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Rapid absorption of SCFA from the rumen is of crucial importance for the prevention of ruminal acidosis, but our understanding of the absorptive pathway is incomplete. Uptake of SCFA leads to a stimulation of sodium absorption via NHE (Gäbel et al., 1991), suggesting that SCFA acidify the intraepithelial space either by uptake of the undissociated SCFA, or via exchange of the anion for HCO3- (Aschenbach et al., 2009). Conversely, basolateral efflux of acetate probably occurs through a recently characterized large conductance anion channel (Stumpff et al., 2009) that has previously been postulated to mediate efflux of chloride (Sehested et al., 1999; Leonhard-Marek et al., 2006). However, acidification of the ruminal epithelium by SCFA has never been directly demonstrated, and the basolateral efflux pathway for SCFAs other than acetate remains unclear. In the current study, the response of intraepithelial pH to the application of SCFA was studied in the intact tissue using pH sensitive microelectrodes. The permeability of the anion channel for propionate was studied on isolated cells from the ruminal epithelium using the patch clamp technique.
The microelectrode experiments confirm the presence of an apical pH microclimate within the stratum corneum and the acidifying effects of SCFA as suggested previously (Gäbel et al., 1991). The patch clamp data show that the ruminal anion channel conducts not only acetate as recently reported (Stumpff et al., 2009), but also propionate. We suggest that in the intact tissue, this channel is basolaterally located and mediates efflux of anions such as chloride, acetate and propionate, driven by the potential generated by the Na+-K+-ATPase. This mechanism should be helpful in maintaining cellular volume and thus, contribute to the ability of the ruminal epithelium to absorb SCFA.