Effects of maleic acid on renal phosphorus transport: role of dietary phosphorus.

The effects of maleic acid on renal phosphate (Pi) transport were examined by clearance and brush-border membrane vesicle (BBMV) transport studies. In normal rats, maleic acid 50 mg.kg body wt-1.h-1 increased the phosphaturia (P less than 0.001). Intraperitoneal administration of a similar dose of maleic acid decreased the BBMV uptake of Pi but not glucose. In rats fed a low-phosphate diet (0.03%), the maleic acid-induced phosphaturia was blunted, but the inhibitory effect on the BBMV transport of Pi persisted. In chronic parathyroidectomized rats fed a low-phosphate diet, where the filtered load of Pi was higher than in the previous groups, the phosphaturia was abolished, but the inhibition of the BBMV transport of Pi was sustained. Both the in vitro incubation of BBMVV and in vivo administration of maleic acid were associated with a competitive inhibition of Pi transport. These studies indicate that the maleic acid-induced phosphaturia is expressed at the apical membrane entry step of Pi, and the enhanced distal tubular reabsorption accounts for the lack of phosphaturia in dietary Pi deprivation.