Influence of external barium and potassium on potassium efflux in depolarized frog sartorius muscles

Summary Efflux of⁴²K⁺ was measured in frog sartorius muscles equilibrated in depolarizing solutions with external K⁺ concentrations (K⁺] _o ) between 75 and 300mm and NaCl concentrations of 60, 120, or 240mm. For several combinations of KCl and NaCl, steady-state internal potentials (V _i) were the same for different K⁺] _o . For the range ofV _i examined, K⁺ efflux occurs principally through the K⁺ inward rectifier channels. When external K⁺ is removedV _i remains constant for 2 to 3 hr because of the high membrane conductance to Cl^–, but K⁺ efflux drops by about one order of magnitude.External Ba²⁺ in the presence or absence of external K⁺ produces an inhibition of K⁺ efflux described by a relation of the formu=(u₁/(1+C)Ba²⁺] _o ))+u ₂, whereu is the uninhibited fraction of K⁺ efflux;u ₁, u₂ andC are constants; andu ₁+u₂=1.C depends both on K⁺] _o andV _i. When K⁺] _o 75mm, increasing K⁺] _o at constantV _i reduces Ba²⁺ sensitivity. For constantV _i–30 mV, Ba²⁺ sensitivity is less when K⁺] _o =0 than when K⁺] _o 75mm. When K⁺] _o =0, Ba²⁺ sensitivity decreases asV _i is made more positive. The dependence of the Ba²⁺ sensitivity onV _i at constant K⁺] _o is greater when K⁺] _o =0 than when K⁺] _o 75mm.Both the activation of K⁺ efflux by external K⁺ and the Ba²⁺ inhibition of K⁺ efflux can be explained on the basis of two membrane control sites associated with each channel. When both sites are occupied by K⁺, the channels are in a high flux state. When one or both sites are empty, the channels are in a low, nonzero flux state. When Ba²⁺ occupies either site, K⁺ efflux is further reduced. The reduction of Ba²⁺-sensitivity by increasing K⁺] _o at high K⁺] _o is attributable to the displacement of Ba²⁺ from the control sites by K⁺. The increased Ba²⁺ sensitivity produced by going from K⁺] _o =0 to K⁺] _o >-75mm whenV _i–30 mV is attributable to states in which Ba²⁺ occupies one site and K⁺ the other when K⁺] _o 0. The smallerV _i dependence of the Ba²⁺ sensitivity when K⁺] _o 75mm compared to K⁺] _o =0 is attributable to the necessity that Ba²⁺ displace K⁺ at the control sites when K⁺] _o is high but not when K⁺] _o =0.