A mathematical model of amphibian skin epithelium with two types of transporting cellular units

Abstract

A computer model of ion transport across amphibian skin epithelium containing two types of cellular units, their relative number and sizes, and a paracellular pathway has been developed. The two cellular units are, a large Na+ transporting compartment representing the major epithelium from stratum granulosum to str. germinativum, and a small, Cl- transporting compartment representing the mitochondria rich cell. The cellular units both contain dissipative and active pathways according to the two-membrane model. The Na+ transporting unit includes a (Na+, K+, 2 Cl-) co-transport system in the inward facing membrane. The outward facing membrane of the Cl- transporting units contains a potential gated Cl- permeability. Effects of ion distributions and changes in gating variables on the time course of transepithelial voltage clamp currents and their steady states are analyzed. The model predicts communication between the two cellular units under open circuit conditions.

The volume of mitochondria-rich cells of frog skin epithelium

“…Chloride removal could cause the loss of cell C1 across the apical membrane. On the basis of model calculations [6,7] C1 efflux across the basolateral membrane might also be expected to continue for some time after removal of CI from the mucosal perfusate. The loss of C1 and an accompanying cation, possibly K, would result in the observed cell shrinkage.…”

“…A striking correlation between the number of MR cells and the magnitude of the Cl conductance across the skin was observed [3,12]. Furthermore, it was noted that the C1 conductance of toad skin epithelium exhibited voltage-dependent gating: depolarization reduced C1 conductance, and hyperpolarization greatly increased the C1 conductance [4][5][6][7]. The Cl conductance of toad skin was found to be * Present address: University of Copenhagen, Institute of Biological Chemistry A, 13 Universitetsparken, DK-2100 Copenhagen 0, Denmark.…”

“…The transepithelial movement of chloride ions across frog skin epithelium may be adequately described by the flux ratio equation for passive movements, but the pathways taken by the ions are unknown [3][4][5][6][7]13]. Two pathways for C1 have been proposed: (i) a paracellular leak across the tight junctions and (ii) transcellular movement across the mitochondria-rich cells (MR) of the skin [5,13].…”

Fast and slow voltage modulation of apical Cl- permeability in toad skin at high [K+]

“…Models based on slow voltage activation of apical Cl -channels have been proposed in order to explain this observation (5,7,9). Also, there is ample evidence that Cl -ions move through a specialized cell compartment consisting of the mitochondrion-rich cells (MR-cells) (1,2,(13)(14)(15).…”

“…It is known that hyperpolarization of toad skin epithelium leads to a slow activation of a Cl -conductive pathway (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12). Models based on slow voltage activation of apical Cl -channels have been proposed in order to explain this observation (5,7,9).…”