Ca 2+ Uptake by the Sarcoplasmic Reticulum in Ventricular Myocytes of the SERCA2 b/b Mouse Is Impaired at Higher Ca 2+ Loads Only

SERCA2a is the cardiac-specific isoform of Ca2+ -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 ofAtp2a2 , 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 Ca2+ handling in isolated cardiac myocytes fromSERCA2 b/b . At 0.25 Hz stimulation, the amplitude of the [Ca2+ ]i transients, SR Ca2+ content, diastolic [Ca2+ ]i , and density ofI CaL were comparable between WT andSERCA2 b/b . However, the decline of [Ca2+ ]i was slower (t1/2 154±7 versus 131±5 ms;P <0.05). Reducing the amplitude of the [Ca2+ ]i transient (eg, SR depletion), removed the differences in [Ca2+ ]i decline. In contrast, increasing the Ca2+ load revealed pronounced reduction of SR Ca2+ uptake at high [Ca2+ ]i . There was no increase in Na+ -Ca2+ exchange protein or function. Theoretical modeling indicated that in theSERCA2 b/b mouse, the higher Ca2+ affinity of SERCA2b partially compensates for the 40% reduction of SERCA expression. The lack of SR depletion in theSERCA2 b/b may also be related to the absence of upregulation of Na+ -Ca2+ exchange. We conclude that for SERCA isoforms with increased affinity for Ca2+ , a reduced expression level is better tolerated as Ca2+ uptake and storage are impaired only at higher Ca2+ loads.