Cardiac Na⁺/K⁺-ATPase (NKA) regulates intracellular Na⁺, which in turn affects intracellular Ca²⁺ and contractility via the Na⁺/Ca²⁺ exchanger. Extracellular [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 1 vs. 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 WT mice (i.e. current due to NKA 1) and mice in which the NKA isoforms have swapped ouabain affinities (1 is ouabain-sensitive and 2 is ouabain-resistant) to assess current due to NKA 2. We found that NKA 1 has a higher affinity for external K⁺ than NKA 2 (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-KO mice (K_0.5=2.0 ±0.2 mM vs. 1.05 ±0.08 mM). However, PKA activation by isoproterenol (1 µM) did not alter the K_0.5 of NKA for external K⁺ in WT myocytes. We conclude that (1) NKA 1 has higher affinity for K⁺ than NKA 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.