P331 INVOLVEMENT OF SODIUM-COUPLED MONOCARBOXYLATE TRANSPORTER-LIKE ACTIVITY IN NICOTINATE UPTAKE BY BRUSH BORDER MEMBRANE VESICLES FROM THE RAT SMALL INTESTINE

Hiroaki Yuasa , Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan
Masahiko Ohkubo , Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan
Kinya Ohta , Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan
Katsuhisa Inoue , Graduate School of Pharmaceutical Sciences, Nagoya City University, Nagoya, Japan
Carrier-mediated transport systems have been suggested to be present in the small intestine for the efficient absorption of nicotinate, a water soluble vitamin (vitamin B3) that has various biological activities including lipid-lowering effects, and indeed recent studies have identified Na+-dependent monocarboxylate transporter 1 (SMCT1) and SMCT2 as those that may be involved in that. However, the roles of these transporters in intestinal nicotinate absorption have not been fully verified yet. Although our latest study using the everted tissue sacs successfully demonstrated the prevailing involvement of Na+-dependent SMCT-like transporter activities in nicotinate uptake in both the jejunal and ileal segments of the rat small intestine, there is an earlier study that contradictorily indicated the absence of SMCT-like transporter activity, with demonstrating the involvement of H+-dependent monocarboxylate transporter 1 (MCT1)-like transporter activity, in brush border membrane vesicles (BBMVs) from the rat small intestine. To address the issue and gain information for optimizing the absorption of nicotinate and relevant drugs, we examined the uptake of [3H]nicotinate in rat intestinal BBMVs. In the present study, BBMVs were prepared from the 20-cm segments of jejunum and ileum, using 8 male Wistar rats for a preparation, by a calcium precipitation method and uptake experiments were performed by a rapid filtration technique. The uptake of nicotinate (0.4 μM) was higher initially in the presence of Na+ than in its absence at pH 7.4 in ileal BBMVs, successfully demonstrating the presence of SMCT-like transporter activity. However, in jejunal BBMVs, the initial uptake of nicotinate at the same concentration was independent of Na+ at pH 7.4 and also at pH 5.5, but was enhanced at the lower pH of 5.5 than at pH 7.4 and saturable only at pH 5.5. These results are indeed in agreement with the earlier indication of the presence of MCT1-like transporter activity alone in BBMVs and conflict with what we recently found in the everted sacs. MCT1-like transporter activity was similarly observed also in ileal BBMVs, but at least we could also observe the presence of Na+-dependent transporter activity at pH 5.5 as well as at pH 7.4. Particularly at pH 7.4, where MCT1-like transporter activity was absent, the Na+-dependent SMCT-like transporter activity was suggested to represent a major uptake component. This is the first to demonstrate SMCT-like transporter activity in intestinal BBMVs, although the reason why SMCT-like transporter activity is less evident in BBMVs than in everted sacs remains to be elucidated.