Phosphorylation of cardiac sodium channel at Ser571 anticipates manifestations of the aging myopathy

Diastolic dysfunction and delayed ventricular repolarization are typically observed in the elderly, but whether these defects are intimately associated with 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 (INa,L) in cardiomyocytes, raising the possibility that INa,L 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 INa,L. At ∼18 mo 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 INa,L inhibition. Prolonged repolarization and impaired LV filling occurred prematurely in adult (∼5 mo) 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 mo) 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 in 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 modulate the modality of myocyte relaxation, constituting the mechanism linking delayed ventricular repolarization and diastolic dysfunction.NEW & NOTEWORTHY We have investigated the impact of the late Na current (INa,L) on cardiac and myocyte function with aging by using genetically engineered animals with enhanced or stabilized INa,L, due to phosphomimetic or phosphoablated mutations of Nav1.5. Our findings support the notion that phosphorylation of Nav1.5 at Ser571 prolongs myocardial repolarization and impairs diastolic function, contributing to the manifestations of the aging myopathy.

The cAMP/PKA signaling pathway conditions cardiac performance in experimental animals with metabolic syndrome

Metabolic syndrome (MetS) increases the risk of coronary artery disease, but effects of this condition on the working myocardium remain to be fully elucidated. In the present study we evaluated the consequences of diet-induced metabolic disorders on cardiac function and myocyte performance using female mice fed with Western diet. Animals maintained on regular chow were used as control (Ctrl). Mice on the Western diet (WesD) had increased body weight, impaired glucose metabolism, preserved diastolic and systolic function, but increased left ventricular (LV) mass, with respect to Ctrl animals. Moreover, WesD mice had reduced heart rate variability (HRV), indicative of altered cardiac sympathovagal balance. Myocytes from WesD mice had increased volume, enhanced cell mechanics, and faster kinetics of contraction and relaxation. Moreover, levels of cAMP and protein kinase A (PKA) activity were enhanced in WesD myocytes, and interventions aimed at stabilizing cAMP/PKA abrogated functional differences between Ctrl and WesD cells. Interestingly, in vivo β-adrenergic receptor (β-AR) blockade normalized the mechanical properties of WesD myocytes and revealed defective cardiac function in WesD mice, with respect to Ctrl. Collectively, these results indicate that metabolic disorders induced by Western diet enhance the cAMP/PKA signaling pathway, a possible adaptation required to maintain cardiac function.