Background— Activation of cellular Ca²⁺ signaling molecules appears to be a fundamental step in the progression of cardiomyopathy and arrhythmias. Myocardial overexpression of the constitutively active Ca²⁺-dependent phosphatase calcineurin (CAN) causes severe cardiomyopathy marked by left ventricular (LV) dysfunction, arrhythmias, and increased mortality rate, but CAN antagonist drugs primarily reduce hypertrophy without improving LV function or risk of death.

Methods and Results— We found that activity and expression of a second Ca²⁺-activated signaling molecule, calmodulin kinase II (CaMKII), were increased in hearts from CAN transgenic mice and that CaMKII-inhibitory drugs improved LV function and suppressed arrhythmias. We devised a genetic approach to “clamp” CaMKII activity in CAN mice to control levels by interbreeding CAN transgenic mice with mice expressing a specific CaMKII inhibitor in cardiomyocytes. We developed transgenic control mice by interbreeding CAN transgenic mice with mice expressing an inactive version of the CaMKII-inhibitory peptide. CAN mice with CaMKII inhibition had reduced risk of death and increased LV and ventricular myocyte function and were less susceptible to arrhythmias. CaMKII inhibition did not reduce transgenic overexpression of CAN or expression of endogenous CaMKII protein or significantly reduce most measures of cardiac hypertrophy.

Conclusions— CaMKII is a downstream signal in CAN cardiomyopathy, and increased CaMKII activity contributes to cardiac dysfunction, arrhythmia susceptibility, and longevity during CAN overexpression.