The skeletal muscle excitation‐contraction coupling leading to tension development and increased mechanical stimuli change in concert, leaving it difficult to study their respective effect on molecular signalling and following adaptations. In the current issue of Acta Physiologica, Rindom and co‐workers¹ further our understanding of these signalling pathways through a series of elegantly designed experiments. They investigated specific effects of the excitation‐contraction coupling and tension development on signals regulating transcription, translation and protein synthesis in isolated rat EDL muscles by performing combinations of: 1) excitation‐induced Ca²⁺‐ release by standardized electrical stimulation; (2) addition of chemical inhibitors of the myosin ATPase in order to block muscle contractions, and; (3) mechanical manipulation to achieve passive tension equal to actively contracting muscles through extensive muscle stretching.

From the series of experiments Rindom et al¹ present three main findings: (1) Puromycin‐labelled peptides were only increased when both excitation and tension development (active or passive) were present, leading to the conclusion that both muscle excitation and tension development is required for protein synthesis. (2) Phosphorylation of mTOR, 4E‐BP1, p70S6K1 and rpS6 increased both after active contraction and passive stretching with or without excitation (no active contraction, yet matched tension development), but not after excitation without tension development, leading to the conclusion that tension development, but not excitation, is required for translation initiation signalling. (3) Phosphorylation of p38MAPK and MSK1, as well as mRNA levels of c‐JUN and c‐FOS were increased only when both excitation and tension development (as in finding 1) were present, leading to the conclusion that both excitation and tension development is required for transcriptional regulation of myofibrillar genes.

Rindom et al¹ suggest that separate signalling pathways are dependent on either excitation or tension development, or both, and that they in sum are all obligatory to increase protein synthesis responsible for hypertrophy (see Figure 1 for summary of findings put into a broader context). One intriguing thought related to their findings, is that succeeding steps in the signalling network projecting from either the muscle excitation or tension development can be modulated by cues from the respective stimuli, like a Yin & Yang of hypertrophic signalling, as well as other interdependent cues such as nutrient availability and other circulating factors. One such modulating cross‐talk could depend on some kind of dose‐response effect of the degree of stretch both at the level of transcriptional regulation of myofibrillar genes and translational regulation, supported by the results in Rindom et al.¹ Another example of such a cross‐talk is that Rindom et al¹ do not find an increased protein synthesis following passive tension without excitation. As pointed out by the authors, this is contrary to previous similar studies using a high‐glucose rather than a low‐glucose medium (references within¹). One plausible explanation to this discrepancy could be that glucose acts as a signalling molecule activating some of the same pathways as the excitation, and that this is necessary to complement the cues from tension development in order to activate protein synthesis without excitation.^{2, 3} Further, other in vivo stretch experiments where excitation is blocked also show effects on protein synthesis (see Kalamgi et al⁴ and references therein). Such an intricate modulatory system could thus also help explain some of the contradicting results in the field of mechanotransduction.^{3, 5}

One important advantage of the present work by Rindom et al¹ is that they were able to standardize a high peak tension and tension‐time integral both with and without excitation, as signalling is highly dependent on these parameters (see references within¹). This is very challenging to achieve intravitally, as the maximum joint excursion limits the amount of passive muscle tension one can obtain.⁶ Yet, a similar approach as used by Rindom et al¹ could be performed with an acute in vivo setup with sedated rats where the muscle is detached distally, but still keeping the blood supply and the rest of the in vivo milieu intact, as, for example, nutrient, energy and oxygen supply can all affect the signalling network.⁷ In a recent rat strength straining study⁶ where the muscle excitation were standardized in order to investigate acute and long‐term effects of a varying tension development per se, increased tension led to an increased myonuclear number and hypertrophic response, but observed fibre type changes were not affected by degree of tension development. Due to limitations in such in vivo setups, the peak tension only differed by 40%‐50%, and although significant long‐term tension‐effects were seen, effects on molecular correlates were not as clear. Increasing the relative load difference between experimental groups by, for example, incorporating an eccentric contraction might clarify. These two works complement each other in an interesting manner, and both results and permutations of their respective experimental approaches should be considered for future experiments to further the understanding of these signalling networks.

An additional emerging mechanotransduction pathway is through a direct mechanical link between the ECM and chromatin in both satellite cells and myonuclei, obtained by linking a number of transmembrane and cytoskeletal proteins, causing chromatin unfolding and activation of gene expression upon mechanical stimuli.³ This route bypasses the much more heavily studied “indirect” intracellular pathways, but changes in gene availability and expression could then as a second wave affect both the Yin & Yang part of the intracellular pathways. On this note, another important aspect regarding skeletal muscle signalling that is rarely considered in the literature, is the effect on the different cell types. Up to 50% of nuclei in skeletal muscle are non‐myonuclei. This is especially interesting in the context of mechanotransduction, given that myonuclear accretion is highly dependent on load per se,⁶ that is, activating satellite cells. However, the vast majority of studies use full muscle homogenates to study only a small selection of signalling factors, the result being a low‐resolution snapshot of the network with many empty pixels. Future studies should thus consider both single fiber⁸ and single nuclei⁹omics in order to differentiate between signalling in different cell types in skeletal muscle.

In conclusion, although the interest for mechanotransduction and deciphering relative effects of tension development and excitation in skeletal muscle has had a steep increase in later years, there is still work to do. Indeed, the present paper by Rindom et al¹ is a valuable contribution to the field of mechanotransduction, as it establishes a better foundation to explore the complex network of skeletal muscle plasticity.

骨骼肌的兴奋与收缩耦合导致紧张的发展和协调一致的机械刺激变化的增加，这使得很难研究它们各自对分子信号传导和后续适应的影响。在本期 《生理学报》中，Rindom及其同事¹通过一系列设计精美的实验进一步了解了这些信号传导途径。他们研究了激发-收缩偶联和张力发展对离体大鼠EDL肌肉中调节转录，翻译和蛋白质合成的信号的特定作用，方法是进行以下组合：1）激发诱导的Ca ²⁺-通过标准化的电刺激释放; （2）添加肌球蛋白ATP酶的化学抑制剂以阻断肌肉收缩，和；（3）机械操作，以达到被动张力，等于通过广泛的肌肉拉伸主动收缩肌肉。

通过一系列实验Rindom等¹目前有三个主要发现：（1）嘌呤霉素标记的肽仅在同时存在兴奋性和张力性发展（主动或被动）的情况下才增加，从而得出结论，肌肉兴奋性和张力性发展都需要蛋白质合成。（2）在有或没有激发的情况下进行主动收缩和被动拉伸后，mTOR，4E‐BP1，p70S6K1和rpS6的磷酸化均增加（无主动收缩，但与张力发展相匹配），但在没有张力发展的激发后则没有磷酸化。翻译起始信号需要张力发展，而不是激发。（3）仅当同时存在激发和张力发展时（如发现1），p38MAPK和MSK1的磷酸化以及c-JUN和c-FOS的mRNA水平才增加，

Rindom等人¹提示单独的信号传导途径取决于兴奋或张力的发展，或两者兼而有之，总之，它们都必须增加引起肥大的蛋白质合成（参见图1，了解更广泛的研究内容）。与他们的发现有关的一个有趣的想法是，信号线网络中从肌肉兴奋或张力发展中投射出来的后续步骤都可以通过来自各自刺激的线索来调节，例如肥大信号的阴阳，以及其他相互依赖的线索。例如养分利用率和其他循环因素。一个这样的调节性串扰可能取决于肌原纤维基因的转录调控水平和翻译调控水平上延伸程度的某种剂量反应效应，¹这种串扰的另一个例子是，Rindom等[ ^1]在被动激励下没有激发就没有发现蛋白质合成增加。正如作者所指出的那样，这与以前使用高葡萄糖而不是低葡萄糖培养基的类似研究相反（参考文献在¹内）。对于这种差异的一个合理的解释可能是葡萄糖充当了一种信号分子，激活了一些与激发相同的途径，这对于补充张力产生的线索是必需的，以便在不激发的情况下激活蛋白质合成。^[2，3]此外，激发被阻断的其他体内拉伸实验也显示了对蛋白质合成的影响（参见Kalamgi等人⁴以及其中的参考文献）。这样复杂的调节系统因此也可以帮助解释机械转导领域中的一些矛盾结果。^{3 5}

Rindom等人¹的当前工作的一个重要优点是，由于信号高度依赖于这些参数（参见^1中的参考文献），因此他们能够标准化高峰值张力和张力时间积分（无论有无激励）。这是很难做到的，因为最大的关节偏移限制了人们可以获得的被动肌肉张力。^[6]然而，可以用镇静大鼠在急性体内建立一种与Rindom等[ ^1]所用的相似方法，在这种镇静大鼠中，肌肉向远端分离，但仍保持血液供应和其余体内环境完整，因为例如，营养，能量和氧气供应都会影响信号网络。⁷在最近的大鼠力量拉紧研究中⁶为了研究本身张力变化的急性和长期影响，标准化了肌肉刺激，张力的增加导致肌核数目和肥大反应的增加，但是观察到的纤维类型变化不受张力发展程度的影响。由于这种体内设置的局限性，峰值张力仅相差40％-50％，尽管可以看到明显的长期张力效应，但对分子相关性的影响尚不清楚。可以通过增加例如偏心收缩来增加实验组之间的相对负荷差异。这两幅作品以有趣的方式相互补充，

另一种新兴的机械转导途径是通过卫星细胞和肌核中ECM和染色质之间的直接机械连接，这是通过连接许多跨膜和细胞骨架蛋白而获得的，从而导致染色质展开并在机械刺激下激活基因表达。³该途径绕过了研究更多的“间接”细胞内途径，但基因可用性和表达的变化可能随后作为第二波影响细胞内途径的阴和阳部分。关于这一点，在文献中很少考虑的关于骨骼肌信号传导的另一个重要方面是对不同细胞类型的影响。骨骼肌中多达50％的细胞核是非肌核细胞。在机械转导的情况下，这尤其有趣，因为肌核的积聚高度依赖于负荷本身，⁶即激活卫星小区。但是，绝大多数研究使用全肌肉匀浆仅研究信号选择因子的一小部分，结果是网络中有许多空像素的低分辨率快照。因此，未来的研究应同时考虑单纤维⁸和单核⁹组学，以区分骨骼肌中不同细胞类型的信号传导。

综上所述，尽管对机械传递和破译骨骼肌张力产生和兴奋的相对作用的兴趣在后来几年急剧增加，但仍有工作要做。的确，Rindom等人[ ^1]的论文为机械转导领域做出了宝贵的贡献，因为它为探索复杂的骨骼肌可塑性网络奠定了更好的基础。