Analysis of a model to describe lead transport by the small intestine

Abstract

Investigations into the mechanism of lead transport across the rat small intestine were conducted using the in vitro everted sac technique. The data obtained were used to construct a model which could competently rationalise some of the reported observations in the literature concerning intestinal lead transport. The model comprises of two individual processes acting in concert. A large rapid adsorption of lead onto the intestinal tissue surface; and a concurrent slow passive diffusion of lead cations across the intestinal epithelium. The presence of methylxanthines, which diminish the size of the intercellular channels, were found to decrease the rate of lead cation transport, and indicated a paracellular route for lead transport. Certain metal cations compete with lead for transport via the paracellular route and hence decrease its rate of transit. Other metal cations increase the tissue binding of lead by a process of coprecipitation, and may ultimately increase the entry of lead into the body. Chelated lead species exhibit different kinetic characteristics to that of the lead cation. The data suggests that chelation accelerates lead absorption, probably by the utilisation of a transcellular transport route. The studies indicate why, under normal conditions, only a small percentage of lead is absorbed, and also suggest the manner by which larger arrounts of lead may gain entry into the body.