Electrical Potential Differences and Electromotive Forces in Epithelial Tissues

Dear Sir: Studies on intracellular electrical potentials in a variety of epithelial tissues which are involved in transmural Na transport have resulted in an apparent dichotomy. In tissues characterized by high transmural resistances such as isolated frog skin (1, 2), toad skin (3), and toad urinary bladder (4), the cell interior is electrically positive with respect to the outer or mucosal solution; the electrical potential profile across these tissues consists of two or more steps in the same direction that sum to give the total transmural potential difference (PD). In contrast, low resistance tissues such as rabbit ileum (5), bullfrog small intestine (6), rat colon (7), and renal tubular epithelium (8) are characterized by intracellular potentials that are electrically negative with respect to the mucosal solution; the electrical potential profile across these tissues consists of two steps in opposite directions that sum to give the transmural PD. These two types of profiles are illustrated in Fig. 1 using data obtained on frog skin (2) (Fig. I a) and rabbit ileum (5) (Fig. 1 b). All of the tissues mentioned above are engaged in active Na transport from the mucosal (outer) to the serosal (inner) solution and share other similarities with respect to ion transport and intracellular ion composition. Thus, it is reasonable to inquire whether the differences in electrical potential profile necessarily reflect fundamentally different properties of the transporting cells. In particular, do these different profiles necessarily imply that the electromotive forces across the mucosal or outer membranes of high resistance epithelia are oriented in a direction opposite to those characteristic of low resistance epithelia? An analysis of the equivalent electrical circuit model illustrated in Fig. 2 provides an answer to this question. In this circuit, Em designates an electromotive force operating across the mucosal or outer membrane, R is the internal resistance of this battery, and R2 is a shunt resistance across this membrane. E, R3 , and R4 are the respective parameters for the serosal or inner membrane. R5 designates the resistance of a transepithelial, extracellular shunt; m, c, and s represent the mucosal, intracellular, and serosal electrodes. All electrical potential differences are given with reference to the potential of the mucosal solution which is taken as zero. The solutions of this circuit for the electrical PD across the mucosal membrane (me) and the transmural PD (m) are: