We have studied potassium currents through a cloned Ca(2+)-dependent K+ channel (hslo) from human myometrium. Currents were recorded in inside-out macropatches from membranes of Xenopus laevis oocytes. In particular, the inactivation-like process that these channels show at high positive potentials was assessed in order to explore its molecular nature. This current inhibition conferred a bell shape to the current-voltage curves. The kinetic and voltage dependence of this process suggested the possibility of a Ba2+ block. There were the following similarities between the inactivation process observed at zero-added Ba2+ and the internal Ba2+ block of hslo channels: (a) in the steady state, the voltage dependence of the current inhibition observed at zero-added Ba2+ was the same as the voltage dependence of the Ba2+ block; (b) the time constant for recovery from current decay at zero-added Ba2+ was the same as the time constant for current recovery from Ba2+ blockade; and (c) current decay was largely suppressed in both cases by adding a Ba2+ chelator [(+)-18-crown-6-tetracarboxylic acid] to the internal solution. In our experimental conditions, we determined that the Kd for the complex chelator-Ba2+ is 1.6 x 10(-10) M. We conclude that the current decay observed at zero-added Ba2+ to the internal solution is due to contaminant Ba2+ present in our solutions (approximately 70 nM) and not to an intrinsic gating process. The Ba2+ blocking reaction in hslo channels is bimolecular. Ba2+ binds to a site (Kd = 0.36 +/- 0.05 mM at zero applied voltage) that senses 92 +/- 25% of the potential drop from the internal membrane surface.

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