Spatio-Temporal Relationship Between T-Tubular Electrical Activity and Ca2+ Release in Heart Failure

Abstract

T-tubules (TT) are invaginations of the surface sarcolemma (SS) that conduct the action potential (AP) to the cardiomyocyte core. Previously, we observed that TT structural remodeling in a rat model of post-ischemic heart failure (HF) was associated with abnormal electrical activity in several t-tubular elements (Sacconi et al. PNAS 2012). To clarify the link between TT abnormalities and Ca2+-dependent arrhythmias, we combined the advantages of an ultrafast random access multi-photon (RAMP) microscope with a double staining approach to optically record t-tubular AP and, simultaneously, the corresponding local Ca2+-transient in different positions across the cardiomyocytes. Despite a uniform AP between SS and TT at steady-state stimulation, in control cardiomyocytes we observed a non-negligible beat-to-beat variability of local Ca2+ transient amplitude and kinetics. The beat-to-beat variability was quantitatively similar to the one observed among different tubules during the same beat (spatial variability) and both were significantly reduced by 0.1μM Isoproterenol, which increases the opening probability of Ca2+-release units. As previously demonstrated, some tubular elements fail to propagate AP in rat failing cardiomyocytes. By separately averaging tubules conducting (AP+) or not conducting AP (AP-), we found that Ca2+-transient amplitude was reduced and Ca2+-transient rise was slower in AP- tubules compared to AP+ tubules. Both beat-to-beat and spatial variability of Ca2+-transient kinetics were increased in HF. Finally, TT that did not show AP, occasionally displayed spontaneous depolarizations that were never accompanied by local Ca2+-release in the absence of any pro-arrhythmogenic stimulation. Simultaneous recording of AP and Ca2+-transient allows us to probe the spatio-temporal variability of Ca2+-release, whereas the investigation of Ca2+-transient in HF discloses an unexpected uncoupling between t-tubular depolarization and Ca2+-release in remodeled tubules.