CHARACTERISTICS OF IONIC TRANSPORT PROCESSES IN FISH INTESTINAL EPITHELIAL-CELLS

A general mathematical version of the cell model of a leaky epithelium for the NaCl absorption is presented, analysed and integrated numeric ally. The model consists in the adequate differential equations that d escribe the rate of change of the intracellular ion concentrations and are expressed in strict accordance with the law of mass conservation. The model includes many state variables representing ion concentratio ns, the cell volume, and membrane potentials. Ion movements are descri bed by the Michaelis-Menten kinetics or by the constant field flux equ ation (Goldman-Hodgkin-Katz). In this paper, we model the intracellula r ion concentrations, change in the cell volume, the transmembrane flu x and membrane potentials of intestinal epithelium of both fresh water and sea water fish, and generate several simulations (in both the ste ady state and the transient slate analysis) that appear to accord with prior experimental data in this area. For the ion movements of the se a water fish intestine, there were included a Na+/K+ pump, a K+-Cl- sy mport system, the K+ and Cl- channels in the basolateral membrane, whe reas a Na+-K+-2Cl(-) cotransporter for NaCl absorption and K+ channels are located in the apical membrane. In the fresh water fish intestina l cells, the NaCl absorption is performed by two coupled antiporters N a+/H+ and Cl-/HCO3- presumably responsible for the intracellular pH re gulation. In this type of cells, Na+ and K+ channels are located withi n the apical membrane, whereas Cl- channels are located within the bas olateral membrane. The osmotically induced water transport across the apical and basolateral membranes has been taken into account as well. The simulations plot the steady state values for membrane potential di fference, short-circuit current and intracellular ionic concentrations using the magnitude of the transmembrane flux through the Na+/K+ pump and Na+ -K+ 2Cl(-) cotransporter, or the basolateral Cl- permeability as dependent variables. The model behaves appropriately with regard t o several experimental studies regarding the hyperpolarization (sea wa ter fish intestine) and depolarization (fresh water fish intestine) of the apical membrane potential and inhibition of the short-circuit flu x with reduced NaCl absorption. The model is also used to make several analytical predictions regarding the response of the membrane potenti al and ionic concentrations to variations in the basolateral Cl- flux. Furthermore, maintaining conservation of both mass and electroneutral ity and taking into account the osmolar forces is an important advanta ge, because it allows a rigorous analysis of the relationship between membrane potential difference, volume and flux. The model can be used in the analysis and planning of the experiments and is capable of pred icting the instantaneous values of ionic fluxes and intracellular conc entrations and of cell volume. (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.