Fromm M, Schultz S G
J Membr Biol. 1981;63(1-2):93-8. doi: 10.1007/BF01969450.
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.
先前研究的结果表明，钾在短路的兔降结肠中的双向通量归因于通过细胞旁途径的被动扩散，并且该途径对钾的通透性比对钠和氯高十倍。然而，在存在较大的跨上皮钠和钾浓度差的情况下，跨上皮扩散电位远低于“恒定场方程”预测的值，并且似乎与这种高钾选择性不一致。本研究旨在解决这一明显矛盾，其结果表明：(a) 在广泛的化学和电位差组合范围内测定的钾双向跨上皮通量的比值与乌辛通量比方程预测的值相当吻合。(b) 根据浓度差存在时的净通量和短路条件下的单向通量确定的钾通透系数（PK）随钾浓度的增加而降低；在低钾浓度下，PK约为PNa的十倍，但在高钾浓度下接近PNa。在这些条件下，PNa不受影响。这些结果解释了在存在较大的跨上皮钠和钾浓度差时未能观察到较大的跨上皮扩散电位的原因。此外，这些结果与钾通过两条平行途径扩散穿过该标本的观点一致，一条途径对钾、钠和氯没有区分（“自由溶液”旁路），另一条途径对钾具有高度选择性，并且涉及与沿该途径的一个或最多两个位点的相互作用。
