Ba2+-Induced conductance fluctuations of spontaneously fluctuating K+ channels in the apical membrane of frog skin (Rana temporaria)

Abstract

We studied the influence of mucosal Ba2+ ions on the recently described (Zeiske & Van Driessche, 1979a, J. Membrane Biol. 47:77) transepithelial, mucosa towards serosa directed K+ transport in the skin of Rana temporaria. The transport parameters G (conductance), PD (potential difference), Isc (short-circuit current, "K+ current"), as well as the noise of Isc were recorded. Addition of millimolar concentrations of Ba/+ to the mucosal K+-containing solution resulted in a sudden but quickly reversible drop in Isc. G and Isc decreased continuously with increasing Ba2+ concentration, (Ba2+)o. The apparent Michaelis constant of the inhibition by Ba2+ lies within the range 40-80 microM. The apical membrane seems to remain permselective for K+ up to 500 microM (Ba2+)o. Higher (Ba2+)o, however, appears to induce a shunt (PD falls, G increases). This finding made an accurate determination of the nature of the inhibition difficult but our results tend to suggest a K+-channel block by K+-Ba2+ competition. In the presence of Ba2+, the power spectrum of the K+ current shows a second Lorentzian component in the low-frequency range, in addition to the high-frequency Lorentzian caused by spontaneous K+-channel fluctuations (Van Driessche & Zeiske, 1980). Both Lorentzian components are only present with mucosal K+ and can be depressed by addition of Cs+ ions, thus indicating that Ba2+ ions induce K+-channel fluctuations. The dependence of the parameters of the induced Lorentzian on (Ba2+)o shows arise in the plateau values to a maximum around 60 microM (Ba2+)o, followed by a sharp and progressive decrease to very low values. The corner frequency which reflects the rate of the Ba2+-induced fluctuations, however, increases quasi-linearly up to 1 mM (Ba2+)o with a tendency to saturate at higher (Ba2+)o. Based on a three-state model for the K+ channel (having one open state, one closed by the spontaneous fluctuation and one blocked by Ba2+) computer calculations compared favorably with our results. The effect of Ba2+ could be explained by assuming reversible binding at the outer side of the apical K+ channel, thereby blocking the open channel in ;competition with K+. The association-dissociation of Ba2+ at its receptor site is thought to cause a chopping of the K+ current, resulting in modulated current fluctuations.