A Na+-Dependent Mechanism Is Involved in Mucosal Uptake of Cinnamic Acid across the Jejunal Brush Border in Rats

Abstract

Phenolic acids are present in all plant-derived foods and in most diets. Indirect evidence indicates substantial absorption of phenolic monomers from the gastrointestinal tract. However, the mechanisms involved in the absorptive process are unknown. The present study investigates mucosal uptake of radioactively labeled cinnamic acid as a model substance for monomeric cinnamic acid derivatives (e.g., cinnamic, ferulic or caffeic acid) in the rat jejunum using an in vitro mucosal uptake technique. The results indicate the existence of a Na+-dependent saturable transport mechanism for uptake of cinnamic acid across the jejunal brush border membrane. The observed Na+ dependence of jejunal cinnamate uptake seems not to be related to the activity of the Na+, H+ exchanger. Lowering the pH of the incubation medium resulted in a pronounced increase in mucosal cinnamate uptake that can be only partially explained by an increase in nonionic diffusion of cinnamic acid. Furthermore, jejunal uptake of cinnamate seems to be influenced by intracellular HCO3- and/or pH, since the addition of methazolamide to a HCO3- and CO2-free incubation medium significantly inhibited mucosal cinnamate uptake, whereas methazolamide was without an effect in the presence of HCO3- and CO2 in the incubation medium. Unlabeled cinnamic and ferulic acid as well as short-chain fatty acids (acetic, propionic and butyric acid) significantly inhibited Na+-dependent uptake of radioactively labeled cinnamic acid. Taken together, the results indicate the involvement of a Na+-dependent, carrier-mediated transport process in the uptake of cinnamic acid and structurally related substrates such as ferulic acid across the brush border membrane of rat jejunum.