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SERCA2a is the cardiac-specific isoform of Ca 2+ -ATPase of the sarcoplasmic reticulum (SR). A reduction of SERCA2a has been implicated in the contractile dysfunction of heart failure, and partial knockout of the SERCA2 gene ( Atp2a2 +/− mice) reiterated many of the features of heart failure. Yet, mice with a mutation of Atp2a2 , resulting in full suppression of the SERCA2a isoform and expression of the SERCA2b isoform only ( SERCA2 b/b ), showed only moderate functional impairment, despite a reduction by 40% of the SERCA2 protein levels. We examined in more detail the Ca 2+ handling in isolated cardiac myocytes from SERCA2 b/b . At 0.25 Hz stimulation, the amplitude of the [Ca 2+ ] i transients, SR Ca 2+ content, diastolic [Ca 2+ ] i , and density of I CaL were comparable between WT and SERCA2 b/b . However, the decline of [Ca 2+ ] i was slower (t 1/2 154±7 versus 131±5 ms; P 2+ ] i transient (eg, SR depletion), removed the differences in [Ca 2+ ] i decline. In contrast, increasing the Ca 2+ load revealed pronounced reduction of SR Ca 2+ uptake at high [Ca 2+ ] i . There was no increase in Na + -Ca 2+ exchange protein or function. Theoretical modeling indicated that in the SERCA2 b/b mouse, the higher Ca 2+ affinity of SERCA2b partially compensates for the 40% reduction of SERCA expression. The lack of SR depletion in the SERCA2 b/b may also be related to the absence of upregulation of Na + -Ca 2+ exchange. We conclude that for SERCA isoforms with increased affinity for Ca 2+ , a reduced expression level is better tolerated as Ca 2+ uptake and storage are impaired only at higher Ca 2+ loads.

The cardiac sarco(endo)plasmic reticulum Ca 2+ -ATPase gene ( ATP2A2 ) encodes the following two different protein isoforms: SERCA2a (muscle-specific) and SERCA2b (ubiquitous). We have investigated whether this isoform specificity is required for normal cardiac function. Gene targeting in mice successfully disrupted the splicing mechanism responsible for generating the SERCA2a isoform. Homozygous SERCA2a −/− mice displayed a complete loss of SERCA2a mRNA and protein resulting in a switch to the SERCA2b isoform. The expression of SERCA2b mRNA and protein in hearts of SERCA2a −/− mice corresponded to only 50% of wild-type SERCA2 levels. Cardiac phospholamban mRNA levels were unaltered in SERCA2a −/− mice, but total phospholamban protein levels increased 2-fold. The transgenic phenotype was characterized by a ≈20% increase in embryonic and neonatal mortality (early phenotype), with histopathologic evidence of major cardiac malformations. Adult SERCA2a −/− animals (adult phenotype) showed a reduced spontaneous nocturnal activity and developed a mild compensatory concentric cardiac hypertrophy with impaired cardiac contractility and relaxation, but preserved β-adrenergic response. Ca 2+ uptake levels in SERCA2a −/− cardiac homogenates were reduced by ≈50%. In isolated cells, relaxation and Ca 2+ removal by the SR were significantly reduced. Comparison of our data with those obtained in mice expressing similar cardiac levels of SERCA2a instead of SERCA2b indicate the importance of the muscle-specific SERCA2a isoform for normal cardiac development and for the cardiac contraction-relaxation cycle.