Effects of cations and pH on apical membrane potential of in vitro Necturus antrum

The surface epithelial cells of the gastric mucosa transport Na⁺ from lumen to serosa. The first step in this process is the entry of Na⁺ through an amiloride-sensitive Na⁺ channel at the apical membrane. The exact function(s) of this Na⁺ transport are unknown, but it has been suggested that it might help the stomach to withstand an acid load. The present study was undertaken to examine the effects of low luminal pH and the alkali cations Li⁺, K⁺, Rb⁺, and Cs⁺ on the amiloride-sensitive Na⁺ permeability of the apical membrane of Necturus antral cells by measuring the changes in apical membrane voltage (V(mc)) using conventional microelectrode techniques. Isosmolar replacement of luminal NaCl (pH 7.25) with LiCl caused a depolarization of the V(mc), a decrease in transepithelial resistance (R(t)), and an increase in the transepithelial potential (V(ms)), whereas replacement with KCl, RbCl, or CsCl caused a hyperpolarization of the V(mc), an increase in R(t), and a decrease in the V(ms). Luminal acidification from pH 7.25 to pH 3.00 caused very similar changes with all the cation solutions tested, hyperpolarizing the V(mc), increasing R(t), and reducing the V(ms) to near 0 mV. Acidification of the luminal NaCl solution from pH 7.25 to pH 2.00 caused a progressive hyperpolarization of the V(mc) similar to the effects seen with luminal amiloride (10^-4 M) in pH 7.25 NaCl solutions or luminal Na⁺-free (N-methyl-D-glucamine) Ringer at pH 7.25. These results demonstrate that 1) of the cations tested only Li⁺ can substitute for Na⁺ in maintaining the V(mc), and 2) external H ions (low luminal pH) block cation permeability of the apical membrane.