Figure 6. Both BK and SK channels contribute to the Ca²⁺ spike AHP.

A, three traces from the same cell show Ca²⁺ spikes evoked under control conditions (left trace), in the presence of apamin (middle trace) and in the combined presence of iberiotoxin and apamin (right trace). Spikes were evoked by applying a 2 nA depolarizing current step for 3 s. B, (upper panel) five sequential Ca²⁺ spikes from each trace in A (spikes indicated by brackets) are shown aligned to peak time. Control traces are black, apamin traces are blue and iberiotoxin + apamin traces are green. The lower panel shows one trace from each condition on an expanded time scale. By 3 ms after the peak of the spike (dashed line), the iberiotoxin + apamin trace was 20 mV more depolarized than the apamin-only trace. C, iberiotoxin by itself (100 nm, red traces) increased the peak amplitude of Ca²⁺ spikes and reduced the AHP compared to control (black traces), but the subsequent addition of apamin to the iberiotoxin-containing bath (green traces) further attenuated the AHP (same scale as the upper traces in B.) D1, for all cells, Ca²⁺ spike AHP amplitudes were measured as the difference between the membrane potential 3 ms before the peak and the membrane potential 3 ms after the peak (when control spikes typically reached minimum voltages). Measurements are compared before and after the addition of a blocker or, in the case of the control group, before and after a 10 min time period. The effect of the different treatments is expressed as the change in AHP amplitude as a percentage of the initial AHP amplitude (100 × (treatment – control)/control). For the control group, the AHP amplitude changed by 0 ± 4 % (n = 5) over the 10 min recording period. Apamin increased the AHP amplitude by 10 ± 9 % (n = 4), but this did not differ significantly from the control group (P > 0.3). Iberiotoxin reduced the AHP amplitude by 31 ± 4 % (n = 5), while the combination of iberiotoxin and apamin reduced the AHP by 45 ± 14 % (n = 4), and both of these differed significantly from the control group (P < 0.05). D2, when BK channels were first blocked with iberiotoxin, the subsequent addition of apamin caused a further reduction of the AHP amplitude by 18 ± 5 % (n = 3) compared to iberiotoxin alone (green and black hatched bar), which was a significant effect (P < 0.05).