Comparison of Phasic Store-Operated Calcium Entry in Rat Slow- and Fast-Twitch Muscle Fibers

Abstract

Aim

This study investigates the activation and regulation of phasic store-operated calcium entry (pSOCE) in fast- and slow-twitch skeletal muscle fibers. Specifically, we aimed to enhance the sensitivity of pSOCE detection in slow-twitch fibers by optimizing ionic conditions and to compare the physiological relevance of pSOCE between fiber types.

Methods

We employed mechanically skinned fast-twitch extensor digitorum longus (EDL) muscle fibers loaded with spectrally distinct Ca²⁺-sensitive dyes to simultaneously measure action potential-induced sarcoplasmic reticulum Ca²⁺ release and t-tubular system Ca²⁺ dynamics with millisecond resolution. Experimental conditions were optimized by reducing cytosolic Mg²⁺ and EGTA buffering to enhance Ca²⁺ release in slow-twitch soleus fibers. Confocal microscopy was used to track t-tubular system Ca²⁺ depletion and reuptake during electric field stimulation.

Results

Skinned soleus fibers exhibited ~8-fold lower Ca²⁺ release per action potential compared to EDL fibers, yet pSOCE amplitudes were comparable. Reducing Mg²⁺ and EGTA levels increased Ca²⁺ release and left pSOCE kinetics in EDL fibers unaltered, but enabled pSOCE measurements in soleus fibers. While pSOCE in EDL fibers followed a linear dependence on the ambient Ca²⁺ concentration in the t-tubular system, such a relationship was violated in soleus fibers.

Conclusion

These findings reveal a novel, fiber-type-specific difference in pSOCE regulation. When compared to EDL fibers, soleus fibers exhibited a higher sensitivity to SOCE activation despite releasing less Ca²⁺ from the sarcoplasmic reticulum upon an action potential. These differences may allow soleus fibers to sustain Ca²⁺ homeostasis more effectively, be more resilient against disruptions in Ca²⁺ handling, and entail protection against disease states.