Suppression of dynamic Ca²⁺ transient responses to pacing in ventricular myocytes from mice with genetic calmodulin kinase II inhibition

The multifunctional Ca²⁺ and calmodulin-dependent protein kinase II (CaMKII) is important for regulating L-type Ca²⁺ current (I _Ca) and cytoplasmic Ca²⁺ (Ca²⁺_i) uptake and release from the sarcoplasmic reticulum (SR), key elements of the 'Ca²⁺-induced Ca²⁺ release' (CICR) mechanism. However, the effects of chronic CaMKII inhibition on Ca²⁺_i responses during CICR are unknown. We hypothesized that chronic CaMKII inhibition significantly affects CICR in ventricular myocytes. We studied CICR by simultaneously measuring Ca²⁺_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 Ca²⁺_i uptake into the SR Ca²⁺ 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 Ca²⁺_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 Ca²⁺ content and slow SR Ca²⁺ 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.