| Abstract: | We have applied transepithelial AC impedance techniques to gastric mucosa to reconcile ultrastructural and electrophysiological findings about gastric acid secretion and the mucosal barrier. By fitting impedance data measured at different HCl secretion rates to equivalent circuit models, we extracted capacitances and resistances (as measures of membrane area and ionic conductance, respectively) for the apical and basolateral membranes. The impedance measurements were found to be incompatible with earlier equivalent circuit models that modeled membrane electrical properties as lumped circuits based on one or two cell types. A distributed circuit model was developed that assumed only one dominant electrical pathway (i.e., one cell type), but that incorporated electrical effects arising from long and narrow membrane-lined structures present in the epithelium (e.g., gastric crypts, tubulovesicles, lateral intercellular spaces). This morphologically based model was found to represent the measured data accurately, and to yield values for membrane capacitances consistent with morphometric measurements of membrane areas. The main physiological conclusions from this analysis were as follows: (a) The dominant transepithelial current pathway may reside in the oxyntic cells. (b) The transepithelial conductance increase associated with the onset of acid secretion is entirely due to increased conductance of the apical membrane. This is in turn due entirely to increased area of this membrane, resulting from incorporation of tubulovesicular membrane. (c) When membrane conductances are normalized to actual membrane area by use of membrane capacitances, it turns out that acid secretion is not associated with a change in specific ionic conductance (change in conductance per unit area) at either the apical or basolateral membrane. (d) The puzzlingly low value of transepithelial resistance (≤400 Ω-cm2) arises because there are hundreds or thousands of square centimeters of actual membrane area per square centimeter chamber area. Apical membrane resistance is 25 kΩ-cm2 (actual membrane area), implying a tight barrier to back-diffusion of protons. |
