Abstract
BACKGROUND: The intracellular Na (+) concentration ([Na (+) ] (i) ) is a crucial but understudied regulator of cardiac myocyte function. The Na (+) /K (+) ATPase (NKA) controls the steady-state [Na (+) ] (i) and thereby determines the set-point for intracellular Ca (2+) . Here, we investigate the nanoscopic organization and local adrenergic regulation of the NKA macromolecular complex and how it differentially regulates the intracellular Na (+) and Ca (2+) homeostases in atrial and ventricular myocytes. METHODS: Multicolor STORM super-resolution microscopy, Western Blot analyses, and in vivo examination of adrenergic regulation are employed to examine the organization and function of Na (+) nanodomains in cardiac myocytes. Quantitative fluorescence microscopy at high spatiotemporal resolution is used in conjunction with cellular electrophysiology to investigate intracellular Na (+) homeostasis in atrial and ventricular myocytes. RESULTS: The NKAα1 (NKAα1) and the L-type Ca (2+) -channel (Ca (v) 1.2) form a nanodomain with a center-to center distance of ∼65 nm in both ventricular and atrial myocytes. NKAα1 protein expression levels are ∼3 fold higher in atria compared to ventricle. 100% higher atrial I (NKA) , produced by large NKA "superclusters", underlies the substantially lower Na (+) concentration in atrial myocytes compared to the benchmark values set in ventricular myocytes. The NKA's regulatory protein phospholemman (PLM) has similar expression levels across atria and ventricle resulting in a much lower PLM/NKAα1 ratio for atrial compared to ventricular tissue. In addition, a huge PLM phosphorylation reserve in atrial tissue produces a high ß-adrenergic sensitivity of I (NKA) in atrial myocytes. ß-adrenergic regulation of I (NKA) is locally mediated in the NKAα1-Ca (v) 1.2 nanodomain via A-kinase anchoring proteins. CONCLUSIONS: NKAα1, Ca (v) 1.2 and their accessory proteins form a structural and regulatory nanodomain at the cardiac dyad. The tissue-specific composition and local adrenergic regulation of this "signaling cloud" is a main regulator of the distinct global intracellular Na (+) and Ca (2+) concentrations in atrial and ventricular myocytes.