K⁺ currents in ventricular cardiomyocytes of p.N98S-calmodulin mutant mice

Missense mutations in calmodulin (CaM)-encoding genes are associated with life-threatening ventricular arrhythmia syndromes. Here, we investigated the role of cardiac K⁺ channel dysregulation in arrhythmogenic long QT syndrome (LQTS) using a knock-in mouse model heterozygous for a recurrent mutation (p.N98S) in the <i>Calm1</i> gene (<i>Calm1</i>^N98S/+). Single-cell patch-clamp technique and whole heart optical voltage mapping were used to assess action potentials and whole cell currents. Ventricular action potential duration (APD) at baseline was similar between genotypes. The β-adrenergic agonist isoproterenol prolonged APD in myocytes and isolated perfused hearts from <i>Calm1</i>^N98S/+, but not wild-type (<i>Calm1</i>^+/+), mice. Current density-voltage relationships for the small-conductance calcium-activated K⁺ (SK) current and the inward rectifier K⁺ current did not significantly differ between <i>Calm1</i>^+/+ and <i>Calm1</i>^N98S/+ ventricular cardiomyocytes ± isoproterenol. Peak densities of other voltage-gated K⁺ currents were significantly larger in <i>Calm1</i>^N98S/+ versus <i>Calm1</i>^+/+ cells at voltages ≥40 mV, both without and with isoproterenol. Isoproterenol reduced outward K_ATP currents more in <i>Calm1</i>^N98S/+ versus <i>Calm1</i>^+/+ myocytes. Dialysis of <i>Calm1</i>^+/+ cardiomyocytes with exogenous wild-type or N98S-CaM protein (5 µmol/L) via the pipette, respectively, increased and eliminated SK currents. The specific SK channel inhibitor apamin did not significantly alter the APD of <i>Calm1</i>^+/+ or <i>Calm1</i>^N98S/+ hearts ± isoproterenol. Thus, dysregulation of SK or voltage-gated K⁺ channels does not contribute to the β-adrenergic-induced LQTS of <i>Calm1</i>^N98S/+ mice, possibly because cardiomyocyte content of endogenous N98S-CaM and/or its affinity for CaM-binding domains may be too low to modulate channel properties. The larger K_ATP current inhibition by isoproterenol may delay <i>Calm1</i>^N98S/+ myocyte repolarization at low intracellular [ATP].<b>NEW & NOTEWORTHY</b> Despite in vitro and in silico evidence implicating cardiac K⁺ channel dysregulation in LQTS associated with missense mutations in genes-encoding calmodulin, their effects on native cardiac K⁺ currents are unknown. Using a knock-in mouse model harboring the p.N98S mutation in the <i>Calm1</i> gene, we found no evidence for dysregulation of major cardiac K⁺ channels. Although these data do not support mechanistic findings from heterologous systems, our finding impacts efforts to improve therapies for calmodulinopathies.