Mitochondria can substitute for parvalbumin to lower cytosolic calcium levels in the murine fast skeletal muscle

Aim

Parvalbumin (PV) is a primary calcium buffer in mouse fast skeletal muscle fibers. Previous work showed that PV ablation has a limited impact on cytosolic Ca²⁺ ([Ca²⁺]_cyto) transients and contractile response, while it enhances mitochondrial density and mitochondrial matrix-free calcium concentration ([Ca²⁺]_mito). Here, we aimed to quantitatively test the hypothesis that mitochondria act to compensate for PV deficiency.

Methods

We determined the free Ca²⁺ redistribution during a 2 s 60 Hz tetanic stimulation in the sarcoplasmic reticulum, cytosol, and mitochondria. Via a reaction–diffusion Ca²⁺ model, we quantitatively evaluated mitochondrial uptake and storage capacity requirements to compensate for PV lack and analyzed possible extracellular export.

Results

[Ca²⁺]_mito during tetanic stimulation is greater in knock-out (KO) (1362 ± 392 nM) than in wild-type (WT) (855 ± 392 nM), p < 0.05. Under the assumption of a non-linear intramitochondrial buffering, the model predicts an accumulation of 725 μmoles/L _fiber (buffering ratio 1:11 000) in KO, much higher than in WT (137 μmoles/L _fiber, ratio 1:4500). The required transport rate via mitochondrial calcium uniporter (MCU) reaches 3 mM/s, compatible with available literature. TEM images of calcium entry units and Mn²⁺ quenching showed a greater capacity of store-operated calcium entry in KO compared to WT. However, levels of [Ca²⁺]_cyto during tetanic stimulation were not modulated to variations of extracellular calcium.

Conclusions

The model-based analysis of experimentally determined calcium distribution during tetanic stimulation showed that mitochondria can act as a buffer to compensate for the lack of PV. This result contributes to a better understanding of mitochondria's role in modulating [Ca²⁺]_cyto in skeletal muscle fibers.