FIGURE 1.

The single channel current amplitude of Kv1.5 does not change between pH 7.4, 6.4, and 5.9. (A) Representative unitary current through a one-channel, outside-out patch at pH 7.4. Current traces were evoked by 1-s depolarizing pulses at 100 mV from a holding potential of −80 mV every 15 s. Kv1.5 unitary currents show flickering behavior. The failure of the channel to open (e.g., last trace in pH 7.4 column) was an infrequent observation at this pH. The corresponding cumulative all-points histogram based on traces in panel A is shown at the bottom of the panel. Fitting the histogram to a three-component Gaussian function gave a value of 1.7 pA for the major conducting level. (B) Unitary current from the same outside-out patch using the same voltage protocol after switching to external solution at pH 6.4. The open channel current does not change but there are more null sweeps. The all-points amplitude histogram shown at the bottom gave a single channel current of 1.7 pA for the major conducting level. The higher proportion of the nonconducting points reflects the higher proportion of null sweeps and the more frequent termination of active sweeps by a long-lived nonconducting state. (C) Unitary current from a different outside-out patch using the same voltage protocol as in panel A but with an external solution at pH 5.9. Channel activity is observed in only one of the 12 sweeps. This dramatic decrease of channel activity was not due to the loss of the channel from the patch since recovery of activity was obtained after returning to pH 7.4 solution (D). The all-points histograms at the bottom of panels C and D give a value of 2.0 pA for the mean open channel current.