Functional coupling of alpha(2)-isoform Na(+)/K(+)-ATPase and Ca(2+) extrusion through the Na(+)/Ca(2+)-exchanger in...

Intracellular Na(+) concentration ([Na(+)]i) is an important regulator of Ca(2+) homeostasis in cardiomyocytes through its influence on the Na(+)/Ca(2+)-exchanger (NCX). [Na(+)]i is regulated by the Na(+)/K(+)-ATPase (NKA) of which two catalytic isoforms are expressed in cardiomyocytes, alpha(1) and alpha(2). Here we report that the NKA alpha(2)-isoform can modulate NCX mediated Ca(2+) transport by establishing a shared microdomain of Na(+) where [Na(+)]i can be different from that in the bulk cytosol. Ca(2+) transients and the corresponding NCX currents were induced by [Ca(2+)]i jumps via UV-flash photolysis of caged Ca(2+) (DM-nitrophen), while Ca(2+) release and uptake by the sarcoplasmic reticulum was blocked pharmacologically. We determined that specific inhibition of 55% of the alpha(2)-isoform was obtained with 0.3 microM ouabain, whereas 5 microM ouabain blocked approximately 95% of alpha(2)-isoform and in addition, approximately 8% of the alpha(1)-isoform. Specific inhibition of the alpha(2)-isoform caused slowing of Ca(2+) 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 alpha(2)-isoform in the "fuzzy space". The results suggest that regulation of NKA alpha(2)-isoform activity is crucial for the regulation of Ca(2+) homeostasis in cardiomyocytes.