Mir-133a-3p is the most abundant myocardial microRNA. The impact of mir-133a-3p on cardiac electrophysiology is poorly explored. In this study, we investigated the effects of mir-133a-3p on the main ionic currents critical for action potential (AP) generation and electrical activity of the heart. We used conventional ECG, sharp microelectrodes and patch-clamp to clarify a role of mir-133a-3p in normal cardiac electrophysiology in rats after in vivo and in vitro transfection. Mir-133a-3p caused no changes to pacemaker APs and automaticity in the sinoatrial node. No significant changes in heart rate (HR) were observed in vivo; however, miR transfection facilitated HR increase in response to β-adrenergic stimulation. Mir-133a-3p induced repolarization abnormalities in the atrial working myocardium and the L-type calcium current (I_Ca,L) was significantly increased. The main repolarization currents, including the transient outward (I_to), ultra-rapid (I_K,ur), and inward rectifier (I_K1) remained unaffected in atrial cardiomyocytes. Mir-133a-3p affected both I_Ca,L and I_to in ventricular cardiomyocytes. Systemic administration of mir-133a-3p induced QT-interval prolongation. Bioinformatic analysis revealed protein phosphatase 2 (PPP2CA/B) and Kcnd3 (encoding K_v4.3 channels generating I_to) as the main miR-133a-3p targets in the heart. No changes in mRNA expression of Cacna1c (encoding Ca_v1.2 channels generating I_Ca,L) and Kcnd3 were seen in mir-133a-3p treated rats. However, the expression of Ppp2cA, encoding PPP2CA, and Kcnip2 encoding KChIP2, a K_v4.3 regulatory protein, were significantly decreased. The accumulation of mir-133a-3p in cardiac myocytes causes chamber-specific electrophysiological changes. The suppression of PPP2CA, involved in adrenergic signal transduction, and Kchip2 may indirectly mediate mir-133a-3p-induced augmentation of I_Ca,L and attenuation of I_to.

A small group of specialized myocardial cells constitutes a natural dominant heart pacemaker in the so-called sinoatrial node (SAN). The SAN determines pro