BackgroundCalmodulin (CaM) mutations have been identified recently in subjects with congenital long QT syndrome (LQTS) or catecholaminergic polymorphic ventricular tachycardia (CPVT), but the mechanisms responsible for these divergent arrhythmia‐susceptibility syndromes in this context are unknown. We tested the hypothesis that LQTS‐associated CaM mutants disrupt Ca2+ homeostasis in developing cardiomyocytes possibly by affecting either late Na current or Ca2+‐dependent inactivation of L‐type Ca2+ current.Methods and ResultsWe coexpressed CaM mutants with the human cardiac Na channel (NaV1.5) in tsA201 cells, and we used mammalian fetal ventricular cardiomyocytes to investigate LQTS‐ and CPVT‐associated CaM mutations (LQTS‐ and CPVT‐CaM). LQTS‐CaM mutants do not consistently affect L‐type Na current in heterologous cells or native cardiomyocytes, suggesting that the Na channel does not contribute to LQTS pathogenesis in the context of CaM mutations. LQTS‐CaM mutants (D96V, D130G, F142L) impaired Ca2+‐dependent inactivation, whereas the CPVT‐CaM mutant N54I had no effect on Ca2+‐dependent inactivation. LQTS‐CaM mutants led to loss of Ca2+‐transient entrainment with the rank order from greatest to least effect: CaM‐D130G~CaM‐D96V>>CaM‐F142L. This rank order follows measured Ca2+‐CaM affinities for wild‐type and mutant CaM. Acute isoproterenol restored entrainment for CaM‐130G and CaM‐D96V but caused irreversible cytosolic Ca2+ overload for cells expressing a CPVT‐CaM mutant.ConclusionsCaM mutations associated with LQTS may not affect L‐type Na+ current but may evoke defective Ca2+‐dependent inactivation of L‐type Ca2+ current.

Rationale: Myocardial infarction (MI) is a leading cause of death in the U.S. A non-contractile infarct compromises the overall mechanical function of the heart, reducing cardiac output and triggering decompensatory ventricular dilation. Rad GTPase, a member of the small GTPase RGK (Rem, Gem, Kir) family, is a calcium channel blocker that is endogenously expressed in the myocardium. We have previously shown that Rad deletion in mice results in increased Ca 2+ handling and a sustained non-pathological improvement in left ventricular function compared to wildtype. Hypothesis: Rad-ablation attenuates post-ischemic loss of function, resulting in reduced remodeling and improved long-term contractility. Methods and Results: We subjected Rad-deficient mice to ligation of the left anterior descending (LAD) coronary artery, and monitored cardiac function using echocardiography. We found that Rad deletion reduces both mortality and contractile dysfunction after MI, as well as ventricular dilation over five weeks. This improvement is also accompanied by preserved calcium handling in isolated myocytes. Histological and MRI examination of both ex vivo global ischemia and in vivo 24 hour LAD ligated myocardium revealed that initial infarct size is comparable between knockout and wildtype. We found that Rad loss reduced scar development and elongation independent of preserving tissue viability. Investigation of inflammatory pathways to account for this revealed increased expression of the anti-inflammatory protein thrombospondin accompanied by a reduction in neutrophil infiltration into the myocardium after MI. Conclusion: Rad deletion results in reduced cardiac remodeling, diminished myocardial inflammation, and improved contractile function after MI.