Shaker B inactivating 'ball' peptide is shown to produce no detectable block of ATP-dependent K+ channels of rat skeletal muscle fibres at concentrations up to 300 microM or membrane potentials up to +30 mV. 2. The peptide does produce a voltage-dependent block of large-conductance Ca(2+)-activated K+ channels at lower concentrations (K1 = 55 microM at 0 mV). An appropriate point mutation (L7E) abolishes block. 3. Mean open times, corrected for missed closures, are little affected by the blocking peptide, but burst durations are substantially reduced. 4. The inactivating peptide increases occupancy of substates, whose amplitudes are 0.27 and 0.64 of fully open.
