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Introduction: The concentration of short chain fatty acids (SCFA) in the gut exceeds levels of 100 mmol·l-1, raising the question how these energy rich substrates are absorbed without impairing cell homeostasis. We have recently identified a large conductance anion channel in isolated cells of the ruminal epithelium permeable to acetate. This channel may facilitate the basolateral efflux of SCFA from the rumen (Stumpff et al. 2008, Pflugers Archive, Epub). In the current study, we investigated the permeability of this conductance for propionate-.
Methods: Isolated cells of the ruminal epithelium were studied using the whole cell configuration of the patch-clamp technique.
Results: When filled with physiological Na-gluconate solution and superfused with NaCl Ringer, cells displayed an outwardly rectifying conductance with inward current level at 33 ± 3 % of the outward current level (n = 33). Reversal potential was -32 ± 1 mV (n = 33). After replacement of 130 mM NaCl with Na-gluconate, outward current level dropped to 38 ± 6% (n = 33, p < 0.001). Subsequent replacement with Na-propionate resulted in a strong rise in outward current, followed by a rupture of the seal in 10 of 16 cells studied. In the six remaining cells, outward current stabilized at 225 ± 76 % of the level in NaCl solution (n = 6, p < 0.05). Reversal potential decreased from 1 ± 2 mV (Na-gluconate) to -11 ± 2 mV (n = 6)(Na-propionate, p < 0.001 vs NaCl, p < 0.01 vs Na-gluconate). Application of DIDS (200 µM) in Na-propionate solution reduced the outward current level (p < 0.05) to a value close to that in Na-gluconate solution (p = 0.9, n = 5) without changing reversal potential. Washout was complete. Reexposure to NaCl solution resulted in a relative rise in outward current by 66 ± 19 % (n = 4). Outward current level in NaCl at the end of the experiment was thus significantly higher (253 ± 83 %, p < 0.01, n = 4) than at the beginning of the experiment (100 %). Relative to the initial level, inward current also rose, but not significantly. Reversal potential returned to - 31 ± 3 mV (p < 0.001 vs. Na-propionate). Similar slow rises in outward current at negative reversal potential could be seen in cells exposed to hypotonic NaCl solution.
Conclusion: We conclude that the anion channel previously identified in ruminal epithelial cells conducts propionate- with p(propionate-)/p(Cl-) = 0.52 ± 0.04. The results further suggest that the channel is activated by osmotic stress. We propose a role for this channel in the basolateral efflux of propionate- and acetate- from the ruminal epithelium, energized by the Na,K-ATPase via the basolateral potential gradient.