Na ⁺ /K ⁺ ATPase-Ca _v 1.2 nanodomain differentially regulates intracellular [Na ⁺ ], [Ca ²⁺ ] and local adrenergic signaling in cardiac myocytes

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.