The depolarization of the sarcolemma is one of the first effects of unloading on skeletal muscle. We hypothesized that unloading-induced activation of the dihydropyridine receptor (DHPR), a voltage-sensitive L-type Ca2+ channel, and depolarization of the sarcolemma trigger intracellular Ca2+ release from the sarcoplasmic reticulum and activation of Ca2+-dependent signaling pathways, resulting in muscle atrophy. Nifedipine, a DHPR calcium channel blocker, was used to study the role of DHPR in the regulation of signaling pathways during three days of rat soleus muscle unloading/hindlimb suspension. Inhibition of the DHPR during unloading attenuates the decrease in soleus muscle contractile properties, prevents the accumulation of ATP, ROS, and Ca2+ content in the sarcoplasm and the mitochondria, and blocks the decrease in PGC1alpha mRNA expression and Junctophilin-1 (JP1) proteolysis. In nifedipine-treated rats, the improvement of the unloaded soleus muscle contractile properties could be mediated by blocking the calpain-mediated degradation of the cytoskeletal proteins. DHPR blocking could be one of the future directions for the preservation of contractile properties of inactive/unloaded muscle.

Aim: Skeletal muscle unloading leads to the upregulation of proteolytic gene expression, downregulation of protein synthesis markers, and the development of muscle atrophy. These changes are accompanied by alterations in calcium signaling. We investigated the role of Inositol 1,4,5-triphosphate (IP3) receptors (IP3Rs) in regulating calcium signaling and controlling gene expression in skeletal muscles during unloading. Methods: Male Wistar rats were randomly assigned to 4 groups (n = 8 per group). C-vivarium control; C+2APB-vivarium control with daily intraperitoneal injections of the IP3 receptor inhibitor 2-aminoethoxydiphenyl borate (2-APB, 10 mg/ kg b.w.); HU-3-day hind limb unloading; HU+2APB-3-day hind limb unloading with daily 2-APB injections. After the intervention, soleus muscles were analyzed for markers of calcium metabolism and proteostasis. Results: Three days of unloading resulted in a significant increase in nuclear phosphorylated Ca2+/calmodulin-dependent protein kinase II (p-CaMK II) content and calcineurin (CaN) expression compared to the C group (P < 0.05); this effect was prevented in the HU+2APB group. In HU+2APB rats, the decline in soleus muscle weight-to-body weight ratio was partially prevented, and the downregulation of protein synthesis markers (as seen in the HU group) was also prevented. However, proteolytic signaling markers were equally upregulated in the HU+2APB and HU groups compared to C. Conclusion: IP3 receptor inhibition during 3-day hind limb suspension in rats partially prevented the decline in m. soleus weight index and protein synthesis markers. This effect may be attributed to alterations in the regulation of nuclear calcium signaling.

Skeletal muscle unloading results in muscle atrophy associated with the upregulation of proteolytic genes and suppression of protein synthesis, often accompanied by altered calcium signaling. Here, we used the inositol trisphosphate receptor (IP3R) inhibitor aminoethoxydiphenyl borate (2-APB) to explore the hypothesis that these changes are mediated by IP3Rs. Male Wistar rats were divided into 4 groups: (i) control, (ii) control with daily injections of 2-APB, (iii) 3 days of hind limb suspension, (iv) 3 days of hind limb suspension with daily administration of 2-APB. At the end-point, soleus muscles from the animals were analyzed by Western blotting for the markers of calcium, anabolic, and catabolic signaling. The 3-day hind limb unloading resulted in a decreased muscle weight index, upregulation of the anabolic suppressor pThr56-eEF2, downregulation of anabolic signaling via the mTOR pathway and rRNA expression, as well as the increase in the content of nuclear pThr286-CaMKII (p < 0.05) and cytosolic calcineurin A. While 2-APB did not affect the mTOR-governed changes in anabolism and catabolism, it significantly attenuated alterations in the calcium-dependent targets, such as CaMKII, calcineurin, and eEF2. By contrast, proteolytic signaling (expression of MuRF1, atrogin-1, Ulk1, and ubiquitin mRNAs) after 3-day hind limb unloading was equally upregulated in the control and 2-APB-treated animals. These results suggest that IP3Rs are involved in the unloading-induced muscle atrophy by controlling the nuclear content of calcium; however, they are dispensable for reduced mTOR activity and altered metabolism.

During skeletal muscle unloading, phosphoinositide 3-kinase (PI3K), and especially PI3K gamma (PI3Kγ), can be activated by changes in membrane potential. Activated IP3 can increase the ability of Ca2+ to enter the nucleus through IP3 receptors. This may contribute to the activation of transcription factors that initiate muscle atrophy processes. LY294002 inhibitor was used to study the role of PI3K in the ATP-dependent regulation of skeletal muscle signaling during three days of unloading. Inhibition of PI3K during soleus muscle unloading slows down the atrophic processes and prevents the accumulation of ATP and the expression of the E3 ubiquitin ligase MuRF1 and ubiquitin. It also prevents the increase in the expression of IP3 receptors and regulates the activity of Ca2+-dependent signaling pathways by reducing the mRNA expression of the Ca2+-dependent marker calcineurin (CaN) and decreasing the phosphorylation of CaMKII. It also affects the regulation of markers of anabolic signaling in unloaded muscles: IRS1 and 4E-BP. PI3K is an important mediator of skeletal muscle atrophy during unloading. Developing strategies for the localized skeletal muscle release of PI3K inhibitors might be one of the future treatments for inactivity and disease-induced muscle atrophy.

The imbalance in the ratio of protein synthesis versus protein degradation results in skeletal muscle atrophy following unloading. The onset of these processes is regulated by the sarcoplasmic concentrations of ATP and calcium (Ca2+). We tested the hypothesis that unloading-induced inactivation of sarcoendoplasmic reticulum calcium ATPase (SERCA) results in raised Ca2+ concentrations, triggering catabolic processes. CDN1163, an activator of SERCA, was used to test this hypothesis. Three groups of male rats were used: control rats with intraperitoneal injection of placebo (C), 3 days of unloading with placebo injection (3HS), and 3 days of unloading injected with CDN1163 (3HSC). Treatment with CDN1163 during 3 days of soleus muscle unloading prevented the upregulation of Ca2+ and ATP and the slow-to-fast shift in muscle fiber composition. This treatment blocked the decrease in the phosphorylation of the anabolic markers [glycogen synthase kinase-3β (GSK3β), eukaryotic translation elongation factor 2 (eEF2), and ribosomal protein S6 (S6, Ser240/244/Ser235/236)], and therefore it is likely that it improved the efficiency of translation in the unloaded muscle, but it did not affect mTORC1-dependent signaling. Treatment with CDN1163 also modulated the regulation of the Ca2+-dependent signaling in muscle during unloading via SERCA1 and calsequestrin 2 (CSQ2) and changes in the calcium/calmodulin-dependent protein kinase II (CaMKII) phosphorylation and the content of inositol 1,4,5-trisphosphate receptor (IP3R). In addition, CDN1163 prevented the upregulation of the mRNA expression of muscle-specific RING finger protein 1 (MuRF1) [but not ubiquitin ligase muscle atrophy F-box (MAFbx)] and attenuated the increase of casitas B lymphoma-b (Cbl-b) and ubiquitin mRNA expression during unloading. Activation of SERCA with CDN1163 prevents the upregulation of Ca2+ and ATP, as well as calcium-dependent and ubiquitin-proteasome pathways markers, and improves protein translation efficiency in 3-day unloaded soleus muscle.NEW & NOTEWORTHY A hypothesis was tested that unloading-induced inactivation of SERCA results in the accumulation of increased Ca2+ concentrations and activation of catabolic processes. CDN1163, an activator of SERCA, was used to test this hypothesis. CDN1163 prevented the decrease in phosphorylation of anabolic markers, which likely improved translation efficiency in unloaded muscle. CDN1163 prevented unloading-induced upregulation of mRNA expression of MuRF1 (but not MAFbx) and attenuated the increase of Cbl-b and ubiquitin mRNA expression.

Phosphoinositide-3-kinase (PI3K) (PI3Kγ, specifically) can be activated by a change in the membrane potential due to muscle functional unloading; as a result, IP3 enhances Ca2+ entry in the nucleus via IS3R. This benefits activation of the transcriptional factors that trigger atrophy. Inhibitor LY294002 was used to explore the PI3K role in ATP-dependent regulation of signaling in rat's muscles after 3-day suspension. PI3K inhibition after functional unloading slows down m. soleus atrophy, prevents ATP deposition, and expression of Е3-ubuquitine ligase MuRF1 and ubuquitine, prevents rise in expression of IP3-receptors, regulates the activity of Ca-dependent signaling pathways by reducing expression of Ca-dependent мРНК CaN markers, and phosphorylation of CaMKII (Са/calmodulin kinase II); the inhibitor modulates also regulation of the anabolic signaling markers IRS1 and 4E-BP in unloaded muscles.

Membrane IP3 receptors (IP3Rs) abound in the sarcoplasmic reticulum, nucleus and mitochondria of muscle fibers. We hypothesized that IP3R activation during muscle unloading may elicit a weak Ca2+ release signal, both cytosolic and nucleoplasmic, which promotes (perhaps in cooperation with other signaling cascades) the activation of transcription factors and thus leads to the expression or repression of genes associated with muscle phenotypes. Here, we tested this hypothesis by blocking IP3Rs with 2-APB (2-aminoethoxydiphenyl borate, 10 mg/kg in 5 of soleus muscle unloading. The blocking of IP3Rs prevented a decrease in the cross-sectional area of both slow and fast soleus muscle fibers and thus slowed down the development of atrophic processes in this postural muscle during 7-day hindlimb suspension. Such an effect of blocking IP3Rs during rat soleus muscle unloading may be due to preventing a decrease in ribosomal biogenesis and an increase in the expression of autophagy markers ULK-1 and IL-6.

The current study aimed to investigate the hypothesis that purinergic receptors P2Y1 and P2Y2 play a regulatory role in gene expression in unloaded muscle. ATP is released from cells through pannexin channels, and it interacts with P2Y1 and P2Y2 receptors, leading to the activation of markers of protein catabolism and a reduction in protein synthesis. To test this hypothesis thirty-two rats were randomly divided into four groups (8 per group): a non-treated control group (C), a group subjected to three days of hindlimb unloading with a placebo (HS), a group subjected to three days of hindlimb unloading treated with a P2Y1 receptor inhibitor, MRS2179 (HSM), and a group subjected to three days of hindlimb unloading treated with a P2Y2 receptor inhibitor, AR-C 118925XX (HSA). This study revealed several key findings following three days of soleus muscle unloading: 1: Inhibition of P2Y1 or P2Y2 receptors prevented the accumulation of ATP, the increase in IP3 receptor content, and the decrease in the phosphorylation of GSK-3beta. This inhibition also mitigated the reduction in the rate of protein synthesis. However, it had no significant effect on the markers of mTORC1-dependent signaling. 2: Blocking P2Y1 receptors prevented the unloading-induced upregulation of phosphorylated p38MAPK and partially reduced the increase in MuRF1mRNA expression. 3: Blocking P2Y2 receptors prevented muscle atrophy during unloading, partially maintained the levels of phosphorylated ERK1/2, reduced the increase in mRNA expression of MAFbx, ubiquitin, and IL-6 receptor, prevented the decrease in phosphorylated AMPK, and attenuated the increase in phosphorylated p70S6K. Taken together, these results suggest that the prevention of muscle atrophy during unloading, as achieved by the P2Y2 receptor inhibitor, is likely mediated through a reduction in catabolic processes and maintenance of energy homeostasis. In contrast, the P2Y1 receptor appears to play a relatively minor role in muscle atrophy during unloading.

IP3 receptors are found in significant quantities in muscle fibers in the sarcoplasmic reticulum, nucleus and mitochondria. We hypothesized that activation of IP3 receptors (IP3Rs) during muscle unloading may induce a weak calcium release signal, both cytosolic and nucleoplasmic, that promotes (possibly with other signaling cascades) the activation of transcription factors, leading to the expression or repression of genes involved in muscle phenotype. This hypothesis was tested by blocking IP3R during unloading of rat muscles by administering 2-APB (2-aminoethoxydiphenyl borate). Wistar rats were administered intraperitoneally at a dose of 10 mg/mg in 5 % DMSO daily. We found that the IP3R state influences the development of atrophic processes in the postural m. soleus during unloading. Administration of the IP3R blocker 2-APB to animals successfully prevented a decrease in m. soleus cross-sectional area (CSA) of both fast and slow muscle fibers. The slowdown in CSA decrease upon administration IP3R inhibitor during 7 days m. soleus unloading is associated with the prevention of a decrease in ribosomal biogenesis and an increase in the expression of autophagy markers ULK-1 and IL-6.

Dysfunction of skeletal muscles and their atrophy during unloading are accompanied by excess calcium accumulation in the myoplasm of muscle fibers. We hypothesized that calcium accumulation may occur, among other reasons, due to inhibition of SERCA activity under muscle unloading. In this case, the use of a SERCA activator will reduce the calcium level in the myoplasm and prevent the consequences of unloading. Male Wistar rats were divided into 3 groups: vivarium control with placebo administration (C, n = 8), 7-day suspension group with placebo administration (7HS, n = 8) and 7-day suspension group with intraperitoneal administration of SERCA CDN1163 activator (50 mg/kg (7HS + CDN), n = 8). One m. soleus of each rat was frozen in liquid nitrogen, the second was tested for functional properties. In the 7HS group, increased soleus fatigue was found in the ex vivo test, a significant increase in mRNA and the number of fast muscle fibers, an increase in the level of calcium-dependent CaMK II phosphorylation and the level of tropomyosin oxidation, as well as a decrease in the content of mitochondrial DNA and protein. All these changes were prevented in the SERCA CDN1163 activator group. Conclusion: 7-day SERCA activator administration does not delay of soleus atrophy, but prevents the development of its fatigue, probably by preventing a decrease in the number of type I fibers and markers of mitochondrial biogenesis.

Skeletal muscle abnormalities and atrophy during unloading are accompanied by the accumulation of excess calcium in the sarcoplasm. We hypothesized that calcium accumulation may occur, among other mechanisms, due to the inhibition of sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activity. Consequently, the use of the SERCA activator will reduce the level of calcium in the sarcoplasm and prevent the negative consequences of muscle unloading. Wistar rats were randomly assigned into one of three groups (eight rats per group): control rats with placebo (C), 7 days of unloading/hindlimb suspension with placebo (7HS), and 7 days of unloading treated with SERCA activator CDN1163 (7HSC). After seven days of unloading the soleus muscle, the 7HS group displayed increased fatigue in the ex vivo test, a significant increase in the level of calcium-dependent CaMK II phosphorylation and the level of tropomyosin oxidation, as well as a decrease in the content of mitochondrial DNA and protein, slow-type myosin mRNA, and the percentage of slow-type muscle fibers. All of these changes were prevented in the 7HSC group. Moreover, treatment with CDN1163 blocked a decrease in the phosphorylation of p70S6k, an increase in eEF2 phosphorylation, and an increase in MuRF-1 mRNA expression. Nevertheless, there were no differences in the degree of fast and slow muscle fiber atrophy between the 7HS and 7HSC groups. Conclusion: SERCA activation during 7 days of unloading prevented an increase in soleus fatigue, the decrease of slow-type myosin, mitochondrial markers, and markers of calcium homeostasis but had no effect on muscle atrophy.

Muscle unloading leads to signaling alterations that cause muscle atrophy and weakness. The cellular energy sensor AMPK can regulate myofiber-type shift, calcium-dependent signaling and ubiquitin-proteasome system markers. We hypothesized that the prevention of p-AMPK downregulation during the first week of muscle unloading would impede atrophy development and the slow-to-fast shift of soleus muscle fibers, and the aim of the study was to test this hypothesis. Thirty-two male Wistar rats were randomly assigned to four groups: placebo control (C), control rats treated with metformin (C + M), 7 days of hindlimb suspension (HS) + placebo (7HS), and 7 days of HS + metformin administration (7HS + M). In the soleus of the 7HS rats, we detected a slow-to-fast fiber-type shift as well as a significant downregulation of MEF-2D and p300 in the nuclei. In the 7HS group, we also found decreases in p-ACC (AMPK target) protein level and in the expression of E3 ubiquitin ligases and p-CaMK II protein level vs. the C group. The 7-day metformin treatment for soleus muscle unloading (1) prevented slow-to-fast fiber-type shift; (2) counteracted changes in the p-ACC protein level; (3) hindered changes in the nuclear protein level of the slow myosin expression activators MEF-2D and p300, but did not affect NFATc1 signaling; and (4) attenuated the unloading-induced upregulation of MuRF-1, atrogin-1, ubiquitin and myostatin mRNA expression, but did not prevent soleus muscle atrophy. Thus, metformin treatment during muscle disuse could be useful to prevent the decrease in the percentage of slow-type fatigue-resistant muscle fibers.

Dysfunction of skeletal muscles and their atrophy under unloading are accompanied by excess calcium accumulation in the myoplasm of muscle fibers. We hypothesized that calcium accumulation may occur, among other reasons, due to inhibition of SERCA activity under muscle unloading. In this case, the use of a sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) activator would reduce the calcium level in the myoplasm and prevent the consequences of unloading. Male Wistar rats were divided into 3 groups: vivarium control with placebo administration (C, n = 8), 7-day suspension group with placebo administration (7HS, n = 8), and 7-day suspension group with intraperitoneal administration of the SERCA activator CDN1163 (7HS + CDN, 50 mg/kg, n = 8). One m. soleus of each rat was frozen in liquid nitrogen, the second was tested for functional properties. 7HS rats demonstrated an increased m. soleus fatigue in the ex vivo test, a significant increase in mRNA and the number of fast-twitch muscle fibers, an increase in the level of calcium-dependent CaMK II phosphorylation and tropomyosin oxidation, as well as a decrease in mitochondrial DNA and protein levels. All these changes were prevented in the 7HS + CDN group administered with SERCA activator. Conclusion: 7-day SERCA activator administration does not delay m. soleus atrophy but prevents the development of its fatigue, probably due to prevention of a reduction in the number of type I fibers and mitochondrial biogenesis markers.

Disuse muscle atrophy is usually accompanied by changes in skeletal muscle structure, signaling, and contractile potential. Different models of muscle unloading can provide valuable information, but the protocols of experiments with complete immobilization are not physiologically representative of a sedentary lifestyle, which is highly prevalent among humans now. In the current study, we investigated the potential effects of restricted activity on the mechanical characteristics of rat postural (soleus) and locomotor (extensor digitorum longus, EDL) muscles. The restricted-activity rats were kept in small Plexiglas cages (17.0 × 9.6 × 13.0 cm) for 7 and 21 days. After this, soleus and EDL muscles were collected for ex vivo mechanical measurements and biochemical analysis. We demonstrated that while a 21-day movement restriction affected the weight of both muscles, in soleus muscle we observed a greater decrease. The maximum isometric force and passive tension in both muscles also significantly changed after 21 days of movement restriction, along with a decrease in the level of collagen 1 and 3 mRNA expression. Furthermore, the collagen content itself changed only in soleus after 7 and 21 days of movement restriction. With regard to cytoskeletal proteins, in our experiment we observed a significant decrease in telethonin in soleus, and a similar decrease in desmin and telethonin in EDL. We also observed a shift towards fast-type myosin heavy chain expression in soleus, but not in EDL. In summary, in this study we showed that movement restriction leads to profound specific changes in the mechanical properties of fast and slow skeletal muscles. Future studies may include evaluation of signaling mechanisms regulating the synthesis, degradation, and mRNA expression of the extracellular matrix and scaffold proteins of myofibers.

A decrease in skeletal muscle contractile activity or its complete cessation (muscle unloading or disuse) leads to muscle fibers’ atrophy and to alterations in muscle performance. These changes negatively affect the quality of life of people who, for one reason or another, are forced to face a limitation of physical activity. One of the key regulatory events leading to the muscle disuse-induced changes is an impairment of calcium homeostasis, which leads to the excessive accumulation of calcium ions in the sarcoplasm. This review aimed to analyze the triggering mechanisms of calcium homeostasis impairment (including those associated with the accumulation of high-energy phosphates) under various types of muscle unloading. Here we proposed a hypothesis about the regulatory mechanisms of SERCA and IP3 receptors activity during muscle unloading, and about the contribution of these mechanisms to the excessive calcium ion myoplasmic accumulation and gene transcription regulation via excitation–transcription coupling.

We tested the hypothesis that in unloaded skeletal muscles the calcium-dependent signaling pathways and E3-ligase expression are controlled by regulation of phosphorylation of AMP-activated protein kinase (AMPK). For a 3-d experiment 32 Wister male rats were divided into 4 groups: placebo control (С), metformin control (300 mg/kg of body, per oral, MC), suspension and placebo (SP), suspension and metformin (SM). Object of the investigation was m. soleus. In comparison to group C, rAMPK in group MC reduced 46 % and ATP increased 49 % (p < 0.05); pCaMK II showed an increase and expression of mRNA CaN, SERCA2a and Calpain 1 grew 483 %, 87 %, 41 % and 62 %, respectively; p < 0.05). In group SP, MuRF1, MAFbx E3-ligase expression and ubiquitin increased 167 %, 146 % and 191 %, respectively (p < 0.05). Per oral metformin prevented these changes in the suspended rats. During 3 days of suspension, metformin prevented changes in rAMK and ATP; also, it influenced regulation of the calcium-dependent signaling pathways through expression and phosphorylation of such markers as CaMK, CaN, SERCA2a and Calpain-1, and prevented to a degree growth in expression of key markers of ubuquitin-proteasome pathway – MuRF1, MAFbx, and uniquitin.

Skeletal muscle atrophy during their unloading is due to a decrease in protein synthesis and an increase in proteolysis. The accumulation of ATP in muscle during unloading, detected at its early stages, may be one of the stimuli triggering this process. It has been shown that pannexin channels allow ATP efflux from the cytoplasm to the extracellular space during muscle unloading. Extracellular ATP can be sensed by P2Y2 receptors. To test the hypothesis on the involvement of P2Y2 receptors in the regulation of signaling processes in skeletal muscles at early stages of unloading, they were inhibited by their selective inhibitor AR-C 18925XX. The inhibition of P2Y2 receptors during 3-day unloading attenuated m. soleus atrophy, prevented ATP accumulation therein, downregulated the expression of MAFbx E3-ligase mRNA, ubiquitin and IL6 receptors, upregulated the level of AMPK phosphorylation and intensity of protein synthesis.

Current study tested a hypothesis that during skeletal muscle unloading, calcium-dependent signaling pathways, markers of protein synthesis, and expression of E3 ubiquitin ligases can be regulated by metformin. Thirty-two male Wistar rats were randomly assigned into one of four groups: nontreated control (3C), control rats treated with metformin (3CM), 3 days of unloading/hindlimb suspension with placebo (3HS), and 3 days of unloading treated with metformin (3HSM). In soleus muscle of HS group level of phospho-AMP-activated protein kinase (p-AMPK) was decreased by 46% while ATP content was increased by 49% when compared with the control group. There was an increase of the level of phospho-CaMK II (483%) and an upregulation of Calcineurin (CaN), SERCA2a, and Calpain-1 mRNA expression (87%, 41%, and 62%, respectively, P < 0.05) in the HS group relative to the control. HS group also had increased mRNA expression of MuRF1, MAFbx, and ubiquitin (167%, 146%, and 191%, respectively, P < 0.05) when compared with the control soleus muscle. Metformin treatment impeded unloading-induced changes in soleus muscle. In conclusion, metformin treatment during 3 days of soleus muscle unloading: 1) prevented the decrease of p-AMPK and increase of ATP content; 2) affected regulation of calcium-dependent signaling pathways via level of CaMK II phosphorylation or CaMK II, CaN, SERCA2a, and Calpain-1 mRNA expression; 3) attenuated an increase in the expression of critical markers of ubiquitin-proteasome pathways MuRF1, MAFbx, and ubiquitin while not affecting the unloading-induced increase of ULK-1 marker of autophagic/lysosomal pathway.NEW & NOTEWORTHY Current study for the first time tested the hypothesis that during 3 days of soleus muscle unloading, calcium-dependent signaling pathways, markers of protein synthesis, and the expression of E3 ubiquitin ligases can be regulated by metformin. Treatment with metformin during unloading: prevented the decrease of p-AMPK and increase of ATP content, affected regulation of calcium-dependent signaling pathways, and attenuated an increase of critical markers of ubiquitin-proteasome pathways. Nevertheless, metformin treatment has not prevented soleus muscle atrophy.

Skeletal muscle unloading leads to the decreased electrical activity and decline of muscle tone. Aims: Current study evaluated the effect of muscle tone preservation achieved by tetanus toxin (TeNT) treatment on signaling pathways regulating atrophic processes during unloading. Main methods: Four groups of rats were used: non-treated control (C), control rats with TeNT administration (CT), 7 days of unloading/hindlimb suspension with placebo (HS), and 7 days of unloading with TeNT administration (HST). Key findings: Absolute and relative force of tetanic contractions was decreased by 65% in soleus muscle of HS rats when compared with C. Treatment with TeNT significantly lessened force decline in soleus muscle of HST rats when compared with HS. TeNT administration increased myosin heavy chain I beta (MyHC Iβ) expression in CT rats and prevented MyHC Iβ loss in HST group when compared with C rats. Desmin content was lower by 31.4% (p < 0.05) in HS group when compared with HST. Calpain-1 expression was increased in HS group when compared with C, CT and HST. There was a decrease in p-p70S6K content (41%, p < 0,05) and an increase in p-eEF2 content (77%, p < 0,05) in HS group when compared with C, while there were no significant differences in the content of these proteins between HST, CT and C groups. Significance: Treatment with TeNT significantly diminished unloading-induced decline of soleus muscle mass and mechanical properties and affected the regulation of MyHC Iβ expression. These effects are mediated by signaling pathways regulating protein synthesis and degradation.

Lowering or full stopping of the skeletal muscles contraction activities leads to atrophy of the muscle fibers and loss of performance. These changes affect the life quality of people who for one or another reason are faced with a limited ability to move. Among the key triggers of events in regulation responsible for the changes due to low muscle activity (disuse) is disturbance of calcium homeostasis and consequent build up of Ca deposits in the fiber myoplasm. The review is devoted to analysis of causes for Ca homeostasis disturbance, including macroergic phosphate accumulation, by the muscle disuse. The authors hypothesize a mechanism regulating the SERCA and IP3-receptors and their role in myoplasmic Ca deposit during functional unloading of muscles.