R-CEPIA1er as a new tool to directly measure sarcoplasmic reticulum [Ca] in ventricular myocytes

In cardiomyocytes, [Ca] within the sarcoplasmic reticulum (SR; [Ca](SR)) partially determines the amplitude of cytosolic Ca transient that, in turn, governs myocardial contraction. Therefore, it is critical to understand the molecular mechanisms that regulate [Ca](SR) handling. Until recently, the best approach available to directly measure [Ca](SR) was to use low-affinity Ca indicators (e.g., Fluo-5N). However, this approach presents several limitations, including nonspecific cellular localization, dye extrusion, and species limitation. Recently a new genetically encoded family of Ca indicators has been generated, named Ca-measuring organelle-entrapped protein indicators (CEPIA). Here, we tested the red fluorescence SR-targeted Ca sensor (R-CEPIA1er) as a tool to directly measure [Ca](SR) dynamics in ventricular myocytes. Infection of rabbit and rat ventricular myocytes with an adenovirus expressing the R-CEPIA1er gene displayed prominent localization in the SR and nuclear envelope. Calibration of R-CEPIA1er in myocytes resulted in a K-d of 609 mu M, suggesting that this sensor is sensitive in the whole physiological range of [Ca](SR). [Ca](SR) dynamics measured with R-CEPIA1er were compared with [Ca](SR) measured with Fluo5-N. We found that both the time course of the [Ca](SR) depletion and fractional SR Ca release induced by an action potential were similar between these two Ca sensors. R-CEPIA1er fluorescence did not decline during experiments, indicating lack of dye extrusion or photo-bleaching. Furthermore, measurement of [Ca](SR) with R-CEPIA1er can be combined with cytosolic [Ca] measurements (with Fluo-4) to obtain more detailed information regarding Ca handling in cardiac myocytes. In conclusion, R-CEPIA1er is a promising tool that can be used to measure [Ca](SR) dynamics in myocytes from different animal species.