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

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 Ca-sensitive dyes to simultaneously measure action potential-induced sarcoplasmic reticulum Ca release and t-tubular system Ca dynamics with millisecond resolution. Experimental conditions were optimized by reducing cytosolic Mg and EGTA buffering to enhance Ca release in slow-twitch soleus fibers. Confocal microscopy was used to track t-tubular system Ca depletion and reuptake during electric field stimulation.

RESULTS

Skinned soleus fibers exhibited ~8-fold lower Ca release per action potential compared to EDL fibers, yet pSOCE amplitudes were comparable. Reducing Mg and EGTA levels increased Ca 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 Ca 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 Ca from the sarcoplasmic reticulum upon an action potential. These differences may allow soleus fibers to sustain Ca homeostasis more effectively, be more resilient against disruptions in Ca handling, and entail protection against disease states.