Figure 4. Characteristics of the M-current in bovine chromaffin cells and its inhibition by histamine.

A, hyperpolarizing voltage steps from a holding potential of −30 mV gave rise to a slow inward current relaxation. This inward current relaxation is characteristic of an M-current and is due to the closure of the M-channels. The inset shows the inward current relaxation evoked by a step to −50 mV, highlighting the increase in membrane resistance occurring during the voltage step, consistent with channel closure, as indicated by the reduction in the ohmic current step represented by a and b. B, average current-voltage relation for seven cells showing a reversal potential for the current of about −80 mV. Current was measured over a 10 ms segment beginning 25 ms after the start of the voltage step. C, M-currents recorded in response to a voltage step to −60 mV from a holding potential of −30 mV in standard bath solution (a), and in the presence of linopirdine (10 and 100 μm, b). Linopirdine blocked the M-current in a concentration-dependent fashion. D, M-currents recorded in response to a voltage step to −60 mV in standard bath solution (a), and after exposure to histamine (5 μm) for 3 min. Histamine inhibited M-currents recorded in response to hyperpolarizing voltage steps by around 40 %. Records in A and D are from the same cell; records in C are from a different cell.