NPPA and Polyuria: However, significant up- or downregulation at various time points, beginning at 7 days post-injury, have been shown for kidney natriuretic peptide receptor-A (NPRA), atrial natriuretic peptide (ANP), kidney vasopressin-2 receptor (V2R), kidney aquaporin-2 (AQP2) channels, and kidney epithelial sodium channels (ENaC; β and γ but not α subunits), suggesting that both central and peripheral mechanisms are involved in the development and maintenance of polyuria.9,14,15 Together, these biomarkers are key for both water/sodium and cardiovascular homeostasis.