Increment of late sodium currents in the left atrial myocytes and its potential contribution to increased susceptibility of atrial fibrillation in castrated male mice

Background

The incidence of atrial fibrillation (AF) is correlated with decreased levels of testosterone in elderly men. Late sodium current may exert a role in AF pathogenesis.

Objective

The purpose of this study was to explore the effect of testosterone deficiency on AF susceptibility and the therapeutic effect of late sodium current inhibitors in mice.

Methods

Male ICR mice (5 weeks old) were castrated to establish a testosterone deficiency model. One month after castration, dihydrotestosterone 5 mg/kg was administered subcutaneously for 2 months. Serum total testosterone level was assessed by enzyme-linked immunosorbent assay. High-frequency electrical stimulation was used to induce atrial arrhythmias. Whole-cell patch-clamp technique was used to for single-cell electrophysiologic study.

Results

Serum dihydrotestosterone levels of castration mice declined significantly but recovered with administration of exogenous dihydrotestosterone. In comparison with sham mice, the number of AF episodes significantly increased by 13.5-fold, AF rate increased by 3.75-fold, and AF duration prolonged in castrated mice. Dihydrotestosterone administration alleviated the occurrence of AF. Action potential duration at both 50% and 90% repolarization were markedly increased in castrated mice compared to sham controls. The late sodium current was enhanced in castrated male mice. These alterations were alleviated by treatment with dihydrotestosterone. Systemic application of the I_Na-L inhibitors ranolazine, eleclazine, and GS967 inhibited the occurrence of AF in castrated mice.

Conclusion

Testosterone deficiency contributed to the increased late sodium current, prolonged action potential repolarization, and increased susceptibility to AF. Blocking of late sodium current is beneficial against the occurrence of AF in castrated mice.

Introduction

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia encountered in clinical practice and represents a major cause of morbidity and mortality. Testosterone deficiency seems to be a causal factor for the increased propensity for AF. The risk of AF incidence increases with increment in age, which is accompanied by a decline in testosterone.¹ In patients with lone AF, the levels of testosterone were lower than those of normal controls.² Moreover, a recent study showed that testosterone is associated with the incidence of AF in a cohort of men. In men older than 80 years, testosterone deficiency is strongly associated with AF risk.³

The causal effect of testosterone deficiency on AF has also been verified in animal studies. Tsuneda et al⁴ showed that castration of male rats increased atrial arrhythmogenicity of Langendorff-perfused hearts that was abolished by administration of testosterone. The increased AF susceptibility of castrated rats was attributed to up-regulation of the ryanodine receptor type 2 and the sodium–calcium exchanger.⁴ In another study, Tsai et al⁵ evaluated the effects of androgen receptor knockout (ARKO) on atrial electrophysiology and arrhythmogenesis. They found that the left atria of ARKO mice had a lower negative resting membrane potential and a >90% action potential duration (APD) than wild-type control by conventional microelectrode technique, with the findings explained by reduced expression of Kir2.1, Kir3.1, and Kv7.1. To date, the functional alteration of ion channels and its contribution to AF susceptibility on testosterone deficiency remain unclear.

The late component of sodium current (I_Na-L) is a depolarizing current that has been shown to participate in the development of AF.6, 7 Activation of I_Na-L increases atrial APD.⁸ In a cardiac hypertrophy model of rabbits, higher density of I_Na-L was recorded in the left atrial myocytes, accompanied by the occurrence of early afterdepolarizations (EADs) and spontaneous automaticity (SA). Blockade of I_Na-L abolished all atrial EADs and SA.⁷ Sossalla et al found that I_Na-L was significantly increased in isolated atrial myocytes from AF patients than in those with sinus rhythm.⁶ Administration of the selective I_Na-L blockers ranolazine, eleclazine, and GS967 dramatically inhibited the occurrence of atrial arrhythmias.6, 9, 10 However, the role of I_Na-L in the testosterone deficiency–related AF remains unclear. In the current study, we explored the susceptibility of AF in a mouse model of testosterone deprivation by castration in vivo and the alteration of I_Na-L in isolated left atrial myocytes. The therapeutic effects of testosterone replacement and I_Na-L blocking on AF in castrated mice were also evaluated.