Phosphate Transport hy Intestinal Endoplasmic Reticulum during Maturation

Abstract

ABSTRAm. The transport of phosphate into jejunal endoplasmic reticulum vesicles isolated from suckling and adolescent rats was investigated using a rapid filtration technique. Intestinal endoplasmic reticulum from both ages were enriched with NADPH cytochrome-C-reductase whereas other markers for brush border, basolateral, mitochondrial, and Golgi apparatus were impoverished. Phosphate uptake represented an energy-dependent process as evident by more than 80% decrease in uptake values at 0" C compared to 25" C. Phosphate uptake was ATP dependent in both age groups, however, mean uptake values were significantly greater in suckling rats compared to adolescent rats. pH optimum for uptake was 7.2. p-Chloromercuribenzoate at 100 pM concentration inhibited phosphate uptake by more than 90%. Initial rate of phosphate uptake was linear up to 45 s. Kinetics of phosphate uptake at 30 s showed a K, of 0.7 + 0.1 and 0.15 2 0.1 suckling and adolescent rats, respectively. V,., was 1.5 + 0.5 and 0.14 f 0.01 nmol/mg protein/30 s for both suckling and adolescent rats, respectively. Herein we provide evidence for the first time for the presence of a phosphate carrier in intestinal endoplasmic reticulum of rats. Endoplasmic reticulum of phosphate uptake was significantly greater in suckling rats compared to adolescent rats. This increase in uptake is due to a greater number and activity of phosphate carriers in suckling rats. (Pediatr Res 23: 612-615, 1988)The transport of phosphate across the small intestine of suckling and adult animals occurs by an active process (1-5). Jejunal phosphate transport take place against an electrochemical gradient and is inhibited by arsenate (1-8). The uptake of phosphate at the brush border membrane consists of two components: a saturable, electroneutral sodium dependent component and an unsaturable, sodium-independent component (1, 2). The energy driving the sodium-dependent entry of phosphate into the cell is provided by an extracellular to intracellular Na+ gradient maintained by the Na+-K+-ATPase at the basolateral membrane (5). Both sodium-dependent and -independent brush border phosphate uptake are higher in suckling rats compared to adult rats (6). Phosphate exit at the basolateral membrane of the enterocyte occurs by a camer-mediated process that is sodium dependent, electroneutral with stoichiometry of 2 Na:l phosphate at pH 7.4 (7). The transcellular movement of phosphate is unknown. Recently, we described an important role for the endoplasmic reticulum in transcellular calcium transport (8). This process is ATP dependent and exhibits characteristic devel- opmental patterns (8). Our study was designed to investigate the role of intestinal endoplasmic reticulum in phosphate transport during maturation in the rat. We used a millipore filtration system to study phosphate uptake by intestinal microsomes. EXPERIMENTAL PROCEDURES Isolation of intestinal endoplasmic reticulum vesicles.The jejunum of two adolescent rats (42 + 2 days) of 12 suckling rats (14 + 1 day) were used to prepare the en...

Glucocorticoid regulation and glycosylation of mouse intestinal type IIb Na-Picotransporter during ontogeny

“…Total intestinal Na-P i uptake in the current studies was highest in 14-day-old suckling mice, lower in 21day-old weanlings, and lowest in older age groups. This observation agreed with previous reports that intestinal Na-P i transport was higher in suckling than in adult rats and rabbits (7,9,17). Moreover, in the current studies, the decline in uptake activity was not constant with age, but instead was rapid (sevenfold) from 14-to 21-day-old mice and then gradually decreased (threefold) from weanling to young adult mice (8-wk-old).…”

“…In laboratory animals, Na-P i absorption across jejunal, apical, and endoplasmic reticular membranes of enterocytes was significantly greater in suckling (2wk-old) rats compared with adolescent (6-wk-old) rats (7,17). A study with rabbits at various ages (2, 4, and 6 wk and 3 mo) demonstrated similar results in which duodenal and jejunal Na-P i uptake was greatest in 2-wk-old animals and severely decreased in older (3mo-old) animals (9).…”

“…Age-related plasma P i levels may result from decreased renal and intestinal P i absorption (5). In intestine, Na-P i absorption across the apical and endoplasmic reticulum membranes of enterocytes is higher in suckling animals than in young adult animals (7,9,17). Conversely, renal Na-P i cotransport is lowest in suckling rats, highest in weanling rats, and declines with age (43).…”