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 < 0.001). Intraperitoneal administration of a similar dose of maleic acid decreased the BBMV uptake of P_i 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 P_i persisted. In chronic parathyroidectomized rats fed a low-phosphate diet, where the filtered load of P_i was higher than in the previous groups, the phosphaturia was abolished, but the inhibition of the BBMV transport of P_i was sustained. Both the in vitro incubation of BBMVV and in vivo administration of maleic acid were associated with a competitive inhibition of P_i transport. These studies indicate that the maleic acid-induced phosphaturia is expressed at the apical membrane entry step of P_i, and the enhanced distal tubular reabsorption accounts for the lack of phosphaturia in dietary P_i deprivation.