Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman

Han F, Tucker AL, Lingrel JB, Despa S, Bers DM. Extracellular potassium dependence of the Na+-K+-ATPase in cardiac myocytes: isoform specificity and effect of phospholemman. Am J Physiol Cell Physiol 297: C699-C705, 2009. First published July 1, 2009; doi:10.1152/ajpcell.00063.2009.-Cardiac Na+-K+-ATPase (NKA) regulates intracellular Na+, which in turn affects intracellular Ca2+ and contractility via the Na+/Ca2+ 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 mu M) 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.