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Background: Rapid absorption of SCFA from the rumen is of crucial importance for the maintanance of ruminal osmolarity, but a comprehensive model of the absorptive pathway is lacking. Uptake of SCFA acidifies the intraepithelial space (1), with protons taken up apically extruded into the rumen in exchange for sodium via NHE. Both uptake of the undissociated HSCFA and uptake via exchange of the SCFA anion for HCO3- have been proposed as an apical uptake pathway (2). Conversely, basolateral efflux of acetate probably occurs through a recently characterized large conductance anion channel (3) that has previously been postulated to mediate efflux of chloride. The permeability of this anion channel for propionate and butyrate was studied.
Methods: Isolated cells of the ruminal epithelium from freshly slaughtered sheep were studies using whole cell and inside out configuration of the patch clamp technique. Patch clamp experiments were performed as described previously (3).
Results: In cells filled with a Na-gluconate solution and superfused with a NaCl bath solution, a strong current could be induced by depolarisation of the pipette potential to 100 mV that was sensitive to the replacement of the dominant anion in the bath, suggesting p(Cl-) > p(acetate-) > p(propionate-) > p(butyrate-) > p(gluconate-). Currents were sensitive to DIDS (200 mol/l) and p-chloromercuribenzoate (pCMB; 1 mmol/l). Relative to chloride, permeability increased with decreasing chain length (P(gluconate)/P(Cl): 0.26 ± 0.01; n = 39, P(butyrate)/P(Cl): 0.36 ± 0.02; n = 32, P(propionate)/P(Cl): 0.39 ± 0.01; n = 47, P(acetate)/P(Cl): 0.42 ± 0.01; n = 16). The conductance for propionate was significantly higher that that for gluconate (p = 0.001), and slightly lower than that for acetate (p = 0.04), with the difference versus butyrate below significance level (p = 0.1, Mann Whitney rank sum test).
In a second set of experiments, membrane patches were investigated in the inside-out configuration of the patch clamp technique. Channels in symmetrical NMDG-propionate solution showed a conductance of 114 ± 10 pS for propionate, which is lower than the conductances for chloride (~350 pS) and acetate (~140 pS) obtained in a previous study (3). The size of channel events at positive pipette potential changed with the dominant anion in the bath, confirming the permeability sequence obtained in the symmetrical solutions and in the whole cell experiments.
Conclusion: The patch clamp data show that the ruminal anion channel not only conducts acetate as recently reported (3), but also propionate, with the conductance for butyrate smaller. We suggest that this channel is basolaterally located in the intact tissue and mediates the efflux of anions such as chloride, acetate and propionate, driven by the potential generated by the Na+-K+-ATPase, while larger anions are metabolized or leave via MCT. This mechanism should be helpful in maintaining cellular volume and thus, contribute to the ability of the ruminal epithelium to absorb SCFA.