Marked
hypophosphatemia is common after major hepatic resection, but the pathophysiologic mechanism remains unknown. We used a partial
hepatectomy (PH) rat model to investigate the molecular basis of
hypophosphatemia. PH rats exhibited
hypophosphatemia and
hyperphosphaturia. In renal and intestinal brush-border membrane vesicles isolated from PH rats, Na(+)-dependent
phosphate (Pi) uptake decreased by 50%-60%. PH rats also exhibited significantly decreased levels of renal and intestinal Na(+)-dependent Pi transporter
proteins (NaPi-IIa [NaPi-4], NaPi-IIb, and NaPi-IIc).
Parathyroid hormone was elevated at 6 hours after PH.
Hyperphosphaturia persisted, however, even after thyroparathyroidectomy in PH rats. Moreover,
DNA microarray data revealed elevated levels of
nicotinamide phosphoribosyltransferase (Nampt)
mRNA in the kidney after PH, and
Nampt protein levels and total
NAD concentration increased significantly in the proximal tubules. PH rats also exhibited markedly increased levels of the Nampt substrate, urinary
nicotinamide (NAM), and NAM catabolites. In vitro analyses using opossum kidney cells revealed that NAM alone did not affect endogenous NaPi-4 levels. However, in cells overexpressing Nampt, the addition of NAM led to a marked decrease in cell surface expression of NaPi-4 that was blocked by treatment with FK866, a specific Nampt inhibitor. Furthermore, FK866-treated mice showed elevated renal Pi reabsorption and hypophosphaturia. These findings indicate that
hepatectomy-induced
hypophosphatemia is due to abnormal NAM metabolism, including Nampt activation in renal proximal tubular cells.