Application of equivalent electrical circuit models to study of sodium transport across epithelial tissues

An equivalent electrical circuit model for Na transport across epithelial tissues under steady-state conditions, which incorporates the flows and forces across the two limiting membranes and the paracellular pathway, is described. The analysis of electrophysiologic data obtained on rabbit colon within the framework of this model provides information regarding the thermodynamic activity of cell Na and the resistance offered by the mucosal membrane to Na entry. The interpretation of the data dealing with the active extrusion of Na from the cell across the basolateral membrane awaits more detailed information regarding the pump mechanism responsible for this movement; until then, the electromotive force across the basolateral membrane and the resistance of that barrier must be considered phenomenologic parameters that relate the Na current to the observed electrical potential difference.