MECHANISMS OF ALUMINUM ABSORPTION IN RATS

Aluminum has become a dietary toxin in modem times but its mechanism o f absorption is poorly understood. After ingestion, the systemic trans fer of aluminum is small but it is greatly affected by the coingestion of certain dietary agents, such as citrate, that complex with the met al in the intestinal lumen or transiently alter the permeability of th e mucosa. Here, mechanisms of aluminum absorption were studied by usin g freshly prepared aluminum hydroxide and aluminum citrate. Everted sa cs of rat gut were used to investigate the site of absorption, effect of chemical charge on absorption of aluminum citrate, and presence of active or passive absorption with use of the metabolic inhibitor ouaba in. Absorption was biphasic with a large tissue uptake that was consis tent with adhesion to mucus-mucosal surface but little tissue transpor t, which was consistent with passive paracellular permeation. Citrate reduced the uptake-transport ratio both by competing with the mucosal uptake and by increasing mucus-mucosal permeation but not by affecting the charge of the luminal aluminum species. Despite the potential for hydroxypolymerization of aluminum at intestinal pH, the small bowel a nd colon absorbed aluminum passively and paracellularly but the stomac h did not. The predominantly proximal absorption of aluminum observed in vivo is a reflection of the proximal absorption, and therefore remo val, of dietary constituents (eg, citrate) that enhance mucosal permea tion of aluminum. The colon should be investigated further as a site o f significant paracellular permeability.