TRANSEPITHELIAL TRANSPORT AND METABOLISM OF THYROTROPIN-RELEASING-HORMONE (TRH) IN MONOLAYERS OF A HUMAN INTESTINAL-CELL LINE (CACO-2) - EVIDENCE FOR AN ACTIVE-TRANSPORT COMPONENT

Cell culture models for gastrointestinal transport and metabolism are important mechanistic tools. Our studies of Caco-2 monolayers demonstrate heterogeneity in transport characteristics depending on passage number and origin of the cells. In accordance with data obtained in animals and humans, TRH shows a carrier-mediated, saturable transport component, which operates parallel to a passive pathway in Caco-2 cells at passage number 89-99. At low TRH concentrations (<3 mM) active transport becomes prominent, as demonstrated by the temperature dependence of TRH transport and inhibition experiments. The Michaelis-Menten parameters of the active, saturable transport component are: K-m = 1.59 mM and V-max = 1.84 mu M/min. The pH optimum was determined to be at pH 6.0. On the other hand an exclusively paracellular passive route was found with Caco-2 cells at passage number 30-34. These results are also in agreement with observations made by others in cell culture experiments. The aspect of rigorously characterizing the specific Caco-2 clone under investigation is emphasized, especially when active transport mechanisms are suspected.