Endosomal Na+, K+/H+ antiporter.Mediates the electroneutral exchange of endosomal luminal H+ for a cytosolic Na+ or K+. By facilitating proton efflux, SLC9A6 counteracts the acidity generated by vacuolar (V)-ATPase, thereby limiting luminal acidification. Responsible for alkalizing and maintaining the endosomal pH, and consequently in, e.g., endosome maturation and trafficking of recycling endosomal cargo.