The dual effect of rubidium ions on potassium efflux in depolarized frog skeletal muscle

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Abstract
The effects of external Rb+ on the efflux of42K+ from whole frog sartorius muscles loaded with 305mm K+ and 120mm Cl− were studied. K+ efflux is activated by [Rb+] o less than about 40mm according to a sigmoid relation similar to that for activation by [K+] o . At [Rb+]o greater than 40mm, K+ efflux declines, although at [Rb+] o =300mm it is still greater than at [Rb+] o =0mm. For low concentrations, the increment in K+ efflux over that in K+- and Rb+-free solution, Δk, is described by the relation Δk=a[X+] o n , for both K+ and Rb+. The value ofa is larger for Rb+ than for K+, while the values ofn are similar; the activation produced by a given [Rb+] o is larger than that by an equal [K+] o for concentrations less than about 40mm. Adding a small amount of Rb+ to a K+-containing solution has effects on K+ efflux which depend on [K+] o . At low [K+] o , adding Rb+ increases K+ efflux, the effect being greatest near [K+] o =30mm and declining at higher [K+] o ; at [K+] o above 40mm, addition of Rb+ decreases K+ efflux. At [K+] o above 75mm, where K+ efflux is largely activated, Rb+ reduces K+ efflux by a factorb, described by the relationb=1/(1+c[Rb+] o ). Activation is discussed in terms of binding to at least two sites in the membrane, and the reduction in K+ efflux by Rb+ at high [K+] o in terms of association with an additional inhibitory site.