Different effects of arsenate and phosphonoformate on P-i transport adaptation in opossum kidney cells

Villa-Bellosta R, Sorribas V. Different effects of arsenate and phosphonoformate on P-i transport adaptation in opossum kidney cells. Am J Physiol Cell Physiol 297: C516-C525, 2009. First published June 24, 2009; doi:10.1152/ajpcell.00186.2009.-The main nonhormonal mechanism for controlling inorganic phosphate (P-i) homeostasis is renal adaptation of the proximal tubular P-i transport rate to changes in dietary phosphate content. Opossum kidney (OK) cell line is an in vitro renal model that maintains the ability of renal adaptation to the extracellular P-i concentration. We have studied how two competitive inhibitors of P-i transport, arsenate [As(V)] and phosphonoformate (PFA), affect adaptation to low and high P-i concentrations. OK cells show very high affinity for As(V) (inhibitory constant, K-i 0.12 mM) when compared with the rat kidney. As(V) very efficiently reversed the adaptation of OK cells to low P-i (0.1 mM), whereas PFA induced adaptation similar to 0.1 mM P-i. Adaptation with 2 mM P-i or As(V) was characterized by decreases in the maximal velociy (V-max) of P-i transport and an abundance of the NaPi-IIa P-i transporter in the plasma membrane, shown by the protein biotinylation. Conversely, PFA and 0.1 mM P-i increased the Vmax and transporter abundance. Changes in the Vmax were limited to a 50% variation, which was not paralleled by changes in the concentration of P-i or of the inhibitor. OK cells are very sensitive to As(V), but the effects are reversible and noncytotoxic. These effects can be interpreted as As(V) being transported into the cell, thereby mimicking a high P-i concentration. PFA blocks the uptake of P-i but is not transported, and it therefore simulates a low P-i concentration inside the cell. To conclude, a mathematical definition of the adaptation process is reported, thereby explaining the limited changes in P-i transport V-max.