Opposite Changes of Ca2+ Wave Threshold and Fractional SR Ca2+ Release during SERCA Stimulation in Cardiomyocytes

Abstract

In cardiac muscle, PKA-dependent phosphorylation of 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 ([Ca2+]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 disinhibition. Ca2+ currents and transients, or cytosolic and intra-SR Ca2+ waves were simultaneously recorded with confocal line-scans in intact and permeabilized mouse cardiomyocytes with rhod-2 and fluo-5-N, respectively. We analyzed changes of several Ca2+ signaling parameters during specific SERCA stimulation by ochratoxin A (OTA) and jasmone. SERCA stimulation resulted in a substantial increase of Ca2+ wave thresholds (30±5.1%) and reduced fractional Ca2+ release. Faster Ca2+ wave decay and SR refilling confirmed SERCA acceleration. In patch-clamped myocytes, a decrease of fractional Ca2+ release together with a slowing of Ca2+ current inactivation and reduced EC-coupling gain was observed. A faster Ca2+ transient decay corroborated the pharmacological SERCA stimulation. These results suggest that SERCA stimulation alone can elevate the intra-SR threshold for the generation of Ca2+ waves, independently of RyR phosphorylation. Unexpectedly, this occurs without noticeably increasing spontaneous and triggered fractional Ca2+ release. This phenomenon could result from an intra-SR mechanism limiting CICR. Supported by SNF.