Different effects of arsenate and phosphonoformate on Pitransport adaptation in opossum kidney cells

Abstract

The main nonhormonal mechanism for controlling inorganic phosphate (Pi) homeostasis is renal adaptation of the proximal tubular Pitransport 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 Piconcentration. We have studied how two competitive inhibitors of Pitransport, arsenate [As(V)] and phosphonoformate (PFA), affect adaptation to low and high Piconcentrations. OK cells show very high affinity for As(V) (inhibitory constant, Ki0.12 mM) when compared with the rat kidney. As(V) very efficiently reversed the adaptation of OK cells to low Pi(0.1 mM), whereas PFA induced adaptation similar to 0.1 mM Pi. Adaptation with 2 mM Pior As(V) was characterized by decreases in the maximal velociy ( Vmax) of Pitransport and an abundance of the NaPi-IIa Pitransporter in the plasma membrane, shown by the protein biotinylation. Conversely, PFA and 0.1 mM Piincreased the Vmaxand transporter abundance. Changes in the Vmaxwere limited to a 50% variation, which was not paralleled by changes in the concentration of Pior 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 Piconcentration. PFA blocks the uptake of Pibut is not transported, and it therefore simulates a low Piconcentration inside the cell. To conclude, a mathematical definition of the adaptation process is reported, thereby explaining the limited changes in Pitransport Vmax.