Phosphate transport by rat intestinal basolateral-membrane vesicles

The characteristics of phosphate transport across intestinal basolateral membranes of the rat were determined by using enriched preparations in which uphill Na⁺-dependent D-glucose transport could not be demonstrated, but ATP-dependent Ca²⁺ transport was present. Phosphate transport was saturable, Na⁺-dependent and exhibited Michaelis-Menten kinetics. V(max.) was 51.1 ± 4.2 pmol/10 s per mg of protein and K(m) was 14 ± 3.9 μM. The transport process was electroneutral. Tracer-exchange experiments and counter-transport studies confirmed the presence of a Na⁺-P(i) carrier at the basolateral membrane. The presence of inside-positive membrane potential did not enhance phosphate uptake, indicating that the Na⁺ effect is secondary to the presence of the Na⁺-P(i) carrier rather than an induction of positive membrane potential. The stoichiometry of this carrier at pH 7.4 was 2 Na⁺:1 phosphate, as shown by direct studies utilizing the static-head method. These studies are the first to determine the presence of a phosphate carrier at the basolateral membrane.