title: Potassium transport across rabbit descending colon in vitro: evidence for single-file diffusion through a paracellular pathway.
abstract: The results of previous studies indicate that the bidirectional fluxes of K across short-circuited rabbit descending colon are attributable to passive diffusion through paracellular pathways and that this route is ten times more permeable to K than to Na and Cl. However, transepithelial diffusion potentials in the presence of large transepithelial Na and K concentration differences are much lower than those predicted by the "constant field equation" and appear to be inconsistent with this high K selectivity. The results of the present studies, designed to resolve this apparent contradiction, indicate that: (a) The ratios of the bidirectional transepithelial fluxes of K determined over a wide range of combined chemical and electrical potential differences conform reasonably well with those predicted by the Ussing flux-ratio equation. (b) The permeability coefficient of K (PK), determined from the net fluxes in the presence of concentration differences and from unidirectional fluxes under short-circuit conditions, decreases with increasing K concentration; in the presence of low K concentrations, PK is approximately ten-times PNa, but it approaches PNa in the presence of high K concentrations. PNa is not affected under these conditions. These results provide an explanation for the failure to observe large transepithelial diffusion potentials in the presence of large transepithelial Na and K concentration differences. In addition, these results are consistent with the notion that K diffuses across this preparation through two parallel pathways, one that does not discriminate among K, Na and Cl (a "free-solution" shunt) and another that is highly K selective and involves an interaction with one, or at most two, sites along the route.
mesh_terms: D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D002462:Cell Membrane / Q000502:physiology; D002794:Choline / Q000494:pharmacology; D003106:Colon / Q000378:metabolism*; D004848:Epithelium / Q000378:metabolism; D008564:Membrane Potentials; D011188:Potassium / Q000378:metabolism*; D011817:Rabbits; D012964:Sodium / Q000494:pharmacology
chemical_list: D012964:Sodium; D002794:Choline; D011188:Potassium