The interaction of “K+-like” cations with the apical K+ channel in frog skin

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Abstract
The apparent permeability of the apical K+ channel in the abdominal skin of the frog (Rana temporaria) for different monovalent cations was tested by comparing the shortcircuit current (SCC) obtained after imposition of serosally directed ionic concentration gradients. Furthermore, the SCC was subjected to noise analysis. Of various cations tested, only the “K+-like” ions NH 4 + , Rb+ and Tl+, besides K+, were found to permeate the apical K+ channel, as reflected by SCC- and fluctuation analysis: (i) The SCC could be depressed by addition of the K+-channel blocker Ba2+ to the mucosal solution. (ii) With the K+-like ions (Ringer's concentration), a spontaneous Lorentzian noise was observed. Plateau values were similar for K+ and Tl+, and smaller for NH 4 + and Rb+. The corner frequencies clearly increased in the order K+4 + +≪Rb+. The SCC dose-response relationships revealed a Michaelis-Menten-type current saturation only for pure K+- or Tl+-Ringer's solutions as mucosal medium, whereas a more complicated SCC behavior was seen with Rb+ and especially, NH 4 + . For K+-Tl+ mixtures an anomalous mole-fraction relationship was observed: At low [Tl+]/[K+] ratios, Tl+ ions appeared to inhibit competitively the K+ current while, at high [Tl+]/[K+] ratios, Tl+ seemed to be a permeant cation. This feature was also detected in the noise analysis of K+−Tl+ mixtures. Long-term exposure to mucosal Tl+ resulted in an irreversible deterioration of the tissue. The SCC depression by Ba2+ was of a simple saturation-type characteristic with, however, different half-maximal doses (NH 4 + ++). Ba2+ induced a “blocker noise” in presence of all permeant cations with corner frequencies that depended on the Ba2+ concentration. A linear increase of the corner frequencies of the Ba2+-induced noise with increasing Ba2+ concentration was seen for NH 4 + , Rb+ and K+. With the assumption of a pseudo two-state model for the Ba2+ blockade the on- and off-rate constants for the Ba2+ interaction with the NH 4 + /Rb+/K+ channel were calculated and showed marked differences, dependent on the nature of the permeant ion. The specific problems with Tl+ prevented such an analysis but SCC- and noise data indicated a comparably poor efficiency of Ba2+ as Tl+-current inhibitor. We attempted a qualitative analysis of our results in terms of a “two-sites, three-barriers” model of the apical K+ channel in frog skin.