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The rumen has long served as a model tissue for studying the transport of short chain fatty acids. In a recent publication, we suggested that basolateral efflux of acetate may involve a large conductance anion channel. The question arises what other complex organic anions can permeate.
The patch clamp technique was used to determine the permeability of ruminal epithelial cells to various anions. Estimates for the atomic or molecular dimensions of the anions studied (anhydrated and hydrated) were obtained from the literature.
Whole cell experiments yielded a permeability sequence of p(NO3-) > p(Br-) [asymp] p(I-) [asymp] p(Cl-) > p(HCO3-) > p(acetate-) > p(F-) > p(propionate-) > p(butyrate-) > p(gluconate-). Single channel data showed a conductance of p(Cl) [asymp] 350 pS, p(acetate-) [asymp] 140 pS and p(propionate-) [asymp] 100 pS.
The low permeability for F- indicates that a removal of the hydration shell is necessary to allow passage through the channel pore, suggesting a pore diameter smaller than that of the smallest hydrated anion (Br-[asymp] 6.6 Å). Acetate (width 5.3 Å, length: 6.4 Å) falls below this limit. To allow passage of propionate (5.6 Å x 7.4 Å) and butyrate (5.5 Å x 8.2 Å), the pore diameter has to be larger than 5.6 Å. Passage is apparently delayed by the long tails, which have to be oriented in parallel to the channel axis. The data suggest that the passage of complex anions such as gluconate (width 8.3 Å) or ATP (width 9.7 Å) is unlikely.