Phosphate depletion in opossum kidney cells: Apical but not basolateral ortransepithelial adaptions of Pi transport

Monolayers of opossum kidney (OK) cells are widely used as models for the r enal proximal tubule. OK cells adapt to phosphate (Pi) depletion by increas ing their capacity for apical and basolateral Nac-dependent Pi uptake. Beca use NMR-visible cell Pi was found to be decreased in Pi-deprived kidney cel ls, we suggested that up-regulation of basolateral Pi efflux also occurs du ring adaptation to Pi deprivation [American Journal of Physiol 1994;267:C91 5-919]. In order to test this hypothesis, we measured the cell Pi pool, bas olateral Pi efflux and transepithelial Pi fluxes in OK cells grown on perme able plastic filters, exposed overnight to solutions containing either 0.5 mM (deprived) or 2.0 mM (replete) Pi or (32)Pi. Following steady state or a cute loading with (32)Pi, the specific activity (SA) of cell Pi, the cell P i pool and the basolateral efflux of (32)Pi were measured. In the steady st ate, a 2-fold increase in Pi uptake sustained the intracellular Pi pool at 85% of the control level (30 +/-5 nmol/mg) in spite of a decrease in extrac ellular Pi from 2 to 0.5 mM When the extracellular Pi was acutely (1 h) red uced to 0.1 mM, the cell Pi pool decreased (to 3 +/-1 nmol/mg) both in cell s previously adapted overnight to either 0.5 or to 2 mM Pi (p >0.3). The ra tes of absolute and fractional basolateral washout of cell (32)Pi after 1h loading with 0.1 mM (32)Pi were similar in cells adapted to 0.5 compared to 2 mM Pi. This indicates that Pi depletion did not affect the effective per meability of the basolateral membranes to Pi. Adaptation for 16h to 0.5 com pared to 2 mM Pi did not alter the rate of net transepithelial transport of 0.1 mM Pi from the apical to the basal compartment but reduced (p < 0.05) the unidirectional fluxes of both (32)Pi and C-14-mannitol. An insufficient driving force (unchanged or low Pi concentration in the transport pool, lo w electrical or coupled-anion gradients) and a constant effective basolater al Pi permeability must have limited basolateral Pi efflux in cells exposed to 0.1 mM Pi. Thus, in OK cells grown on plastic support there are no adap tive increases in either basolateral Pi efflux, or in transcellular and par acellular Pi transport, in response to Pi depletion. Adaptations are limite d to increases in apical and basolateral sodium-dependent Pi uptakes that c an maintain the cell Pi pool as long as apical Pi is not too low (<greater than or equal to> 0.5 mM). The OK cells adapt to low Pi concentrations cons erving cell Pi but not increasing basolateral Pi efflux nor transepithelial Pi transport. Copyright (C) 2001 S. Karger AG, Basel.