Figure 5

The density of functional BK channels at the basolateral pole increases with age. (A) Whole-cell recordings were made from embryonic and posthatch hair cells to confirm the onset of BK currents around E18/19. Example traces from three ages are shown. In these examples, currents were elicited by voltage steps from -80 mV to -20 or 0 mV. A fast-activating outward current was present at E18 under whole-cell recording conditions, but no such current could be recorded from earlier ages. (B) Single-channel current traces are shown for a cell-attached patch from a posthatch hair cell held at three different pipette potentials. Bath and pipette salines consisted of standard ECF and high potassium electrode solutions, respectively. Channel openings are downward. In this configuration, more negative pipette voltages correspond to increasing depolarization. The channel was voltage-sensitive, as evidenced by an increased open probability for more depolarized voltages. (C) Single-channel current amplitudes were estimated from all-points histograms using traces in (B). Linear regression to these data (dotted line) indicates a unitary conductance of 150 pS. Reversal was estimated to be about -60 mV, near the presumed resting potential of the cell. Therefore, all inclusion criteria were satisfied, establishing the identity of this channel as BK. (D) The percent of cell-attached patches (C.A.P.) exhibiting channel fluctuations attributed to BK are plotted for cells from embryonic and posthatch basilar papilla. The total number of attempts is shown in parentheses. The breakdown for the E17-E19 data set was: E17 (6%), E18 (7%), E19 (17%).