Oligonucleotide primers based on the human heart monocarboxylate transporter (MCT1) cDNA sequence were used to isolate a 544 bp cDNA product from human colonic RNA by reverse transcription‐polymerase chain reaction (RT‐PCR). The sequence of the RT‐PCR product was identical to that of human heart MCT1. Northern blot analysis using the RT‐PCR product indicated the presence of a single transcript of 3.3 kb in mRNA isolated from both human and pig colonic tissues. Western blot analysis using an antibody to human MCT1 identified a specific protein with an apparent molecular mass of 40 kDa in purified and well‐characterized human and pig colonic lumenal membrane vesicles (LMV). Properties of the colonic lumenal membrane l‐lactate transporter were studied by the uptake of L‐[U‐14C]lactate into human and pig colonic LMV. l‐lactate uptake was stimulated in the presence of an outward‐directed anion gradient at an extravesicular pH of 5.5. Transport of l‐lactate into anion‐loaded colonic LMV appeared to be via a proton‐activated, anion exchange mechanism. l‐lactate uptake was inhibited by pyruvate, butyrate, propionate and acetate, but not by Cl− and SO42−. The uptake of l‐lactate was inhibited by phloretin, mercurials and α‐cyano‐4‐hydroxycinnamic acid (4‐CHC), but not by the stilbene anion exchange inhibitors, 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonic acid (DIDS) and 4‐acetamido‐4′‐isothiocyanostilbene‐2,2′‐disulphonic acid (SITS). The results indicate the presence of a MCT1 protein on the lumenal membrane of the colon that is involved in the transport of l‐lactate as well as butyrate across the colonic lumenal membrane. Western blot analysis showed that the abundance of this protein decreases in lumenal membrane fractions isolated from colonic carcinomas compared with that detected in the normal healthy colonic tissue.

Luminal membrane vesicles (LMV) were isolated from human and pig colonic tissues. They were characterized in terms of purity and ability to transport [14C]butyrate. The activity of cysteine‐sensitive alkaline phosphatase, and the abundance of villin, NHE2 and NHE3 proteins, markers of the colonic luminal membrane, were significantly enriched in the LMV compared with the original cellular homogenate. The LMV were free from contamination by other cellular organelles and basolateral membranes, as revealed by the negligible presence of either specific marker enzyme activity or characteristic immunogenic protein. The transport of butyrate into the luminal membrane vesicles was enhanced 5‐fold at pH 5.5 compared with pH 8.0. Butyrate transport was temperature dependent, and was stimulated in the presence of an outward‐directed anion gradient in the order of butyrate > bicarbonate > propionate > chloride. Kinetic analysis of increasing substrate concentration showed saturation kinetics with an apparent Km value of 14.8 ± 3.6 mM and a Vmax of 54 ± 14 nmol min−1 (mg protein)−1. Butyrate transport was significantly reduced in the presence of short chain fatty acids (SCFA), acetate, propionate and other monocarboxylates (pyruvate and L‐lactate). Butyrate uptake was inhibited by several cysteine group modifying reagents such as p‐chloromercuribenzosulphonic acid (pCMBS), p‐chloromercuribenzoate (pCMB), mersalyl acid and HgCl2, but not by the stilbene anion exchange inhibitors, 4,4′‐diisothiocyanostilbene‐2,2′‐disulphonate (DIDS) and 4,4′‐dinitrostilbene‐2,2′‐disulphonate (SITS). The described properties of butyrate transport across the luminal pole of the colon suggest the involvement of a carrier protein, in the form of a pH‐activated anion exchange process. The transporter is distinct from the erythrocyte band‐3 type anion exchanger and may belong to the monocarboxylate‐type transport proteins (MCT1).

Linagliptin administration led to attenuation of postprandial glucose excursions and, despite a low HbA1c at baseline, statistically significant reductions in HbA1c after only 4 weeks of treatment. Linagliptin had a safety and tolerability profile similar to placebo in T2DM patients.

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