MECHANISMS OF PHOSPHATE-UPTAKE INTO BRUSH-BORDER MEMBRANE-VESICLES FROM GOAT JEJUNUM

This study concerns the uptake of inorganic phosphate into brush-borde r membrane vesicles prepared from jejunal tissues of either control or Ca- and/or P-depleted goats. The brush-border membrane vesicles showe d a time-dependent accumulation of inorganic phosphate with a typical overshoot phenomenon in the presence of an inwardly directed Na+ gradi ent. The Na+-dependent inorganic phosphate uptake was completely inhib ited by application of 5 mmol . l(-1) sodium arsenate. Half-maximal st imulation of inorganic phosphate uptake into brush-border membrane ves icles was found with Na+ concentrations in the order of 5 mmol . l(-1) . Inorganic phosphate accumulation was not affected by a K+ diffusion potential (inside negative), suggesting an electroneutral transport pr ocess, Stoichiometry suggested an interaction of two or more Na ions w ith one inorganic phosphate ion at pH 7.4. Na+-dependent inorganic pho sphate uptake into jejunal brush-border membrane vesicles from normal goats as a function of inorganic phosphate concentration showed typica l Michaelis-Menten kinetic with V-max = 0.42 +/- 0.08 nmol . mg(-1) pr otein per 15 s(-1) and K-m = 0.03 +/- 0.01 mmol . l(-1) (n = 4, x +/- SEM). Long-term P depletion had no effect on these kinetic parameters. Increased plasma calcitriol concentrations in Ca-depleted goats, howe ver, were associated with significant increases of V-max by 35-80%, ir respective of the level of P intake. In the presence of an inwardly di rected Naf gradient inorganic phosphate uptake was significantly stimu lated by almost 60% when the external pH n as decreased to 5.4 (pH(out )/pH(in) = 5.4/7.4). The proton gradient had no effect on inorganic ph osphate uptake in absence of Na+. In summary, in goats Na+ and calcitr iol-dependent mechanisms are involved in inorganic phosphate transport into jejunal brush-border membrane vesicles which can be stimulated b y protons.