Ionic Conductances of Extracellular Shunt Pathway in Rabbit Ileum

Abstract

The unidirectional influxes of Na, K, and C1 into isolated strips of rabbit ileum are comprised of movements across the mucosal membrane of the epithelial cells and ionic diffusion into an extracellular shunt pathway. A large fraction of the Na influx across the mucosal membrane alone is inhibited by Li, suggesting the participation of a carrier mechanism in the influx process. The partial ionic shunt conductances of Na, K, and C1 account for at least 82 % of the total tissue conductance. The calculated shunt permeabilities (P) are (in centimeters per hour) P = 0.040, PN = 0.035, and Pcl = 0.019, so that Px:PN:PclI = 1.14:1.00:0.55. Diffusion potentials across the tissue resulting from isotonic replacement of NaCl in the mucosal solution with mannitol or KC1 are described by the Goldman constant-field equation together with the above permeabilities of the shunt pathway. These observations are not consistent with permeation through a fixed-charge pore but can be explained by a model featuring constant ionic partition into a neutral-polar pore that traverses the tight junction. Such a pore may be lined with either fixed dipoles or fixed dipolar ions oriented such that electronegative groups influence the permselective properties of the diffusion pathway. The essential feature of both models is that electroneutrality is maintained by means of fixed membrane components and does not depend upon the presence of mobile counterions. In recent years, increasing attention has been focused on the role of transmural, extracellular pathways in the transport of solutes and water by a variety of epithelial tissues. However, the conductance properties of these pathways have not been defined directly because studies of transmural ionic fluxes, diffusion potentials, or streaming potentials do not clearly distinguish between the properties of extracellular and transcellular routes for ion flow. THE JOURNAL OF GENERAL PHYSIOLOGY VOLUME 59, 1972 pages 318-346 318 FRaIZELL AND SCHUmTZ Extracellular Shunt Pathway in Rabbit Ileum 319 As a result of recent studies of the electrical potential profile across rabbit ileum, Rose and Schultz (1) have suggested an equivalent electrical circuit model for this tissue that features a low-resistance, transepithelial, extracellular shunt. The relative resistive properties of the shunt and of the mucosal and serosal cell membranes suggested the present investigation which is concerned with a direct evaluation of the partial ionic conductances of the extracellular pathway. Evidence is presented that at least 85% of the transmural conductance can be attributed to this extracellular pathway, and the implications of these findings with respect to the interpretation of transmural electrical potential differences are discussed. THEORETICAL CONSIDERATIONS The essential resistive elements of the model proposed by Rose and Schultz (1) are illustrated in Fig. 1. R' is the resistance of the mucosal membrane, MUCOSAL SEROSA L SEROSAL SOLUTION EPITHELIAL CELL TISSUES SOLUTION

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@article{Frizzell1972IonicCO, title={Ionic Conductances of Extracellular Shunt Pathway in Rabbit Ileum }, author={Raymond A Frizzell and Stanley G. Schultz}, journal={The Journal of General Physiology}, year={1972}, volume={59}, pages={318 - 346} }