Figure 3.

Rectification of the hyperpolarization-activated current is associated with a submillisecond outward transient that does not arise from HCN2 channel deactivation. (A and C) Schematic representation of the complete voltage paradigms (A) and expanded views of the interval during the middle of the record (C, as indicated by the red ovals in A). In the top “leak” paradigm, the patch was stepped to −135 mV for 5 ms (a duration sufficient to allow uncompensated capacity transients to decay but insufficient to permit channels to open) and then stepped to +80 mV for 1 or 3 ms before being returned to −135 mV for 2 s. In the bottom “active” paradigm, channels were first opened at −135 mV for 2 s before the 1- or 3-ms sojourn at +80 mV and the return to −135 mV. Each sweep was filtered at 20 kHz and sampled at 100 kHz with “leak” and “active” sweeps recorded interlaced. (B) Averages of seven “leak” sweeps (top) and their cognate interleaved “active” sweeps (bottom) recorded with K as the only alkali metal. (D) Expanded views of the total current (including uncompensated capacity and ionic components) observed during the step to (top left) and return from (top right) +80 mV in the presence (dark gray lines) and absence (light gray lines) of activated HCN2 channels and of the “active” minus “leak” subtracted records before, during, and after the step to +80 mV for 1 ms (thin black line, bottom panel only) or 3 ms (thick black lines). Data are representative of five similar recordings of which three were collected at the widest bandwidth (filtered at 100 kHz and sampled at 200 kHz).