Lactate and ketone bodies play important roles as alternative energy substrates, especially in conditions with a decreased utility of glucose. Short-chain fatty acids (acetate, propionate, and butyrate), produced by bacterial fermentation, supply most of the energy substrates in ruminants such as the cow and sheep. These monocarboxylates are transfered through the plasma membrane by proton-coupled monocarboxylate transporters (MCTs) and sodium-coupled MCTs (SMCTs). To reveal the metabolism and functional significance of monocarboxylates, the cellular localization of MCTs and SMCTs together with the expressed intensities holds great importance. This paper reviews the immunohistochemical localization of SMCTs and major MCT subtypes throughout the mammalian body. MCTs and SMCTs display a selective membrane-bound localization with porality. In contrast to the limited expression of SMCTs in the intestine and kidney, MCTs display a broader distribution pattern than GLUTs. The brain, kidney, placenta, and male genital tract express multiple subtypes of the MCT family. Determination of the cellular localization of MCTs is most controversial in the brain, possibly due to regional differences and the transcriptional modification of MCT proteins. Information on the localization of MCTs and SMCTs aids in understanding the nutrient absorption and metabolism throughout the mammalian body. In some cases, the body may use monocarboxylates as signal molecules, like hormones.
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