Na /K ATPase-Ca 1.2 nanodomain differentially regulates intracellular [Na ], [Ca ] and local adrenergic signaling in cardiac myocytes.

Background: The intracellular Na concentration ([Na ] ) is a crucial but understudied regulator of cardiac myocyte function. The Na /K ATPase (NKA) controls the steady-state [Na ] and thereby determines the set-point for intracellular Ca . 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 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 -channel (Ca 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 , 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 in atrial myocytes. ß-adrenergic regulation of I is locally mediated in the NKAα1-Ca 1.2 nanodomain via A-kinase anchoring proteins.

Conclusions: NKAα1, Ca 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 concentrations in atrial and ventricular myocytes.