Intracellular Dyssynchrony of Diastolic Cytosolic [Ca2+] Decay in Ventricular Cardiomyocytes in Cardiac Remodeling and Human Heart Failure

Rationale: Synchronized release of Ca2+ into the cytosol during each cardiac cycle determines cardiomyocyte contraction. Objective: We investigated synchrony of cytosolic [Ca2+] decay during diastole and the impact of cardiac remodeling. Methods and Results: Local cytosolic [Ca2+] transients (1-mu m intervals) were recorded in murine, porcine, and human ventricular single cardiomyocytes. We identified intracellular regions of slow (slowCaR) and fast (fastCaR) [Ca2+] decay based on the local time constants of decay (TAU(local)). The SD of TAU(local) as a measure of dyssynchrony was not related to the amplitude or the timing of local Ca2+ release. Stimulation of sarcoplasmic reticulum Ca2+ ATPase with forskolin or istaroxime accelerated and its inhibition with cyclopiazonic acid slowed TAU(local) significantly more in slowCaR, thus altering the relationship between SD of TAU(local) and global [Ca2+] decay (TAU(global)). Na+/Ca2+ exchanger inhibitor SEA0400 prolonged TAU(local) similarly in slowCaR and fastCaR. FastCaR were associated with increased mitochondrial density and were more sensitive to the mitochondrial Ca2+ uniporter blocker Ru360. Variation in TAU(local) was higher in pig and human cardiomyocytes and higher with increased stimulation frequency (2 Hz). TAU(local) correlated with local sarcomere relengthening. In mice with myocardial hypertrophy after transverse aortic constriction, in pigs with chronic myocardial ischemia, and in end-stage human heart failure, variation in TAU(local) was increased and related to cardiomyocyte hypertrophy and increased mitochondrial density. Conclusions: In cardiomyocytes, cytosolic [Ca2+] decay is regulated locally and related to local sarcomere relengthening. Dyssynchronous intracellular [Ca2+] decay in cardiac remodeling and end-stage heart failure suggests a novel mechanism of cellular contractile dysfunction.