Potentials and Effective Ionic Mobilities in Frog Skin

FROG skin has been used by many workers in the study of bioelectric phenomena of living membranes. The isolated living skin mounted with Ringer's solution on both sides maintains a potential of from 10 to 100 mV., the inside of the skin being positive. This potential can be modified by changing the chemical or physical conditions. Many workers have attempted to interpret the results of such changes in terms of simple membrane potentials but without much success. Dean & Gatty (1937) point out some of the difficulties of such interpretation and review the literature. There is evidence of at least two sources of potential in the skin which may at times be opposed (Steinbach, 1933), and also that the active surfaces are not uniform but present a mosaic structure (see also Dean, 1938). While it is not possible to determine actual mobilities solely from changes in potential following ionic changes in the medium, we can interpret such changes in terms of relative mobilities. If, for example, Cl~ is replaced in the outside solution by an anion that has a higher mobility through some part of the skin, this ion will tend to diffuse inwards more rapidly than Cl~ diffuses outwards. This makes the outside solution less negative and decreases the skin potential. The magnitude of the change will depend not only on the charge and relative mobilities of the ions through the layers causing the potential, but also on the thickness, extent and the chemical nature of these layers as well as on the magnitude of the concentration gradients across them. It is not valid to assume that the entire concentration gradient across the skin is operative across that part of it that has selective ion permeability. Therefore, we can only deduce an averaged apparent ionic mobility and the absolute values for the mobilities of the ions as calculated by a Henderson equation have little meaning. We can say that one ion appears to have a greater or smaller mobility than another.