Background The sinus node (SN) is the primary pacemaker of the heart. SN myocytes possess distinctive action potential morphology with spontaneous diastolic depolarization because of a unique expression of ion channels and Ca 2+ ‐handling proteins. MicroRNAs (miRs) inhibit gene expression. The role of miRs in controlling the expression of genes responsible for human SN pacemaking and conduction has not been explored. The aim of this study was to determine miR expression profile of the human SN as compared with that of non‐pacemaker atrial muscle. Methods and Results SN and atrial muscle biopsies were obtained from donor or post‐mortem hearts (n=10), histology/immunolabeling were used to characterize the tissues, TaqMan Human MicroRNA Arrays were used to measure 754 miRs, Ingenuity Pathway Analysis was used to identify miRs controlling SN pacemaker gene expression. Eighteen miRs were significantly more and 48 significantly less abundant in the SN than atrial muscle. The most interesting miR was miR‐486‐3p predicted to inhibit expression of pacemaking channels: HCN1 (hyperpolarization‐activated cyclic nucleotide‐gated 1), HCN4, voltage‐gated calcium channel (Ca v )1.3, and Ca v 3.1. A luciferase reporter gene assay confirmed that miR‐486‐3p can control HCN4 expression via its 3′ untranslated region. In ex vivo SN preparations, transfection with miR‐486‐3p reduced the beating rate by ≈35±5% ( P <0.05) and HCN4 expression ( P <0.05). Conclusions The human SN possesses a unique pattern of expression of miRs predicted to target functionally important genes. miR‐486‐3p has an important role in SN pacemaker activity by targeting HCN4, making it a potential target for therapeutic treatment of SN disease such as sinus tachycardia.

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The sinus node (SN) is the heart’s primary pacemaker and has a unique expression of pacemaking ion channels and immune cell markers. The role of microribonucleic acids (miRNAs) in control of ion channels and immune function of the sinus node is not well understood. We have recently shown that hsa-miR-486-3p downregulates the main pacemaking channel HCN4 in the SN. In addition, we recently demonstrated that immune cells are significantly more abundant in the SN compared to the right atrium. The aim of this study was to validate the previously predicted interactions between miRNAs and mRNAs of key Ca2+ ion channels (involved in peacemaking) and mRNA of TPSAB1—(a mast cells marker) using luciferase assay. We now show that miR-486 significantly downregulates Cav1.3, Cav3.1, and TPSAB1-mediated luciferase activity, while miR-938 significantly downregulates only TPSAB1-mediated luciferase activity. This makes miR-486-3p a potential therapeutic target in the treatment of SN dysfunctions.