Molecular Identification and Messenger RNA Levels of a Monocarboxylate Transporter (MCT) in Fundulus Heteroclitus, a Hypoxia Tolerant Teleost Fish
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Monocarboxylate transporters (MCTs) are bi-directional symporters that couple proton movement to a variety of glycolytic metabolites, including lactate and pyruvate. As such, MCTs not only play a crucial role in pH regulation, but could also function in the distribution of substrates between organelles, cells and tissues. This function may be critically important during hypoxia, when tissues depend on glycolytic flux for energy. We have examined the role of MCTs during hypoxia in an estuarine teleost, the common killifish, Fundulus heteroclitus, that routinely encounters environmental hypoxia. We cloned and sequenced a killifish MCT that resembled the MCT-2 isoform of mammals, and examined its expression pattern and regulation during hypoxia using real-time quantitative PCR. Killifish MCT mRNA levels were highest in the intestine (~2x) and lowest in the heart (~0.2x) as compared to the expression in the liver. Exposure to hypoxia (0.5 mg L-1) resulted in a significant increase in plasma lactate and glucose within 4 hours. However pyruvate and glycogen concentration in white muscle did not change significantly during this time. We observed no change in the mRNA levels of this MCT isoform in any tissue during 16h of hypoxia. This could indicate the regulation of this MCT at another level, a secondary role in hypoxia, or a role for this isoform unrelated to hypoxia tolerance.