Abstract 1880: Molecular Basis of Inward-Rectifier Potassium Current Upregulation in Human Atrial Fibrillation: Contrasting Mechanisms for IK1 versus Constitutive IKACh
Introduction: Upregulation of Kir-based inward rectifiers IK1 and constitutively active acetylcholine-regulated current (IKACh,c) is a prominent contributor to atrial fibrillation (AF) maintenance. This study assessed occurrence and molecular basis of altered IK1 and IKACh,c in man.
Methods: Left atrial samples from mitral valve repair patients (n = 21 AF; n = 20 SR) were studied. Patch-clamp was applied to record currents. Immunoblots and confocal microscopy were used to study protein content and localization. Quantitative real-time PCR was performed to study microRNA and mRNA levels.
Results: IK1 and IKACh,c densities were greater in cells from AF than SR patients (at −120 mV: 3.5 ± 2.6 vs. 2.1 ± 1.1 pA/pF for IK1 and 1.3 ± 0.8 vs. 0.4 ± 0.3 pA/pF for IKACh,c). Kir2.1 protein (underlying IK1) was increased by AF (Fig.⇓) but Kir2.3 was unchanged. Kir2.1 mRNA was increased and Kir2.3 mRNA reduced. Kir3.1 and Kir3.4 (underlying IKACh,c) protein was unaffected. The inhibitory regulatory G-protein Gαi-3 was downregulated, potentially explaining increased free Gαγ and IKACh,c. No change in other regulatory proteins (RGS4, calmodulin) or subcellular channel and regulatory proteins localization occurred with AF. Kir2.1 protein expression is known to be downregulated by the microRNA miR-1: miR-1 expression was reduced by AF (Fig.⇓).
Conclusions: Both inward rectifier currents IK1 and IKACh,c are increased in human AF. IK1 changes are caused by increased protein expression through transcriptional and post-transcriptional modifications related to miR-1 downregulation, whereas the increase in IKACh,c is functional, possibly mediated by decreased inhibitory G-protein expression.