Rapid determination of intraepithelial resistance barriers by alternating current spectroscopy. II. Test of model circuits and quantification of results.

The impedance of Necturus gallbladder epithelium was measured with transepithelial and intracellular microelectrodes in different transport states. The data are analysed with five electrical equivalent circuits, which differ with respect to the configuration of the paracellular shunt path (lumped vs distributed model of the lateral space), and of the apical cell membrane (non-ideal capacitance or surface amplification by micro-tubular infoldings). Least square fits indicate: 1. that the lumped model cannot represent the epithelium properly, even under control conditions; 2. that the distributed model, which considers the lateral intracellular space separately as a cable-like structure, describes the data well, both under control conditions and during collapse of the lateral spaces; and 3. that the above indicated variations of the apical membrane configuration improve the fits, but have little effect on the magnitude of the calculated circuit parameters. Quantitatively the analysis of 214 measurements on 25 gallbladders under control conditions yields the following results: The resistances of the tight junctions, of the lateral intercellular space, and of the apical and basal cell membrane are Rj = 123, Rlis = 35.5, Ra approximately 3,500, and Rb = 225 (all in omega cm2), and the capacitances of the cell membranes are Ca = 4.95 and Cbl = 26.5 (mu F/cm2). In oxygen deficiency and after cessation of chamber perfusion transport decreased, the spaces collapsed, and Rlis increased to approximately 130 omega cm2. Although the accuracy of the estimated Ra values is still limited, the analysis shows that it is possible to determine Rj and Rlis separately and to follow their changes in response to experimental maneuvers.