Dynamic calcium movement inside cardiac sarcoplasmic reticulum during release.

RATIONALE

Intra-sarcoplasmic reticulum (SR) free [Ca] (Ca) provides the driving force for SR Ca release and is a key regulator of SR Ca release channel gating during normal SR Ca release or arrhythmogenic spontaneous Ca release events. However, little is known about Ca spatiotemporal dynamics.

OBJECTIVE

To directly measure local Ca with subsarcomeric spatiotemporal resolution during both normal global SR Ca release and spontaneous Ca sparks and to evaluate the quantitative implications of spatial Ca gradients.

METHODS AND RESULTS

Intact and permeabilized rabbit ventricular myocytes were subjected to direct simultaneous measurement of cytosolic [Ca] and Ca and FRAP (fluorescence recovery after photobleach). We found no detectable Ca gradients between SR release sites (junctional SR) and Ca uptake sites (free SR) during normal global Ca release, clear spatiotemporal Ca gradients during isolated Ca blinks, faster intra-SR diffusion in the longitudinal versus transverse direction, 3- to 4-fold slower diffusion of fluorophores in the SR than in cytosol, and that intra-SR Ca diffusion varies locally, dependent on local SR connectivity. A computational model clarified why spatiotemporal gradients are more detectable in isolated local releases versus global releases and provides a quantitative framework for understanding intra-SR Ca diffusion.

CONCLUSIONS

Intra-SR Ca diffusion is rapid, limiting spatial Ca gradients during excitation-contraction coupling. Spatiotemporal Ca gradients are apparent during Ca sparks, and these observations constrain models of dynamic Ca movement inside the SR. This has important implications for myocyte SR Ca handling, synchrony, and potentially arrhythmogenic spontaneous contraction.