Apical membrane potassium and chloride permeabilities in surface cells of rabbit descending colon epithelium

The apical membranes of surface cells in the rabbit descending colon possess a significant ionic conductance in parallel to amiloride-blockable Na+ channels. The identity of the ion(s) responsible for the amiloride-insensitive conductance is unknown. The purpose of the present paper was to assess the permeability and net driving forces for K+ and Cl- across this membrane using conventional and ion-sensitive micro-electrode techniques. Intracellular Cl- activity (aiCl) averaged 23 +/- 2 mM with an equilibrium potential (ECl) of -38 +/- 2 mV. This value is less than previous estimates of the electromotive force (e.m.f.) of the amiloride-insensitive pathway (ca. -50 mV). Consequently, Cl- alone cannot account for the amiloride-insensitive conductance. Replacement of Cl- by gluconate in the serosal solution decreased aiCl to 17 +/- 2.8 mM. aiCl was lowered to approximately 1 mM by replacement in the mucosal bath or by replacement in both solutions. The results indicate a low Cl- conductance in the basolateral membrane, in agreement with previous electrophysiological studies of this epithelium. In contrast to Cl-, the chemical driving force for K+ was large enough to support the e.m.f. of the amiloride-insensitive pathway (K+ equilibrium potential, EK = -66 mV). The basolateral membrane potential (Vbl), EK and the intracellular K+ activity (aiK) were decreased in parallel following inhibition of the basolateral Na-K pump, providing evidence that Vbl is largely due to a K+ diffusion potential. In the presence of serosal 10(-4) M-ouabain, aiK appeared to remain above equilibrium and more than doubled after addition of Ba2+ to the serosal bath. Replacement of the mucosal bathing solution with KCl or gluconate Ringer solution largely restored Vbl and the transepithelial potential (VT) in tissues which had been previously treated with ouabain. The restoration of VT was decreased and the transepithelial resistance (RT) was increased by addition of tetraethylammonium to the mucosal bath. The above results suggest that there are at least four routes for ion movement across the apical membrane of rabbit colon surface epithelial cells. These are: (1) an amiloride-sensitive Na+ channel, (2) a K+ conductance, (3) electroneutral uptake of Cl- from lumen to cell interior and (4) an active K+ transport mechanism, also from lumen to cell interior.