📄 systems biology

Interplay between store-operated calcium entry and mitochondrial phosphate handling modulates force and fatigue during exercise

This study employs a quantitative biophysical model to demonstrate that store-operated calcium entry (SOCE) enhances skeletal muscle force during resistance exercise by sustaining calcium release, but can exacerbate fatigue during high-intensity training due to phosphate accumulation, a trade-off that is modulated by mitochondrial phosphate uptake.

Original authors: Francis, E. A., Hamid, J., Kumar, A., Rangamani, P.

Published 2026-07-24

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CC BY 4.0

Original authors: Francis, E. A., Hamid, J., Kumar, A., Rangamani, P.

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

Imagine your body as a high-performance sports car. To make that car move, you need a spark plug to ignite the engine, fuel to keep it running, and a cooling system to stop it from overheating. In the human body, our "engines" are muscle fibers, and the spark plug is a tiny electrical signal that tells the muscle to squeeze. But here's the tricky part: muscles are like busy factories that run out of their internal fuel reserves very quickly when they work hard. To keep the factory running, they need a delivery truck to bring in more supplies from the outside world. In biology, this delivery truck is a mechanism called "Store-Operated Calcium Entry" (SOCE). It's the muscle's way of saying, "Hey, we're empty! Open the gates and let more calcium in!"

Calcium is the magic ingredient that makes muscles contract. When you decide to lift a heavy box, your brain sends an electrical signal that opens the doors inside the muscle cells, releasing calcium from a storage room called the sarcoplasmic reticulum (SR). This calcium rushes out, hits the muscle fibers, and boom—contraction! But once that storage room is empty, the muscle needs to refill it to keep going. That's where SOCE comes in: it opens a gate in the cell's outer wall to let fresh calcium from the outside world pour in, refilling the storage room so the muscle can keep working. Scientists have long wondered if this refilling system is the secret to superhuman endurance or if it might actually be a double-edged sword that causes muscles to tire out faster under certain conditions.

This paper dives into that mystery using a super-detailed computer simulation, acting like a virtual laboratory where the researchers can tweak the rules of physics without needing a real gym or a lab full of mice. The team built a complex digital model of a muscle fiber, complete with its electrical signals, calcium storage, and even its mitochondria (the tiny power plants inside cells). They wanted to see exactly how the "refilling truck" (SOCE) interacts with the muscle's waste management system, specifically how it handles phosphate, a byproduct of energy use that can build up and make muscles feel heavy and tired.

The researchers found that the role of SOCE isn't a simple "good" or "bad" story; it depends entirely on how you're exercising. In their simulations, when the muscle was doing steady, resistance-style work (like lifting weights), turning up the volume on SOCE was a great idea. It kept the calcium levels high, allowing the muscle to generate more force and keep pushing. It was like having a delivery truck that arrived just in time to keep the factory running at full speed.

However, the story changed when they simulated high-intensity interval training (HIIT), where the muscle is firing rapidly and intensely. In this scenario, cranking up SOCE too high actually made the muscle tire out faster. Why? Because the extra calcium coming in made the muscle work so hard that it produced a massive amount of phosphate waste. This phosphate piled up in the muscle, acting like a clog in the pipes, which reduced the force the muscle could produce. The simulation suggested that SOCE was essentially flooding the factory with raw materials, but the waste disposal system couldn't keep up, leading to a backup that caused fatigue.

The paper also suggests that the solution to this high-intensity fatigue might lie in the mitochondria. If the mitochondria could get better at sucking up that excess phosphate (acting like a super-efficient vacuum cleaner), the muscle could handle the high SOCE activity without getting tired. The authors propose that the balance between how fast calcium enters the cell and how fast mitochondria clean up the resulting waste is the key to understanding why muscles sometimes feel strong and other times feel like lead.

In short, this study uses computer modeling to suggest that SOCE is a context-dependent hero. It's a champion for steady, heavy lifting, but it can become a liability during explosive, high-speed bursts of activity unless the cell's internal cleanup crew (the mitochondria) is working overtime to manage the waste. The findings help explain why some muscles might fatigue differently depending on the type of exercise, offering a new way to think about how our bodies manage energy and exhaustion.

Technical Summary: Interplay between Store-Operated Calcium Entry and Mitochondrial Phosphate Handling Modulates Force and Fatigue During Exercise

Problem Statement
Skeletal muscle contraction relies on the precise regulation of intracellular calcium ( $Ca^{2+}$ ) dynamics, linking electrochemical activation to contractile force. While the role of store-operated calcium entry (SOCE)—the influx of extracellular $Ca^{2+}$ triggered by depletion of sarcoplasmic reticulum (SR) stores—is well-established in non-excitable cells, its function in excitable skeletal muscle fibers remains complex. Recent experimental data presents apparent contradictions: some studies suggest SOCE delays fatigue and enhances force by maintaining SR $Ca^{2+}$ levels, while others indicate that SOCE deficiency can mitigate fatigue under specific conditions. Furthermore, muscle fatigue is multifactorial, involving rapid phosphate ( $P_i$ ) accumulation in the myoplasm, reduced SR $Ca^{2+}$ release, and decreased myofibrillar $Ca^{2+}$ sensitivity. A comprehensive quantitative framework is needed to reconcile these findings and determine how SOCE interacts with mitochondrial phosphate handling to modulate force production and fatigue onset during different exercise regimes.

Methodology
The authors developed a quantitative, well-mixed, multi-compartment computational model of a fast-twitch myofiber. The model integrates five distinct modules to simulate the biochemical and biophysical events of muscle contraction:

Action Potential Generation: A Hodgkin-Huxley-type formulation describing $Na^+$ and $K^+$ fluxes across the sarcolemma and T-tubules.
$Ca^{2+}$ Release: Modeling the allosteric interaction between dihydropyridine receptors (DHPR) and ryanodine receptors (RyR) to facilitate SR $Ca^{2+}$ release.
$Ca^{2+}$ Removal and SOCE: Incorporating ATP-dependent pumps (SERCA, PMCA), exchangers (NCX), and a phenomenological description of SOCE. SOCE is modeled via STIM1 sensing SR $Ca^{2+}$ depletion, oligomerizing, and gating Orai1 channels, described by a Hill equation dependent on junctional SR $Ca^{2+}$ concentration.
Cross-Bridge Cycling: A six-state cross-bridge model where force generation is proportional to the density of post-power stroke cross-bridges. Crucially, this module explicitly accounts for the dependence of power stroke reversal on inorganic phosphate ( $P_i$ ) concentration, a key mechanism for fatigue.
Mitochondrial Handling: Modeling mitochondrial $Ca^{2+}$ uptake (via MCU), phosphate uptake (via the phosphate carrier, PiC), and ATP generation via oxidative phosphorylation.

The model consists of 65 ordinary differential equations (ODEs). Parameters were calibrated in three stages: first, mitochondrial parameters were fitted to in vitro data from guinea pig heart mitochondria; second, voltage-related parameters were fitted to action potential data; and finally, the full model was calibrated to simultaneous myoplasmic $Ca^{2+}$ and voltage data from mouse fast-twitch fibers. Sensitivity analysis (Morris method) identified the most influential parameters. The model was then used to simulate resistance exercise (40–70 Hz stimuli) and high-intensity interval training (HIIT) (70–120 Hz stimuli), varying Orai1 conductance ( $g_{Orai1}$ ) and mitochondrial PiC permeability ( $\nu_{PiC}$ ).

Key Contributions and Results
The study provides a systems-level explanation for the context-dependent role of SOCE in muscle performance:

SOCE Enhances Force via Sustained $Ca^{2+}$ : The model predicts that increasing SOCE activity leads to higher peak myoplasmic $Ca^{2+}$ concentrations and sustained $Ca^{2+}$ release during repeated stimuli. This results in increased troponin binding and enhanced cross-bridge cycling, thereby increasing contractile force.
The Phosphate Trade-off: The model reveals a competing mechanism. High SOCE activity drives increased ATP hydrolysis (due to higher cross-bridge cycling and pump activity), leading to rapid accumulation of inorganic phosphate ( $P_i$ ) in the myoplasm. High $P_i$ promotes the reversal of the power stroke, reducing contractile force (fatigue).
Frequency-Dependent Outcomes:
- Resistance Exercise (Lower Frequencies): During resistance exercise (simulated at 40–70 Hz), increased SOCE generally enhances force production. The benefit of sustained $Ca^{2+}$ outweighs the negative effects of phosphate accumulation, provided mitochondrial phosphate uptake is functional.
- High-Intensity Interval Training (Higher Frequencies): During HIIT (simulated at 70–120 Hz), the relationship becomes non-linear. At very high stimulus frequencies, excessive SOCE can exacerbate fatigue. The rapid $Ca^{2+}$ influx leads to such high rates of ATP consumption that phosphate accumulates faster than mitochondria can clear it, causing a net reduction in force.
Mitochondrial Buffering: The capacity of mitochondria to uptake phosphate via the PiC carrier is a critical modulator. The simulations show that enhanced mitochondrial phosphate uptake can mitigate the fatigue induced by high SOCE activity during high-frequency stimulation. Conversely, impaired mitochondrial phosphate handling (e.g., reduced PiC activity) in SOCE-deficient cells leads to significant force reduction at high frequencies, aligning with experimental observations of dominant-negative Orai1 mice.

Significance
The paper claims to resolve apparent contradictions in the literature regarding SOCE's role in muscle fatigue. It posits that SOCE is not universally beneficial or detrimental; rather, its impact is determined by the interplay between stimulus frequency, the rate of phosphate accumulation, and the efficiency of mitochondrial phosphate clearance.

The study demonstrates that while SOCE is generally advantageous for maintaining force during moderate-intensity or resistance exercise by sustaining $Ca^{2+}$ levels, it can accelerate fatigue during high-frequency, high-intensity activation if mitochondrial phosphate handling cannot keep pace with the metabolic demand. This suggests that the "optimal" level of SOCE is context-dependent, tuned by the metabolic capacity of the mitochondria. The work highlights the necessity of considering mitochondrial phosphate dynamics when modeling excitation-contraction coupling and fatigue, offering a mechanistic basis for understanding how structural and functional changes in the SR-mitochondria interface influence athletic performance and muscle failure.

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