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(2+)-sensitive dyes to simultaneously measure action potential-induced sarcoplasmic reticulum Ca(2+) release and t-tubular system Ca(2+) dynamics with millisecond resolution. Experimental conditions were optimized by reducing cytosolic Mg(2+) and EGTA buffering to enhance Ca(2+) release in slow-twitch soleus fibers. Confocal microscopy was used to track t-tubular system Ca(2+) depletion and reuptake during electric field stimulation. RESULTS: Skinned soleus fibers exhibited ~8-fold lower Ca(2+) release per action potential compared to EDL fibers, yet pSOCE amplitudes were comparable. Reducing Mg(2+) and EGTA levels increased Ca(2+) 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(2+) 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(2+) from the sarcoplasmic reticulum upon an action potential. These differences may allow soleus fibers to sustain Ca(2+) homeostasis more effectively, be more resilient against disruptions in Ca(2+) handling, and entail protection against disease states.