Discordant Ca 2+ release in cardiac myocytes: characterization and susceptibility to pharmacological RyR2 modulation.

Systolic Ca 2+ transients are shaped by the concerted summation of Ca 2+ sparks across cardiomyocytes. At high pacing rates, alterations of excitation-contraction coupling manifest as pro-arrhythmic Ca 2+ alternans that can be classified as concordant or discordant. Discordance is ascribed to out-of-phase alternation of local Ca 2+ release across the cell, although the triggers and consequences of this phenomenon remain unclear. Rat ventricular cardiomyocytes were paced at increasing rates. A discordance index (SD of local alternans ratios) was developed to quantify discordance in confocal recordings of Ca 2+ transients. Index values were significantly increased by rapid pacing, and negatively correlated with Ca 2+ transient amplitude change, indicating that discordance is an important contributor to the negative Ca 2+ transient-frequency relationship. In addition, the largest local calcium transient in two consecutive transients was measured to build a potential "best release" profile, which quantitatively confirmed discordance-induced Ca 2+ release impairment (DICRI). Diastolic Ca 2+ homeostasis was also observed to be disrupted by discordance, as late Ca 2+ release events elicited instability of resting Ca 2+ levels. Finally, the effects of two RyR2 inhibitors (VK-II-86 and dantrolene) were tested. While both compounds inhibited Ca 2+ wave generation, only VK-II-86 augmented subcellular discordance. Discordant Ca 2+ release is a quantifiable phenomenon, sensitive to pacing frequency, and impairs both systolic and diastolic Ca 2+ homeostasis. Interestingly, RyR2 inhibition can induce discordance, which should be considered when evaluating pharmacological RyR2 modulators for clinical use.