Enhanced RyR2 Channel Activity but Reduced Ca2+ Spark Occurrence In Failing Mice Cardiomyocytes

Abstract

Alteration in intracellular calcium handling contributes to contractile dysfunction in heart failure (HF). In vitro lipid bilayer studies with reconstituted ryanodine receptor (RyR) reveal that these channels are more spontaneously active in HF compared to control.Here, spontaneous Ca2+ sparks have been characterized to evaluate RyR cluster activity in situ in a mice post-myocardial infarction (PMI) HF model. PMI mice showed reduced ejection fraction and their cardiomycytes were hypertrophied. Ca2+ transient amplitude and SR Ca2+ load were depressed, as well as the spontaneous Ca2+ spark frequency in intact cardiomycytes of PMI mice. However, both the fractional release and Ca2+ wave frequency were elevated, suggesting an increased RyR activity. In agreement, in permeabilized cardiomyocytes, which enabled internal solution controlling, Ca2+ sparks displayed a higher frequency in HF cells, whereas SR Ca2+ content were comparable under these conditions. To resolve the discrepancy, Ca2+ spark frequency in intact cells was normalized to SR Ca2+ load in each cell. While the reduced SR Ca2+ content in intact cells might account for the reduced Ca2+ spark frequency, it could also be attributable to the cytosolic environment in intact cells. In fact, HF is associated to metabolic changes that may affect RyR function. This was further strengthened by the observation that Ca2+ spark frequency was remarkably decreased by using an internal failing solution containing less ATP as found in HF models.Together, although the RyR channels isolated from failing heart may display more activity, our results suggest that in intact cells, concomitant alterations in intracellular media composition together with SR Ca2+ load are likely to mask those effects at the Ca2+ spark level. However, once one spark is generated, the probability of triggering an arrhythmogenic Ca2+ wave is elevated in PMI cardiomyocytes.