Functional coupling of α2-isoform Na+/K+-ATPase and Ca2+ extrusion through the Na+/Ca2+-exchanger in cardiomyocytes

Intracellular Na+ concentration ([Na+]i) is an important regulator of Ca2+ homeostasis in cardiomyocytes through its influence on the Na+/Ca2+-exchanger (NCX). [Na+]i is regulated by the Na+/K+-ATPase (NKA) of which two catalytic isoforms are expressed in cardiomyocytes, α1 and α2. Here we report that the NKA α2-isoform can modulate NCX mediated Ca2+ transport by establishing a shared microdomain of Na+ where [Na+]i can be different from that in the bulk cytosol. Ca2+ transients and the corresponding NCX currents were induced by [Ca2+]i jumps via UV-flash photolysis of caged Ca2+ (DM-nitrophen), while Ca2+ release and uptake by the sarcoplasmic reticulum was blocked pharmacologically. We determined that specific inhibition of 55% of the α2-isoform was obtained with 0.3 μM ouabain, whereas 5 μM ouabain blocked ≈95% of α2-isoform and in addition, ≈8% of the α1-isoform. Specific inhibition of the α2-isoform caused slowing of Ca2+ extrusion by the NCX. However, this NCX inhibition could not be explained by an increase in global [Na+]i as assessed by the Na+ indicator SBFI, but rather by Na+ microdomains built up by the NKA α2-isoform in the “fuzzy space”. The results suggest that regulation of NKA α2-isoform activity is crucial for the regulation of Ca2+ homeostasis in cardiomyocytes.