- This study reveals angiotensin II impacts muscle atrophy signaling pathways without morphological changes. Aerobic exercise effectively prevents this by mitigating inflammation and activating the IGF-1/AKT/mTOR pathway, with high-intensity exercise showing the strongest protective effects.

Skeletal muscle is the most abundant tissue in the human body and plays an essential role in maintaining homeostasis, including movement, respiration, eating, and energy balance, by serving as a protein reservoir [1]. Metabolic adaptations in skeletal muscle can modulate disease risk, and disorders such as muscle mass loss are important predictors of mortality [1]. Muscle atrophy is a natural aspect of aging, with consistent and observable muscle loss typically beginning around age 40; if not prevented or addressed early, this process can increase the risk of various diseases [2]. Muscle mass is regulated by the rates and balance of protein synthesis and breakdown [2]. Although muscle atrophy can develop in otherwise healthy individuals, the underlying mechanisms —especially in the early stages — remain under active investigation [3-5].

Angiotensin II (Ang II) is a central factor in hypertension, known to raise blood pressure through vasoconstriction, sympathetic nervous system activation, and increased aldosterone production [6]. Ang II also contributes to the development of atherosclerosis and heart failure associated with hypertension [6]. Prior studies indicate that Ang II induces muscle wasting via an AT1 receptor-dependent mechanism independent of its hypertensive effects [7]. While it is established that Ang II administration increases protein degradation, suppresses protein synthesis, and promotes inflammation, research specifically addressing Ang II-induced muscle atrophy is limited [8, 9]. The exact mechanisms by which Ang II influences muscle morphology and atrophy-related molecular pathways remain to be elucidated.

Exercise is a potent stimulus for preventing muscle loss under various pathological conditions. Exercise-induced activation of specific genes and proteins leads to structural adaptations within skeletal muscle [10]. These adaptations and the altered expression of multiple genes and proteins are well documented with repeated exercise [11]. Additionally, exercise can elicit diverse changes in signaling pathways and cellular metabolism depending on intensity [12]. High-intensity exercise (HIE) in particular is known to enhance protein synthesis signaling and mitochondrial function [12]. Although exercise is recognized for its effectiveness in reducing muscle atrophy, its impact on muscle morphology and the molecular pathways altered by Ang II-induced muscle atrophy has not been fully characterized.

Therefore, the aim of this study was to determine whether aerobic exercise can provide protection against muscle atrophy in a model of Ang II-induced skeletal muscle loss.

The IGF-1/AKT/mTOR signaling pathway is critical for regulating skeletal muscle protein synthesis and cellular growth, serving as a key mechanism in counteracting muscle atrophy [14]. Impairment of this pathway contributes to muscle wasting, while its activation is essential for maintaining muscle mass and preventing atrophic conditions [14]. In our study, Ang II injection did not significantly alter skeletal muscle morphology. However, IGF-1/AKT/mTOR signaling — a central pathway associated with muscle atrophy—was suppressed by Ang II, and exercise prevented this reduction. Although no morphological changes in muscle were observed, the reduction of the IGF-1/AKT/mTOR pathway after only 4 weeks of Ang II injection is notable. Importantly, HIE was more effective than LIE in mitigating these changes. We compared the weights of the GA and tibialis anterior muscles 4 weeks after Ang II injection to assess muscle atrophy, but found no statistically significant differences among groups (Fig. 1). Likewise, there were no significant differences in total or myofibrillar protein content among all groups (Fig. 2).

Despite the lack of phenotypic differences, a key finding of our study is that factors promoting muscle atrophy were down-regulated following 4 weeks of Ang II injection, and exercise was effective in counteracting these changes. IGF-1/AKT/mTOR signaling, which is closely linked to muscle atrophy, was significantly decreased in the Ang II group compared to the exercise groups, and exercise had a protective effect (Fig. 4). IGF-1 is crucial for regulating protein synthesis and breakdown, cell proliferation and survival, as well as glucose uptake and energy production [14]. Previous studies have shown that IGF-1 decreases as muscle atrophy progresses [15, 16]. Reduced IGF-1 levels lead to increased protein breakdown and decreased synthesis, ultimately resulting in muscle atrophy [15]. Thus, preventing or reversing the decline in IGF-1 is important in the context of muscle atrophy. AKT is known to play a key role in muscle hypertrophy, and is phosphorylated by IGF-1 activation [17, 18]. Previous studies have reported that AKT activation in mouse muscle can prevent muscle atrophy in neurodegenerative disease models [19, 20]. mTOR, a downstream target of AKT, is also vital for IGF-1 action in muscle [21], and its activation has been shown to play a central role in protein anabolic and catabolic balance [21]. The importance of mTOR in muscle atrophy has been demonstrated in studies where muscle-specific mTOR knockout mice developed severe myopathy and died prematurely at 22–38 weeks of age [22]. In summary, the IGF-1/AKT/mTOR pathway plays an important role in muscle atrophy, and preventing or ameliorating its reduction is crucial.

In our study, exercise was shown to protect against the decline in IGF-1/AKT/mTOR signaling in Ang II-induced muscle atrophy (Fig. 4). Notably, changes in IGF-1 and mTOR protein expression were improved by both exercise intensities (Fig. 4A and C), whereas AKT protein changes were observed only with HIE (Fig. 4B). The Ang II group exhibited a significant decrease in mTOR protein, which is essential for muscle synthesis (Fig. 4C). Previous studies have shown that HIE is particularly effective in stimulating the mTOR pathway, potentially leading to greater muscle growth than lower intensities [23, 24]. Nonetheless, in our study, exercise exerted a protective effect against Ang II-induced muscle atrophy regardless of intensity. While resistance exercise is generally considered more effective than aerobic exercise for preventing muscle atrophy due to its direct stimulation of protein synthesis [25], previous studies comparing aerobic and resistance exercise have also found that aerobic exercise can improve the IGF-1/AKT pathway and help maintain skeletal muscle mass [26].

Inflammatory factors can directly bind to their receptors, inhibit muscle protein synthesis, and promote protein breakdown, thereby contributing to skeletal muscle atrophy [27]. Our study demonstrated that Ang II infusion induced muscle atrophy and directly stimulated TNF-α production. TNF-α enhances muscle wasting and reduces IGF-1-mediated protein synthesis [28, 29]. Notably, HIE was more effective than LIE in reducing TNF-α levels (Fig. 3A). While HIE may increase inflammatory cytokines during the initial phase of exercise [30], chronic adaptation to exercise over time results in decreased levels of these cytokines [31]. Thus, our findings suggest that sustained HIE over 4 weeks was effective in reducing TNF-α. TGF-β can increase the expression of muscle atrophy proteins and promote muscle protein breakdown, which can cause muscle fiber atrophy [32]. It can also promote fibrosis formation in muscle tissue, which can further impair muscle function [32]. TGF-β, similar to TNF-α, was also only effectively induced by HIE (Fig. 3B). These findings may be related to PGC-1α. In skeletal muscle, decreased PGC-1α induces the expression of inflammatory cytokines such as TNF-α, while increased PGC-1α has been shown to upregulate inflammatory cytokines in muscle cells [33, 34]. We propose that our results support the idea that only HIE was effective in reducing TNF-α.

Another notable aspect of our study is that continuous Ang II injection, combined with exercise, activated autophagy. Autophagy maintains core cellular metabolism, removes damaged components, and promotes cellular repair and stress resistance. Dysfunction in autophagy leads to muscle degeneration and weakness [35]. Previous studies have reported that autophagy suppresses inflammatory responses by alleviating oxidative stress, thus providing a protective effect against sarcopenia [35]. Our results confirmed that exercise groups exhibited greater changes in autophagy-related protein expression compared to the Ang II group (Fig. 5), supporting the notion that activation of autophagy protects against muscle atrophy and excessive inflammatory responses. Nevertheless, ongoing debate remains regarding the effects of autophagy overactivation or dysfunction in muscle atrophy [14].

In summary, this study demonstrated that Ang II injection did not significantly alter skeletal muscle morphology, but did cause marked changes in signaling pathways associated with muscle atrophy. Aerobic exercise improved IGF-1/AKT/mTOR signaling in Ang II-induced muscle atrophy. Furthermore, reductions in inflammatory cytokines were more pronounced with HIE than with LIE, suggesting greater protective effects. These results suggest that aerobic exercise may offer protection during the early stages of muscle atrophy and may provide a foundation for future research.