Disuse Conditions

“…Preliminary evidences suggest that PKD1 and PKA do not contribute to a HU-induced decrease in GSK-3β Ser9 phosphorylation, at least within the first 24h of unloading, since the activity of these kinases is upregulated in rat soleus during an acute stage of HU [ 64 ] (Vilchinskaya et al, unpublished work). It is also possible that a reduction in GSK-3β Ser9 phosphorylation during disuse conditions could be associated with NO-dependent (cGMP)/PKG signaling pathway, since HU promotes a significant decrease in NO content in rat soleus muscle [ 65 , 66 ].…”

“…Moreover, immunohistochemical analysis showed that application of PMS during 7-day unloading prevented slow-to-fast fiber-type transition in rat soleus. All these effects of PMS were completely abrogated in the group of animals in which PMS sessions were accompanied by administration of NOS inhibitor L-NAME [ 66 ]. Direct inhibition of GSK-3β activity during 7-day HS as well as NO donor L-arginine administration also led to NFATc1 nuclear accumulation and prevention of a decrease in MyHC I mRNA expression in rat soleus muscle [ 113 ].…”

“…Thus, it was shown that NO may serve as a key regulator of activity-dependent GSK-3β inhibition, which is critical for NFAT/MyHCI signaling in rat soleus muscle under mechanical unloading. Both L-arginine administration and PMS during 7-day HS partially prevented downregulation of MyHC IIa mRNA transcription [ 66 , 114 ]. Previously it was shown that GSK-3β inhibition is able to upregulate the expression of slow myosin in chicken skeletal muscle [ 115 ] and MyHC IIa in goat satellite cells [ 116 ], however it is unclear why 3 days of PMS during unloading led to GSK-3β inhibition but did not activate MyHC IIa transcription in rat soleus muscle.…”

“…Transcription of myh7b mRNA (as well as MyHC I) is significantly reduced in rat soleus muscles after 24 h of HS [ 121 ] and remains decreased after 3 and 7-day HS. SOX6 mRNA elevates after the 3rd day of HS and remains increased after 7-day unloading [ 66 , 122 ]. It was shown that overexpression of microRNA-499 in C2C12 myotubes leads to an increased expression of the slow isoform of troponin (TNNI1), myoglobin, and PGC-1α, thus this microRNA plays an important role in the realization of the oxidative phenotype of muscle fiber (Xu, Chen et al 2018).…”

NOS‐dependent effects of plantar mechanical stimulation on mechanical characteristics and cytoskeletal proteins in rat soleus muscle during hindlimb suspension

“…Animals from P and LN groups received PMS as earlier described. [40][41][42] In brief, each sole was stimulated with a frequency of 1-s inflation/1-s deflation with a total of 20 min followed by 10 min rest. This cycle was repeated eight times within 4 h each day of HS.…”