Changes in phosphate transporter activity evaluated by phosphonoformic acid binding in cadmium-exposed renal brush-border membranes

ABSTRACT

Direct exposure of renal tubular brush-border membranes (BBM) to free cadmium (Cd) causes a reduction in phosphate (Pi) transport capacity. Biochemical mechanism of this reduction was investigated in the present study. Renal proximal tubular brush-border membrane vesicles (BBMV) were isolated from rabbit kidney outer cortex by Mg precipitation method. Vesicles were exposed to 50~200 muM CdCl2 for 30 min, then the phosphate transporter activity was determined. The range of Cd concentration employed in this study was comparable to that of the unbound Cd documented in renal cortical tissues of Cd-exposed animals at the time of onset of renal dysfunction. The rate of sodium-dependent phosphate transport (Na+-Pi cotransport) by BBMV was determined by 32P-labeled inorganic phosphate uptake, and the number of Na+/-Pi cotransporters in the BBM was assessed by Pi-protectable 14C-labeled phosphonoformic acid ((14C)PFA) binding. The exposure of BBMV to Cd decreased the Na+-Pi cotransport activity in proportion to the Cd concentration in the preincubation medium, but it showed no apparent effect on the Pi-protectable PFA binding. These results indicate that an interaction of renal BBM with free Cd induces a reduction in Na+-Pi cotransport activity without altering the carrier density in the membrane. This, in turn, suggest that the suppression of phosphate transport capacity (Vmax) observed in Cd-treated renal BBM is due to a reduction in Na+-Pi translocation by existing carriers, possibly by Cd-induced fall in membrane fluidity.