Introduction: Microbial fermentation of carbohydrates in forestomach of ruminants produces large amounts of short-chain fatty acids (SCFA, mainly acetic acid, propionic acid, and n-butyric acid). The majority of these substrates is taken up directly across the ruminal wall. After luminal uptake into the epithelial cells, SCFA mainly occur in the dissociated form due to the intracellular pH of ~7.4. Moreover, a big portion of SCFA is metabolised within the cytosol. Main end products of epithelial SCFA metabolism are ketone bodies (D-3-hydroxybutyric acid and acetoacetic acid) and lactic acid. Both intact SCFA and ketone bodies and lactate need to be efficiently extruded from the ruminal epithelial cells to prevent a lethal drop of intracellular pH and counteract osmotic load of the cytosol. All these substances are less lipophilic in comparison to the undissociated form of SCFA. Thus, dissociated SCFA (SCFA-) and their metabolites need Protein mediated mechanisms for the extrusion across the basolateral side of ruminal epithelium. One mechanism suggested to be involved in the extrusion of SCFA- across basolateral membrane of the ruminal epithelium is the monocarboxylate transporter 1 (MCT1). Functionally, MCT1 was first assumed to operate as proton-coupled transporter for monocarboxylates including SCFA. Nonetheless, a recent study found a bicarbonate dependent anion exchange mechanism which turned out to be sensitive to MCT1 Inhibitors at the basolateral side of the ruminal epithelium pointing to the ability of MCT1 to act as an anion exchanger. However, in these experiments the inhibition of MCT1 abolished bicarbonate dependent transport only by half. This suggests the involvement of further anion exchanger(s) in the transport of SCFA across the basolateral membrane of ruminal epithelium. Promising candidates to underlie this exchange are the putative Anion exchanger 1 (PAT1) and a transport protein designated „down-regulated in adenoma“ (DRA). Materials and Methods: Sheep rumen epithelium was mounted in Ussing Chambers under short-circuit conditions. Radioactively labelled acetate (ac) was added to the serosal side. Serosal to mucosal flux of ac (Jsm ac) was measured with or without anion Exchange inhibitors (50 mM NO3- or 1 mM DIDS) or the MCT1 inhibitor p-hydroxy mercuribenzoic acid (pHMB; 1.5 mM) in the serosal buffer solution. The inhibitors were added alone or in combination with each other. Furthermore, mucosal to serosal flux of radioactivelly labelled ac or butyrate (bu) (Jms ac, bu) was measured in the presence or absence of SO42-, Cl- or NO3- (50 mM respectively) as exchange substrate in the serosal buffer solution. Immunohistochemical staining was conducted to locate PAT1 and DRA by use of commercially available antibodies. Results: NO3- and pHMB significantly reduced Jsm ac by 57 % and 51 %, respectively. When pHMB was applied after pre-incubation with NO3- an additional inhibition of Jsm ac was observed. Vice versa, NO3- further inhibited Jsm ac when epithelia were pre-incubated with pHMB before. DIDS had no inhibitory effect on SCFA flux. Serosal presence of SO42- or Cl- enhanced Jms ac significantly. Regarding bu, Cl- or SO4 2- also enhanced Jms bu significantly. The different anions available in the serosal buffer solution numerically enhanced Jms in the order of SO4 2- > Cl- for both ac and bu, which corresponds to the known affinity sequence of PAT1 and DRA. Immunohistochemistry revealed localization of PAT 1 in the stratum basale, whereas DRA was not detectable using this method. Conclusions: Basically, this study supports the suggestion that MCT1 works as an Anion exchanger in ruminal epithelium. In addition, it clearly shows that there is at least one further anion exchanger involved in the basolateral extrusion of SCFA and their metabolites. The functional and immunohistochemical findings suggest that PAT1 holds a significant role in this respect.

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