The pathogenesis of renal
phosphate (Pi) leak in
Fanconi syndrome is unknown. Disorders of apical
membrane transporters, leaky apical membrane, depleted cellular Pi and
ATP, and impaired
sodium (Na) pumps have been proposed as underlying defects. The present study examined the role of type II Na-Pi cotransport system (NaPi-2) in experimental
Fanconi syndrome in rats. Following a single injection of
maleic acid (MA), 75 mg/kg
body weight i.p., rats were sacrificed after 90 min, 4 h, and 24 h. Renal cortical expression of NaPi-2
mRNA was determined by Northern blotting, and brush border membrane (BBM) NaPi-2
protein by Western blotting. Increased urinary excretion of
phosphate was demonstrated as soon as 90 min after MA injection, and was sustained at 4 and 24 h, NaPi-2
mRNA expression and NaPi-2
protein were not decreased after 90 min. NaPi-2
mRNA decreased after 4 h, while NaPi-2
protein decreased only at 24 h. Hence, the immediate
phosphaturia in experimental
Fanconi syndrome may be independent of NaPi-2 downregulation, possibly resulting from energy depletion or membrane dysfunction. The decrease in NaPi-2
mRNA expression and the subsequent NaPi-2
protein decrease may account for the second-phase
phosphaturia.