Suppression of dynamic Ca(2+) transient responses to pacing in ventricular myocytes from mice with genetic calmodulin kinase II inhibition

The multifunctional Ca(2+) and calmodulin-dependent protein kinase II (CaMKII) is important for regulating L-type Ca(2+) current (I(Ca)) and cytoplasmic Ca(2+) (Ca(2+)(i)) uptake and release from the sarcoplasmic reticulum (SR), key elements of the 'Ca(2+)-induced Ca(2+) release' (CICR) mechanism. However, the effects of chronic CaMKII inhibition on Ca(2+)(i) responses during CICR are unknown. We hypothesized that chronic CaMKII inhibition significantly affects CICR in ventricular myocytes. We studied CICR by simultaneously measuring Ca(2+)(i) transients and I(Ca) in voltage-clamped ventricular myocytes isolated from a recently developed genetic mouse model of cardiac CaMKII inhibition. These measurements were repeated in ventricular myocytes from novel mice with cardiac CaMKII inhibition lacking phospholamban (PLN), a known CaMKII substrate and a negative regulator of Ca(2+)(i) uptake into the SR Ca(2+) store. CaMKII inhibition eliminated a pattern of I(Ca) increases called facilitation and significantly reduced beat-to-beat and cell-to-cell variability of peak Ca(2+)(i) transients in ventricular myocytes with PLN. PLN ablation eliminated I(Ca) facilitation even in the absence of CaMKII inhibition and the effects of CaMKII inhibition to reduce SR Ca(2+) content and slow SR Ca(2+) uptake were lost in the absence of PLN. PLN ablation significantly reduced I(Ca) beat-to-beat variability in cells with CaMKII inhibition. These findings show that chronic CaMKII inhibition reduces variability of CICR responses in a manner that is partly dependent on the presence of PLN.