Abstract

Our objective was to evaluate the utility of an in vitro cell culture system (Caco-2) to study the pathways for the metabolism of L-alpha-methyldopa (MD) in the intestinal mucosa during its transepithelial transport. Caco-2 cell monolayers grown onto polycarbonate membranes in Transwells have been reported to exhibit morphological and biochemical properties similar to those of the human intestinal mucosa. After oral administration, MD has been reported to undergo extensive first-pass metabolism during its transport across the intestinal mucosa. In earlier studies, our laboratory showed that MD was rapidly transported across Caco-2 cell monolayers consistent with a carrier-mediated mechanism involving the large neutral amino acid transporter (Hu and Borchardt, 1990). Reversed-phase, ion-pair HPLC coupled to an electrochemical (amperometric) detector operating in the oxidative mode was used in this study to detect MD and its metabolites in the transport and cellular media. After rapid cellular uptake from the apical surface of Caco-2 cell monolayers, MD was predominantly metabolized to form MD-sulfate and MD-glucuronide and 3-O-methyl-methyldopa and its sulfated conjugate. Although methyldopamine was detected in minor quantities, it was present mainly as the sulfate and glucuronide conjugate. Although the cellular levels of MD and its metabolites rapidly reached steady-state, the corresponding basolateral levels continuously increased after an initial small lag-time. We observed that the 25-day-old Caco-2 cell monolayers used in this study were able to metabolize MD to a greater extent than those cultured for 11 days (p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)