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 a 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 Calm1 gene (Calm1^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 Calm1^N98S/+, but not wild-type (Calm1^+/+), 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 Calm1^+/+ and Calm1^N98S/+ ventricular cardiomyocytes ± isoproterenol. Peak densities of other voltage-gated K⁺ currents were significantly larger in Calm1^N98S/+ versus Calm1^+/+ cells at voltages ≥40 mV both without and with isoproterenol. Isoproterenol reduced outward K_ATP currents more in Calm1^N98S/+ versus Calm1^+/+ myocytes. Dialysis of Calm1^+/+ 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 APD of Calm1^+/+ or Calm1^N98S/+ hearts ± isoproterenol. Thus, dysregulation of SK or voltage-gated K⁺ channels does not contribute to the β-adrenergic-induced LQTS of Calm1^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 Calm1^N98S/+ myocyte repolarization at low intracellular [ATP].