In vivo Ca dynamics induced by Ca injection in individual rat skeletal muscle fibers.

In contrast to cardiomyocytes, store overload-induced calcium ion (Ca) release (SOICR) is not considered to constitute a primary Ca releasing system from the sarcoplasmic reticulum (SR) in skeletal muscle myocytes. In the latter, voltage-induced Ca release (VICR) is regarded as the dominant mechanism facilitating contractions. Any role of the SOICR in the regulation of cytoplasmic Ca concentration ([Ca]) and its dynamics in skeletal muscle in vivo remains poorly understood. By means of in vivo single fiber Ca microinjections combined with bioimaging techniques, we tested the hypothesis that the [Ca] dynamics following Ca injection would be amplified and fiber contraction facilitated by SOICR. The circulation-intact spinotrapezius muscle of adult male Wistar rats (=34) was exteriorized and loaded with Fura-2 AM to monitor [Ca] dynamics. Groups of rats underwent the following treatments: (1) 0.02, 0.2, and 2.0 mmol/L Ca injections, (2) 2.0 mmol/L Ca with inhibition of ryanodine receptors (RyR) by dantrolene sodium (DAN), and (3) 2.0 mmol/L Ca with inhibition of SR Ca ATPase (SERCA) by cyclopiazonic acid (CPA). A quantity of 0.02 mmol/L Ca injection yielded no detectable response, whereas peak evoked [Ca] increased 9.9 ± 1.8% above baseline for 0.2 mmol/L and 23.8 ± 4.3% ( < 0.05) for 2.0 mmol/L Ca injections. The peak [Ca] in response to 2.0 mmol/L Ca injection was largely abolished by DAN and CPA (-85.8%, -71.0%, respectively, both  < 0.05 vs. unblocked) supporting dependence of the [Ca] dynamics on Ca released by SOICR rather than injected Ca itself. Thus, this investigation demonstrates the presence of a robust SR-evoked SOICR operant in skeletal muscle in vivo.