Choline uptake in human intestinal Caco-2 cells is carrier-mediated

Abstract

The objective of the current investigation was to examine the transport characteristics of choline, an endogenous quaternary ammonium compound, into human intestinal Caco-2 cells; the transport of choline has not been characterized in human intestine. The cellular accumulation of choline was independent of an inwardly directed Na+ gradient and demonstrated temperature dependence and saturability. Using the initial uptake rates, choline accumulation was best characterized by a Michaelis-Menten equation and a diffusion component with a Km and Vmax of 110 ± 3 μmol/L and 2800 ± 250 pmol/(mg protein · 10 min), respectively. Choline uptake was significantly inhibited by an excess of choline itself and by hemicholinium-3, a structural analog of choline. However other hydrophilic organic cations, such as tetraethylammonium (TEA) and N-methylnicotinamide (NMN), did not affect choline uptake in Caco-2 cells. Additionally, two typical p-glycoprotein substrates, daunomycin and verapamil, both inhibited choline accumulation. However the opposite was not true: choline did not inhibit DNM accumulation in Caco-2 cells. These results indicate the presence of a carrier-mediated transport system for choline in Caco-2 cells. The substrate specificity of this carrier is unlike that seen in the rat intestinal epithelium, and the human transport protein is distinct from those for TEA and NMN. P-glycoprotein substrates may inhibit choline uptake through specific or nonspecific interactions with the choline transporter.