Calmodulin (CaM) regulates Na⁺ channel gating through binding to an IQ-like motif in the C-terminus. Ca²⁺/CaM-dependent protein kinase II (CaMKII) regulates Ca²⁺ handling, and chronic overactivity of CaMKII is associated with left ventricular hypertrophy and dysfunction and lethal arrhythmias. However, the acute effects of Ca²⁺/CaM and CaMKII on cardiac Na⁺ channels are not fully understood.

Purified Na_V1.5–glutathione-S-transferase fusion peptides were phosphorylated in vitro by CaMKII predominantly on the I–II linker. Whole-cell voltage-clamp was used to measure Na⁺ current (I_Na) in isolated guinea-pig ventricular myocytes in the absence or presence of CaM or CaMKII in the pipette solution. CaMKII shifted the voltage dependence of Na⁺ channel availability by ≈+5 mV, hastened recovery from inactivation, decreased entry into intermediate or slow inactivation, and increased persistent (late) current, but did not change I_Na decay. These CaMKII-induced changes of Na⁺ channel gating were completely abolished by a specific CaMKII inhibitor, autocamtide-2-related inhibitory peptide (AIP). Ca²⁺/CaM alone reproduced the CaMKII-induced changes of I_Na availability and the fraction of channels undergoing slow inactivation, but did not alter recovery from inactivation or the magnitude of the late current. Furthermore, the CaM-induced changes were also completely abolished by AIP. On the other hand, cAMP-dependent protein kinase A inhibitors did not abolish the CaM/CaMKII-induced alterations of I_Na function.

Ca²⁺/CaM and CaMKII have distinct effects on the inactivation phenotype of cardiac Na⁺ channels. The differences are consistent with CaM-independent effects of CaMKII on cardiac Na⁺ channel gating.