The Effects of Alkali Metal Cations and Common Anions on the Frog Skin Potential

Abstract

The effects on the potential difference across isolated frog skin (R. catesbeiana, R. pipiens) of changing the ionic composition of the bathing solutions have been examined. Estimates of mean values and precision are presented for the potential changes produced by substituting other alkali metal cations for Na at the outside border and for K at the inside border. In terms of ability to mimic Na at the outside border of bullfrog skin, the selectivity order is Li > Rb, K, Cs; at the outside border of leopard frog skin, Li > Cs, K, Rb. In terms of ability to mimic K at the inside border of bullfrog and leopard frog skin: Rb > Cs > Li ~ Na. Orders of anion selectivity in terms of sensitivity of the potential for the outside border of bullfrog skin are Br ~ C1 > NOs > I > SO4, isethionate and of leopard frog skin are Br, C1 > I, NOs, SO4. An effect of the solution composition (ionic strength?) on the apparent Na-K selectivity of the outside border is described. The results of the investigation have been interpreted and discussed in terms of the application of the constant field equation to the Koefoed-Johnsen-Ussing frog skin model. These observations may be useful in constructing and testing models of biological ionic selectivity. I N T R O D U C T I O NIn the recent literature, models of selectivity for alkali metal cations have appeared (1, 2). Inasmuch as such suggestions appear to offer pathways toward a more fundamental understanding of the basis of bioelectric phenomena, an attempt to furnish data for testing the validity of the hypotheses is in order. Experimental work with the alkali metal cations has a long history (3, 4), but there is a scarcity of studies in which members of the series have been quantitatively compared under conditions formulated in the light of modern theories of bioelectric phenomena (but see Sjodin (5, 6) on frog skeletal muscle).The suitability of frog skin for such a study lies in two facts. It maintains a 749

K+-permeability of the outer border of the frog skin (R. temporaria)

“…Data on the uptake of K § compared to that of the extracellular marker (PEG [12,13]) across the outer border also suggest that this border is permeable to K--. On the other hand, extensive electrophysiological studies provide evidence of a low but nonetheless finite permeability of the outer border of amphibian skins and bladders to K § [8,4]. The discrepancy between the present findings and those of others [8,4] could be ascribed to species differ- ences.…”

“…On the other hand, extensive electrophysiological studies provide evidence of a low but nonetheless finite permeability of the outer border of amphibian skins and bladders to K § [8,4]. The discrepancy between the present findings and those of others [8,4] could be ascribed to species differ- ences. In the present investigation, permeability of the apical membrane td K + was observed only in skins of R. temporaria, and similar observations have been made by others [23].…”

“…This permeability pattern has been repeatedly confirmed [4,8,22]. In fact, the only condition under which a high K+-permeability had been found is that which prevails after treatment of the apical membrane of the frog skin with polyene antibiotics such as amphotericin B [19] or Filipin [20] which were reported to induce the formation of K § permeable channels in these membranes.…”

Volume-regulatory responses of Amphiuma red cells in anisotonic media. The effect of amiloride.

“…Third, Na transport is a saturable function of external Na (Ussing, 1948;Frazier et al ., 1962 ;Kinsella and Aronson, 1981). Fourth, Li can substitute for Na, whereas K cannot (Zerahn, 1955 ;Lindley and Hoshiko, 1964 ;Herrera, 1972 ;Kinsella and Aronson, 1981). In addition, amiloride appears to act as a competitive inhibitor of Na uptake in the Amphiuma red cell VRI response .…”

Ionic selectivity revisited: The role of kinetic and equilibrium processes in ion permeation through channels

“…Sieving concepts were extended by Boyle and Conway [9] and by Ling [89], who used hydrated radii to calculate the Na vs. K selectivity to be expected from the differences in electrostatic interactions at their distance of closest approach to an anionic site. Considerations based upon particular stable hydration states were further ingeniously developed by Mullins [100,101] and by Lindley and Hoshiko [88]. Such mechanical theories can, at least in principle, be unified with affinity theories by specifying friction in terms of the activation energy of the transition states for migration.…”