Effect of cadmium on Na-Pi cotransport kinetics in rabbit renal brush-border membrane vesicles

Chronic exposure to cadmium impairs various renal functions, including phosphate (Pi) transport. To further investigate the mechanism of cadmium-induced alterations in renal Pi transport, kinetics of Na+-dependent Pi uptake were studied in renal cortical brush-border membrane vesicles (BBMV) exposed to CdCl2 in vitro. BBMVs isolated from rabbit renal outer cortex were preincubated in a buffer containing CdCl2 (50 microM in most cases) for 60 min at 37 degrees C and then tested for Pi uptake at 25 degrees C. CdCl2 treatment resulted in a marked attenuation of Na+-dependent Pi uptake with no changes in Na+-independent Pi uptake and membrane permeability to Na+. CdMt treatment induced no changes in Pi transport. The inhibition required preincubation of vesicles with CdCl2 for more than 30 min and was not reversed by extravesicular EDTA, suggesting that cadmium affects the transport system at the internal side of the membrane. Kinetic analysis indicated that two sodium ions and one phosphate ion interact with a carrier, and this stoichiometry was not altered by cadmium treatment. Cadmium treatment did not change the apparent Km for Na+ (K(Na)) and that for phosphate (K(Pi)), but it markedly reduced the Vmax of the Na+-dependent Pi transport. These results indicate that exposure of proximal tubular brush border membranes to cadmium impairs the Pi transport capacity, probably by reducing the effective number of Na-Pi cotransporter units without altering substrate affinities of the carrier.