Adaptation of phosphate transport to low phosphate diet in renal and intestinal brush border membrane vesicles: influence of sodium and pH

The possible role of changes in the sodium (Na) affinity of the carrier for inorganic phosphate (P_i) in the adaptation of P_i transport to low P_i diet was examined in both renal and intestinal brush border membranes vesicles (BBMV) obtained from the same animal. This role was assessed by measuring the Na concentration resulting in half maximal activation of P_i transport (K_0.5Na) in renal and intestinal BBMV prepared from animals adapted to either low (LPD) or high (HPD) phosphorus diet for 7 days. TheK_0.5Na was not modified by dietary P_i, in both renal and intestinal BBMV. LPD increased maximal P_i transport from 1794.8±198.0 to 296.4±362.0 in renal and from 28.2±3.4 to 80.5±7.2 pmol/mg 10 s in intestinal BBMV. For both LPD and HPD lowering pH from 7.4 to 6 dramatically increasedK_{0.5 Na} in renal and intestinal BBMV. As compared to pH 7.4, it was enhanced by approximately 200% in both renal and intestinal membranes. This change of Na affinity with acidic pH prevented the expression of P_i transport adaptation at 100 mM Na concentration. However, at saturating Na concentrations (500 mM for renal, 300 mM for intestinal membranes), P_i transport adaptation was equally expressed at pH 6 and 7.4 in both types of membranes. Hill coefficient analysis indicates a 2:1 stoichiometry of Na to P_i in renal and intestinal membranes isolated from high or low P_i diet animals. This ratio was not modified by changes of the medium pH.