Fig. 6. Cumulative addition of channel blockers demonstrates selective coupling of N-type Ca²⁺ and BK channels. (Ai) Diary plot of the duration of the first action potential in the evoked train from a single hippocampal neuron recorded in a slice preparation. Application of the L-type Ca²⁺ channel dihydropyridine antagonist isradipine (10 µM) had no effect on action potential duration. By contrast, subsequent application of the BK channel blocker iberiotoxin (IbTx, 100 nM) in the presence of the DHP antagonist slowed action potential repolarisation. (Aii) Examples of action potentials taken from the cell used in Ai, recorded before (control) and after addition of isradipine (10 µM) and isradipine (10 µM) + IbTx (100 nM). A clear prolongation of action potential duration is seen after IbTx addition (superimposed traces in black, control; light grey, isradipine; dark grey, isradipine+IbTx). In addition, a bar chart is shown of normalised action potential duration showing the broadening of action potentials during a train in the absence and presence of channel blockers. No effect of isradipine (10 µM) is seen, while further concomitant addition of IbTx (100 nM) slowed action potential repolarisation throughout the train (n=3, 10 spikes per cell). (Bi) Diary plot of duration of the first action potential in the evoked train showing that isradipine (10 µM) had no effect. In contrast, addition of the N-type Ca²⁺ channel blocker -conotoxin GVIA (-Ctx GVIA, 300 nM) slowed action potential repolarisation. (Bii) Example action potentials from the experiment shown in Bi, showing the action potential broadening only after application of -Ctx GVIA (300 nM). Normalised pooled data showed that the effect of -Ctx GVIA was observed throughout the train of action potentials (n=5 cells, 10 spikes per cell). (Ci) Diary plot of action potential duration showing that -Ctx GVIA (300 nM) had no effect if BK channels were pre-blocked by IbTx (100 nM). (Cii) Example action potentials from the experiment illustrated in Ci, showing action potential duration was prolonged by IbTx (100 nM) with no further effect of -Ctx GVIA (300 nM). The normalised pooled data showed that the effect of IbTx was observed throughout the train of action potentials, with no effect of subsequent addition of -Ctx GVIA (n=4, 10 spikes per cell).