Ontogeny of phosphate transport by rat liver plasma membrane vesicles

The present studies were designed to characterize the developmental aspects of Na⁺-dependent phosphate transport, across the hepatocyte basolateral membranes of the suckling and weanling rats. A well validated technique of plasma membrane vesicles (BLMV) was utilized. Phosphate uptake was driven into an osmotically active intravesicular space as evident by a linear relationship between uptake and 1/osm with no binding component y = 0.04 x-0.03, r² = 0.99 and y = 0.035 x +0.01, r² = 0.95 in suckling and weanling BLMV's respectively. The presence of inwardly directed Na⁺ and pH gradient stimulated phosphate uptake in suckling and weanling BLMV's, however, uptake values under Na⁺ and pH gradients were greater in weanling rats compared to suckling rats. Kinetics of Na⁺-dependent phosphate uptake were 0.14 ± 0.01 and 0.28 ± 0.035 at pH 6.1 (P < 0.05) and 0.1 ± 0.007 and 0.15 ± 0.03 nmoles/mg protein/10s (P < 0.05) at pH 7.4 in suckling and weanling rats respectively. K(m) values were not significantly different. Na⁺ arsenate and phosphonoformonic acid inhibited Na⁺-dependent phosphate uptake, whereas ATP and P-MB (para-chlomercuribenzoic acid) did not effect Na⁺-dependent phosphate uptake. These studies demonstrate for the first time the presence of a specialized transport system for phosphate across the basolateral membranes of the rat liver during development. This transport system exhibit ontogenic characteristics in regard to its transport capacity.