Abstract 496: Increased Trans-Sarcolemmal Calcium Flux and Myofilament Responsiveness in Mice With Cardiomyocyte-Specific Deletion of the Serca2 Gene
Heart failure is associated with reduced activity of the sarcoplasmic reticulum (SR) Ca2+ ATPase (SERCA) in cardiomyocytes. To investigate the contribution of loss of SERCA to the cardiac phenotype, we used a new genetically modified mouse which allows conditional cardiomyocyte-specific excision of the Serca2 gene. Four weeks after gene deletion SERCA2 knockout mice (KO) showed minimal signs of heart failure even though the expression level of SERCA2 protein was strongly reduced (<5% of controls). We examined the endogenous mechanisms that compensated for reduction in SERCA expression in isolated left ventricular cardiomyocytes. KO myocytes exhibited markedly reduced contraction magnitudes relative to flox-flox controls (FF) during 1 Hz field stimulation (KO=27% FF values, P<0.05). Relaxation was also much slower in KO cells (maximal relaxation rate =7% of FF). Parallel alterations were observed in Ca2+ homeostasis; Ca2+ transients (fluo-4 AM) were markedly smaller in KO than FF (FF=786 nM, KO=290 nM) and the rate of Ca2+ decline was reduced. Releasable SR Ca2+ content was decreased in KO to 27% of FF values. Comparison of transients in the absence and presence of caffeine indicated that SR Ca2+ release during each beat was minute in KO (FF=649 nM, KO=34 nM), and that the Ca2+ transient in KO was supported by increased trans-sarcolemmal Ca2+ flux. Indeed, integrated L-type Ca2+ current and outward Na+-Ca2+ exchange current were approximately double FF values in KO. Action potential duration at 20, 50, 70, and 90% repolarization were not different in FF and KO, as were the transient outward K+ current and IK1. Thus, increased trans-membrane Ca2+ cycling in KO was not further facilitated by alterations in action potential configuration. At a high stimulation rate (6 Hz), Ca2+ transient magnitude remained markedly reduced in KO (66 nM vs 290 nM in FF), although contraction magnitudes were similar in FF and KO. Simultaneous measurements of [Ca2+]i and cell length indicated greater fractional shortening / unit increase in [Ca2+]i in KO than FF (0.030 %nM−1 vs 0.016 %nM−1). Thus, increased myofilament responsiveness to Ca2+ together with greater cycling of Ca2+ across the sarcolemma effectively compensate for loss of SR function in SERCA2 KO myocytes.