Expression and cellular localization of monocarboxylate transporters (MCT2, MCT7, and MCT8) along the cattle gastrointestinal tract.

Fourteen members of the monocarboxylate transporter (MCT, SLC16) family have been identified, each having a different tissue distribution and substrate specificity. The expression of monocarboxylate transporters MCT1 and MCT4 have been studied in the gastrointestinal tract of ruminants; however, details of the expression of other MCT isoforms in the various parts of ruminant gastrointestinal tract are lacking. Reverse transcription with the polymerase chain reaction was used to study the regional distribution of MCT2, MCT3, and MCT5-MCT14 in the cattle gastrointestinal tract and verified the existence of MCT mRNA transcripts for MCT2, MCT3, MCT4, MCT7, MCT8, MCT9, MCT10, MCT13, and MCT14 in the ruminal and abomasal epithelia, mRNA transcripts for MCT2, MCT3, MCT4, MCT7, MCT8, MCT10, MCT13, and MCT14 in the jejunum, and mRNA transcripts for MCT2, MCT3, MCT4, MCT7, MCT8, MCT13, and MCT14 in the caecum of cattle. At the cellular level, immunohistochemical studies localized MCT2, MCT7, and MCT8 proteins in the cattle rumen, abomasum, jejunum, and caecum. This is the first study to detect the expression of various MCT isoforms in the gastrointestinal tract of a ruminant species. Our data suggest that these transporter proteins are involved in essential physiologic processes and are possible molecular targets for studying the regulation of the transport of short-chain monocarboxylates, aromatic amino acids, and thyroid hormones across the gastrointestinal tract of cattle.