Intestinal transport of methylmercury and inorganic mercury in various models of Caco-2 and HT29-MTX cells.

Food is the main pathway of exposure to mercury for most of the population. In food, mercury is generally present as inorganic mercury [Hg(II)] or methylmercury [MeHg]. Both chemical forms have some degree of toxicity, especially MeHg, which is considered a powerful neurotoxicant during development and is classified as a possible human carcinogen. Since the main exposure pathway is oral, gastrointestinal absorption is a decisive step in the process by which mercury reaches the systemic circulation. However, there are few studies that characterize this absorption process. The present work evaluates transport and cellular retention of Hg(II) and MeHg, using various models of the intestinal epithelium (Caco-2 monocultures and Caco-2/HT29-MTX co-cultures in various proportions). Additionally, a study was made of the influence of the mucus secreted by HT29-MTX cells and of substances normally present in the gastrointestinal tract (l-cysteine, bile salts and food components) on mercury transport and accumulation. The results show that incorporation of HT29-MTX reduces the permeability coefficient of Hg(II) and MeHg. This decrease coincides with an increase in cellular accumulation, since mercury is retained in the layer of mucus secreted by HT29-MTX cells [Hg(II): 40%; MeHg: 70%]. The presence of l-cysteine, bile salts and food matrix components increases the percentage of both species that is not absorbed. It is noteworthy that in all the conditions assayed the intracellular accumulation of mercury was very high (37-77%). This study shows the importance of the cell model and assay conditions for an in vitro evaluation of intestinal transport of mercury species.