Cardiac signaling cloud regulates intracellular [Na+], [Ca2+] and adrenergic signaling in atrial myocytes.

Abstract

An important but often overlooked "silent" partner of atrial myocyte function is intracellular Na+ concentration ([Na+]i). We report here on four surprising and inter-related discoveries concerning [Na+]i in atrial myocytes. (1). Using highly sensitive, calibrated fluorescence measurements we report that the heart rate-dependent [Na+]i is significantly lower (∼40%) in atrial myocytes than the benchmark values reported for ventricular myocytes. (2). Na+/K+ ATPase (NKA), the main cytosolic Na+ extrusion pump, has much higher protein expression levels (3 X higher) in atrial tissue and NKA current densities (2 X higher) than expected in atrial myocytes. (3). NKA is exquisitely sensitive to ß-adrenergic activation in atrial tissue due to a huge phosphorylation capacity of its regulatory protein phospholemman (PLM). This enhances the high baseline Na+ extrusion rate in atrial myocytes during adrenergic stress. (4). Two-color super-resolution microscopy (dSTORM) reveals a novel ion channel nanodomain in atrial and ventricular myocytes that is formed by the co-clustering of NKA (NKAα1) with the L-type Ca2+ channel (Cav1.2) at the cardiac dyad. In atrial myocytes, each signaling nanodomain consists of large NKAα1 and Cav1.2 "superclusters". Functional studies further reveal that the regulatory proteins associated with NKAα1 and Cav1.2 locally regulate channel function in a proximity-dependent "signaling cloud" that is in direct apposition to ryanodine receptor (RyR2) clusters. Taken together these atrial Na+ signaling features provide robust signaling with an increased margin of safety for a system with known arrhythmogenic vulnerability. Finally, the novel cardiac NKAα1-Cav1.2 signaling cloud localizes the Na+ pump to the cardiac dyad, where it contributes to the regulation of the subsarcolemmal [Na+]i and Ca2+ concentration.