Stabilization of Ca 2+ signaling in cardiac muscle by stimulation of SERCA.

AIMS: In cardiac muscle, phosphorylation of the RyRs is proposed to increase their Ca2+ sensitivity. This mechanism could be arrhythmogenic via facilitation of spontaneous Ca2+ waves. Surprisingly, the level of Ca2+ inside the SR needed to initiate such waves has been reported to increase upon β-adrenergic stimulation, an observation which cannot be easily reconciled with elevated Ca2+ sensitivity of the RyRs. We tested the hypothesis that this change of Ca2+ wave threshold could occur indirectly, subsequent to SERCA stimulation.

METHODS AND RESULTS: Cytosolic and intra-SR Ca2+ waves were simultaneously recorded with confocal line-scan imaging in intact and permeabilized mouse cardiomyocytes using Rhod-2 and Fluo-5-N, respectively. We analyzed changes of several Ca2+ signaling parameters during specific SERCA stimulation by ochratoxin A (OTA), jasmonate or the Fab fragment of a phospholamban antibody. SERCA stimulation resulted in a substantial increase of the threshold for Ca2+ wave initiation. Faster Ca2+ transient decay and SR refilling confirmed SERCA acceleration.

CONCLUSIONS: These results suggest that isolated SERCA stimulation can elevate the intra-SR threshold for the generation of Ca2+ waves, independently of RyR phosphorylation. Simultaneously, fractional Ca2+ release and wave amplitudes are reduced. Thus, SERCA stimulation appears to exert a negative feed-back on the Ca2+ -induced Ca2+ release mechanisms sustaining the waves. Thereby, it may be profoundly antiarrhythmic. This may be clinically relevant when therapies are applied to stimulate the SERCA activity (e.g. SERCA overexpression with gene therapy, future small molecule SERCA stimulators).