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

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Abstract
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.