Associates with and regulates the activity of the sodium/potassium-transporting ATPase (NKA) which transports Na(+) out of the cell and K(+) into the cell (PubMed:17077088). Reduces glutathionylation of the NKA beta-1 subunit ATP1B1, thus reversing glutathionylation-mediated inhibition of ATP1B1 (PubMed:21454534). Induces a hyperpolarization-activated chloride current when expressed in Xenopus oocytes (PubMed:7836447). {ECO:0000269|PubMed:17077088, ECO:0000269|PubMed:21454534, ECO:0000269|PubMed:7836447}. [Isoform 1]: Decreases the apparent K+ and Na+ affinity of the sodium/potassium-transporting ATPase over a large range of membrane potentials. . [Isoform 2]: Decreases the apparent K+ affinity of the sodium/potassium-transporting ATPase only at slightly negative and positive membrane potentials and increases the apparent Na+ affinity over a large range of membrane potentials. . This is the function of Ensembl gene identifier ENSG00000089356 (FXYD3, FXYD domain containing ion transport regulator 3).