Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman.
Cardiac Na(+)-K(+)-ATPase (NKA) regulates intracellular Na(+), which in turn affects intracellular Ca(2+) and contractility via the Na(+)/Ca(2+) exchanger. Extracellular K(+) concentration ([K(+)]) is a central regulator of NKA activity. Phospholemman (PLM) has recently been recognized as a critical regulator of NKA in the heart. PLM reduces the intracellular Na(+) affinity of NKA, an effect relieved by PLM phosphorylation. Here we tested whether the NKA alpha(1)- vs. alpha(2)- isoforms have different external K(+) sensitivity and whether PLM and PKA activation affects the NKA affinity for K(+) in mouse cardiac myocytes. We measured the external [K(+)] dependence of the pump current generated by the ouabain-resistant NKA isoform in myocytes from wild-type (WT) mice (i.e., current due to NKA-alpha(1)) and mice in which the NKA isoforms have swapped ouabain affinities (alpha(1) is ouabain sensitive and alpha(2) is ouabain resistant) to assess current due to NKA-alpha(2). We found that NKA-alpha(1) has a higher affinity for external K(+) than NKA-alpha(2) [half-maximal pump activation (K(0.5)) = 1.5 +/- 0.1 vs. 2.9 +/- 0.3 mM]. The apparent external K(+) affinity of NKA was significantly lower in myocytes from WT vs. PLM-knockout mice (K(0.5) = 2.0 +/- 0.2 vs. 1.05 +/- 0.08 mM). However, PKA activation by isoproterenol (1 microM) did not alter the K(0.5) of NKA for external K(+) in WT myocytes. We conclude that 1) NKA-alpha(1) has higher affinity for K(+) than NKA-alpha(2) in cardiac myocytes, 2) PLM decreases the apparent external K(+) affinity of NKA, and 3) phosphorylation of PLM at the cytosolic domain does not alter apparent extracellular K(+) affinity of NKA.
Han
Fei
F
Department of Pathology, Northwestern University, Feinberg School of Chicago, Chicago, Illinois, USA.
Tucker
Amy L
AL
Lingrel
Jerry B
JB
Despa
Sanda
S
Bers
Donald M
DM
D000818:Animals; D005786:Gene Expression Regulation / Q000502:physiology; D007527:Isoenzymes; D008565:Membrane Proteins / Q000235:genetics / Q000378:metabolism*; D051379:Mice; D018345:Mice, Knockout; D032383:Myocytes, Cardiac / Q000201:enzymology*; D010750:Phosphoproteins / Q000235:genetics / Q000378:metabolism*; D010766:Phosphorylation; D011188:Potassium / Q000378:metabolism*; D011485:Protein Binding; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*
D007527:Isoenzymes; D008565:Membrane Proteins; D010750:Phosphoproteins; C069141:phospholemman; D012964:Sodium; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium