Abstract 5307: Cardiomyocyte Sodium Accumulation Promotes Diastolic Dysfunction in Serca2 Knockout Mice
Terminal decompensation during heart failure involves deterioration of diastolic function. We investigated the mechanisms underlying this functional decline in mice with cardiomyocyte-specific, conditional excision of the Serca2 gene (KO). At 4 weeks following gene deletion, SERCA levels in KO cardiomyocytes were reduced by more than 95% from flox-flox (FF) controls. Surprisingly, echocardiographic measurements indicated only moderate impairment of in vivo function, as systolic and diastolic tissue velocities were 62% and 72% of FF values, respectively. Diastolic heart failure developed in KO between 6 and 7 weeks, as diastolic tissue velocity rapidly declined to 51% of FF values. We compared cardiomyocyte contractions and Ca2+ cycling at the 4 and 7 week time points. In KO cells, contractions were reduced between 4 and 7 weeks (from 40% to 14% of FF values), and the rate of relaxation was slowed (from 11% to 3% of FF values). Similar alterations were observed in Ca2+ transients. Sarcoplasmic reticulum (SR) Ca2+ content was markedly reduced in 4-week KO, although a minute thapsigargin-sensitive SR Ca2+ release could be induced. SR content was further decreased in 7-week KO and SR Ca2+ release was not detectable, although Western blots showed no difference in SERCA levels between 4 and 7 week KO. Ca2+ influx via Ca2+ channels was enhanced in KO (integrated current ≈200% of FF) at both time points. However, greater NCX-mediated Ca2+ extrusion in 4-week KO was partially reversed in 7-week KO due to elevation in cytosolic [Na+] (34 mM vs 25 mM in FF). Normalizing cytosolic [Na+] using patch clamp increased the rate of decline of the Ca2+ transient in 7-week KO to 4-week KO values. Thus, KO mice compensate for loss of SR function by increasing trans-sarcolemmal Ca2+ flux. However, in the longer term, cytosolic Na+ accumulation impairs NCX-mediated Ca2+ extrusion, which promotes development of diastolic heart failure.