Nicotinate uptake by two kinetically distinct Na÷-dependent carrier-mediated transport systems in the rat small...

Recent studies have identified monocarboxylate transporter 1 (MCT1), sodium-coupled monocarboxylate transporter 1 (SMCT1) and SMCT2 as those that may be involved in the carrier-mediated intestinal absorption of nicotinate, but their roles have not been fully clarified yet. To address the issue, we examined the uptake of nicotinate in the rat small intestine by using everted tissue sacs. The uptake of nicotinate was Na⁺-dependent and saturable at pH 7.4 in both the jejunum and ileum. The saturable transport consisted of a single component with the Michaelis constant (K(m)) of 1.18 mM in the jejunum, while in the ileum it consisted of the high and the low affinity components with the K(m) values of 8.62 µM and 2.36 mM, respectively, and the latter was prevailing in transport capacity and similar to the jejunal transport component. Nicotinate uptake activity attributable to a H⁺-dependent transporter like MCT1 was, however, only minimal in the two intestinal sites. These results suggest that a low affinity type of SMCT2-like transporter would be in operation with high capacity throughout the small intestine, playing the role as the major intestinal nicotinate uptake transporter, and a high affinity type of SMCT1-like transporter would be additionally in operation in the ileum.