TRANSEPITHELIAL PHOSPHATE-TRANSPORT IN RABBIT PROXIMAL TUBULAR CELLS ADAPTED TO PHOSPHATE DEPRIVATION

Both renal and nonrenal cells in culture adapt to deprivation of P-i b y increasing Na-dependent P-i uptake. We studied whether this change i n uptake is reflected in an increased renal transepithelial P-i transp ort. We grew primary cultures of rabbit renal cortical cells in plasti c flasks and subcultured them onto Millicell-HA filters. This produced cell monolayers, which structurally and functionally resembled proxim al tubule. These cells performed Na-dependent net transepithelial tran sport of P-32(i) in the apical-to-basolateral direction that was inhib ited by phosphonoformic acid in the apical fluid or by ouabain in the basolateral fluid or by preincubation with parathyroid hormone. Overni ght incubation at low P-i concentrations led to a progressive increase in 5-min Na-dependent P-i uptake into cell monolayers. Na-dependent P -i uptake was threefold higher following overnight incubation at 25 mu M P-i, compared with 3 mM P-i, and the increase was one-half maximal with incubation at an extracellular P-i concentration ([P-i]) of 300 m u M. This was associated with a decrease in Na-dependent transepitheli al P-i flux to the basolateral fluid by the same cells, which fell dra matically following incubation at less than or equal to 300 mu M P-i. There was no change in Na-dependent uptake or transepithelial transpor t of L-glutamine. This adaptation to P-i deprivation in vitro appears to serve to restore depleted cell stores of P-i rather than to regulat e transepithelial P-i transport.