Altered Na+/Ca2+-exchanger activity due to downregulation of Na+/K+-ATPase alpha2-isoform in heart failure.
OBJECTIVE
The Na+/K+-ATPase (NKA) alpha2-isoform is preferentially located in the t-tubules of cardiomyocytes and is functionally coupled to the Na+/Ca(+-exchanger (NCX) and Ca2+ regulation through intracellular Na+ concentration ([Na+]i). We hypothesized that downregulation of the NKA alpha2-isoform during congestive heart failure (CHF) disturbs the link between Na+ and Ca2+, and thus the control of cardiomyocyte contraction.


RESULTS
NKA isoform and t-tubule distributions were studied using immunocytochemistry, confocal and electron microscopy in a post-infarction rat model of CHF. Sham-operated rats served as controls. NKA and NCX currents (I NKA and I NCX) were measured and alpha2-isoform current (I NKA,alpha2) was separated from total I NKA using 0.3 microM ouabain. Detubulation of cardiomyocytes was performed to assess the presence of alpha2-isoforms in the t-tubules. In CHF, the t-tubule network had a disorganized appearance in both isolated cardiomyocytes and fixed tissue. This was associated with altered expression patterns of NKA alpha1- and alpha2-isoforms. I NKA,alpha2 density was reduced by 78% in CHF, in agreement with decreased protein expression (74%). When I NKA,alpha2 was blocked in Sham cardiomyocytes, contractile parameters converged with those observed in CHF. In Sham, abrupt activation of I NKA led to a decrease in I NCX, presumably due to local depletion of [Na+]i in the vicinity of NCX. This decrease was smaller when the alpha2-isoform was downregulated (CHF) or inhibited (ouabain), indicating that the alpha2-isoform is necessary to modulate local [Na+]i close to NCX.


CONCLUSIONS
Downregulation of the alpha2-isoform causes attenuated control of NCX activity in CHF, reducing its capability to extrude Ca2+ from cardiomyocytes.
Swift
Fredrik
F
Institute for Experimental Medical Research, Ullevaal University Hospital, Kirkeveien 166, N-0407 Oslo, Norway. fredrik.swift@medisin.uio.no
Birkeland
Jon Arne Kro
JA
Tovsrud
Nils
N
Enger
Ulla H
UH
Aronsen
Jan Magnus
JM
Louch
William E
WE
Sjaastad
Ivar
I
Sejersted
Ole M
OM
D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects; D003599:Cytoskeleton / Q000378:metabolism; D004195:Disease Models, Animal; D015536:Down-Regulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006333:Heart Failure / Q000201:enzymology* / Q000209:etiology / Q000473:pathology / Q000503:physiopathology; D007150:Immunohistochemistry; D008297:Male; D008564:Membrane Potentials; D018613:Microscopy, Confocal; D008854:Microscopy, Electron; D009119:Muscle Contraction / Q000187:drug effects; D009203:Myocardial Infarction / Q000150:complications* / Q000201:enzymology / Q000473:pathology / Q000503:physiopathology; D032383:Myocytes, Cardiac / Q000187:drug effects / Q000201:enzymology* / Q000648:ultrastructure; D010042:Ouabain / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism*; D019831:Sodium-Calcium Exchanger / Q000378:metabolism*; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors / Q000378:metabolism*
D004791:Enzyme Inhibitors; D019831:Sodium-Calcium Exchanger; D010042:Ouabain; D012964:Sodium; C499075:Atp1a1 protein, rat; C498532:Atp1a2 protein, rat; D000254:Sodium-Potassium-Exchanging ATPase