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Am J Physiol Gastrointest Liver Physiol (May 29, 2002). doi:10.1152/ajpgi.00268.2001
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Articles in PresS, published online ahead of print May 29, 2002
Am J Physiol Gastrointest Liver Physiol, 10.1152/ajpgi.00268.2001
Submitted on June 19, 2001
Accepted on May 23, 2002

Transport of ketone bodies and lactate in the sheep ruminal epithelium by monocarboxylate transporter 1

Frank Muller1*, Korinna Huber2, Helga Pfannkuche1, Jorg R Aschenbach1, Gerhard Breves2, and Gotthold Gabel1

1 Department of Veterinary Physiology, Leipzig University, Leipzig, Saxony, Germany
2 Department of Physiology, School of Veterinary Medicine, Hannover, Lower Saxony, Germany

* To whom correspondence should be addressed. E-mail: muellerf{at}rz.uni-leipzig.de.

Due to intensive intracellular metabolism of short-chain fatty acids, ruminal epithelial cells generate large amounts of D-ß-hydroxybutyric acid, acetoacetic acid, and lactic acid. These acids have to be extruded from the cytosol to avoid disturbances of intracellular pH (pH (pHi). ). To evaluate acid extrusion, intracellular pH was studied in cultured ruminal epithelial cells of sheep using the pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5(6')-carboxyfluorescein (BCECF). Extracellular addition of D-ß-hydroxybutyrate, acetoacetate, or lactate (20 mM) resulted in intracellular acidification. Vice versa, removing extracellular D-ß-hydroxybutyrate, acetoacetate, or lactate after pre-incubation with the respective monocarboxylate induced an increase of pHi. Initial rate of pHi decrease as well as of pHi recovery was strongly inhibited by pCMBS (400 µM) and phloretin (20 µM). Both cultured cells and intact ruminal epithelium were tested for the possible presence of proton-linked monocarboxylate transporter (MCT) on both the mRNA and protein levels. Using RT-PCR, mRNA encoding for MCT1 isoform was demonstrated in cultured ruminal epithelial cells and the ruminal epithelium. Immunostaining with MCT1 antibodies intensively labeled cultured ruminal epithelial cells and cells located in the stratum basale of the ruminal epithelium. In conclusion, our data indicate that MCT1 is expressed in the stratum basale of the ruminal epithelium and may function as a main mechanism for removing ketone bodies and lactate together with H+ from the cytosol into the blood.




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