Observations on Ionic Movement Through the Gastric Mucosa

From studies on the isolated gastric mucosae of amphibia and of the stomach in the intact mammal, there has emerged a simple description of the secretion of acid. There are two primary active transport systems, one for chloride and the other for the hydrogen ion. Chloride is transported somewhat more rapidly than the hydrogen ion. The difference between these two rates is substantially independent of the rate of acid production and constitutes a net current of negative ions moving from serosa to mucosa. Consequently a relatively constant electrical current is generated which accounts for the characteristic transepithelial potential difference. Water is assumed to move passively in response to the transport of these solutes, but clearly much more needs to be known about the kinetics of solvent flow (1). The variation in the composition of gastric juice as a function of flow, which led to the unresolved controversy between Teorell and Hollander, indicates that there is a more or less constant leak or secretion of sodium. Perhaps potassium may approach a diffusion equilibrium between interstitial fluid and gastric juice, though in species other than man the electrochemical potential of potassium in the secretion is less than that in plasma. In search for a more sufficient explanation of the electrophysiology of the gastric mucosa, Harris and Edelman (2) observed the consequence of varying the concentrations of potassium and chloride in the two solutions that bathe the mucosa. Increasing the potassium concentration of the solution bathing the serosal, submucosal, or interstitial face led to a reduction of the transmucosal potential difference. A reduction of the chloride concentration of the serosal solution also depresses the potential difference. By contrast, changing the potassium or chloride concentrations in the solution in contact with the mucosal or luminal surface has little effect on the potential difference. Because the potential change due to an altered serosal solution closely follows that predicted from the Hodgkin-Horowicz equation, it was concluded that the serosal face of the gastric epithelium is selectively permeable to potassium and chloride. Others have applied similar reasoning or the Carroll equation to the results of ionic substitution (3). But before one can proceed to the interpretation of potential changes that follow ionic substitution, it must be appreciated that the gastric mucosa is continuously generating current. A steady-state change of potential difference must derive from a change in current production and/or resistance. Though Harris and Edelrnan recognized that the necessary condition, zero net charge transfer, for the application of the