Potassium Transport Across Rabbit Descending Colon in Vitro: Evidence for Single-file Diffusion Through a Paracellular Pathway

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.