Fig. 4.

Ca²⁺ influx through I_h channels in DRG neurons. (A) Enhancement of I_h by increasing [K⁺]_o from 5 to 40 mM in a neuron. The pulse protocol is shown below. (B) Normalized I_h is increased by 277 ± 34% in 40 mM vs. 5 mM KCl (P < 0.001, n = 6). (C) Inhibition of hyperpolarization-induced I_h by extracellular Cs⁺ (2 mM) or ZD7288 (30 μM). (D) I_h was blocked significantly by 2 mM CsCl (84 ± 3%; P < 0.01, n = 11) or 30 μM ZD7288 (75 ± 5%, P < 0.01, n = 5). (E) Ca²⁺ signals in response to hyperpolarizing pulses of –120 mV for 10 s (arrows 1 and 3) or –90 mV for 50 s (arrow 2) in a DRG neuron (n = 15). (F) I_h at –120 mV for 10 s (traces 1 and 3) and at –90 mV for 50 s (trace 2). Protocols are shown in the text. (G) Fd signals corresponding to arrows 1, 2, and 3 in E and F. Similar to HCN4 in HEK293 cells, the fractional Ca²⁺ current of I_h was 0.5 ± 0.1% in DRG neurons (n = 3).