Phosphorylation of Cardiac Sodium Channel at Ser571 Anticipates Manifestations of the Aging Myopathy

Abstract

Diastolic dysfunction and delayed ventricular repolarization are typically observed in the elderly, but whether these defects are intimately associated in the progressive manifestation of the aging myopathy remains to be determined. In this regard, aging in experimental animals is coupled with increased late Na⁺ current (I_NaL) in cardiomyocytes, raising the possibility that I_NaL conditions the modality of electrical recovery and myocardial relaxation of the aged heart. For this purpose, aging male and female wild-type (WT) C57Bl/6 mice were studied together with genetically engineered mice with phosphomimetic (gain-of-function, GoF) or ablated (loss-of-function, LoF) mutations of the sodium channel Nav1.5 at Ser571 associated with, respectively, increased and stabilized I_NaL. At ~18 months (m) of age, WT mice developed prolonged duration of the QT interval of the electrocardiogram and impaired diastolic left ventricular (LV) filling, defects that were reversed by I_NaL inhibition. Prolonged repolarization and impaired LV filling occurred prematurely in adult (~5 m) GoF mutant mice, whereas these alterations were largely attenuated in aging LoF mutant animals. Ca²⁺ transient decay and kinetics of myocyte shortening/relengthening were delayed in aged (~24 m) WT myocytes, with respect to adult cells. In contrast, delayed Ca²⁺ transients and contractile dynamics occurred at adult stage in GoF myocytes and further deteriorated at old age. Conversely, myocyte mechanics were minimally affected in aging LoF cells. Collectively, these results document that Nav1.5 phosphorylation at Ser571 and the late Na⁺ current modulates the modality of myocyte relaxation, constituting the mechanism linking delayed ventricular repolarization and diastolic dysfunction.