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

Summary 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, k, is described by the relation k=aX⁺] _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+cRb⁺] _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.