Figure 3. Kir2.1 and HCN4 channels reconstitute two levels of resting membrane potential in CHO cells that express both channels in 2 mM [K⁺]_e.

(A-C) Resting membrane potentials and whole-cell ramp currents of three populations of CHO cells that expressed both human Kir2.1 and HCN4 channels before and after reversible changes of [K⁺]_e from 5 mM to 2 mM.

Top raw: (A) 50% of the cells remained on hyperpolarization in 2 mM [K⁺]_e, (B) 21% of the cells spontaneously jumped into permanent depolarization, and (C) 30% of the cells fluctuated between two levels. Blue numbers indicate phases 1 and 2 of changes in resting membrane potential. Time scale, 100 s.

Second row: (A) Whole-cell ramp currents in 5 mM (black) and 2 mM (pink) [K⁺]_e; (B-C) Pink, blue, and green lines indicate “a”, “b”, and “c” types of whole-cell ramp currents that were described in Fig. 2. The first and third zero-current potentials of “a” type currents and reversal potential of “b” type currents were −82.9 ± 3.8 mV (n=5), −22.2 ± 4.6 mV (n=5), and −19.3 ± 3.1 mV (n=7) for B and −88.9 ± 1.4 mV (n=15), −21.9 ± 1.9 mV (n=15), and −24.3 ± 2.3 mV (n=4) for C. Reversal potential of “c” type currents was −93.5 ± 1.3 mV (n=10).

Third row: Purple and orange lines indicate the currents after sequential application of 0.5 mM Ba²⁺ and both 0.5 mM Ba²⁺ and 2 mM Cs⁺ in 2 mM [K⁺]_e, respectively. The HCN2 currents at −100 mV were −20.2 ± 13.4 pA (n=5) for A, −35.6 ± 18.5 pA (n=3) for B, and −42.1 ± 7.8 pA (n=9) for C.

Bottom raw: (A) Resting membrane potentials (filled black and pink bars) of the cells and reversal potentials (open black and pink bars) of whole-cell ramp currents in 5 mM and 2 mM [K⁺]_e; (B-C) Two levels of resting membrane potential (patterned black bars), the first and third zero-current potentials (patterned pink bars) of ‘a’ type N-shaped ramp currents, and reversal potentials of ‘b’ (open blue bars) and ‘c’ (open green bar) types of ramp currents in 2 mM [K⁺]_e (n=4–30).