Short chain fatty acids (SCFA) are absorbed by carrier mediated uptake in the small intestine by pH-dependent SCFA/HCO₃ ⁻ exchangers on the apical membrane of epithelial cells. Conventional assumption is that MCT1 mediates SCFA/HCO₃ ⁻ exchange in the intestine. Further, due to the presence of multiple such anion exchangers, the identity of the intestinal SCFA/HCO₃ ⁻ has been controversial.

The aim of this study was to determine the identities of the butyrate transporter in the intestinal epithelial cells (IEC-18).

IEC-18 cells were treated with specific siRNAs for MCT1 and MCT4, and butyrate and lactate uptake studies were performed.

Alpha-cyano-4-hydroxycinnamic acid inhibited lactate uptake but not butyrate uptake in IEC-18 cells, indicating that these two substrates are transported via two different transporter systems. MCT1 siRNA treatment abolished both MCT1 mRNA by more than 95 % and protein expression by 83 % as evidenced by RTQ-PCR and western blotting experiments. However, MCT1 siRNA treatment inhibited butyrate uptake upto 24 %, whereas it inhibited lactate uptake significantly by 70 %. Treatment with MCT4 siRNA inhibited MCT4 mRNA expression by 75 % and protein expression by 85 % in these cells. MCT4 siRNA inhibited butyrate uptake by 40 %. Further, several non-steroidal anti-inflammatory drugs (NSAIDs) are transported by the butyrate transporter. Finally, MCT4 siRNA inhibited salicylate uptake by 27 % indicating direct evidence for the transport of salicylate by MCT4.

These data indicate that MCT1 is the high affinity lactate transporter and MCT4 is the high affinity butyrate transporter in the intestinal epithelial cell line IEC-18.