Figure 4. HCN channels contribute to two levels of resting membrane potential in K2P1-deficient, Kir2.1-expressing human iPSC-derived cardiomyocytes in 2 mM [K⁺]_e.

(A-B) Resting membrane potentials of two populations of K2P1-deficient, Kir2.1-expressing human iPSC-derived cardiomyocytes before and after [K⁺]_e was reversibly changed from 5.4 mM to 2 mM.

(C) Three types of whole-cell ramp currents recorded in 2 mM [K⁺]_e in the human iPSC-derived cardiomyocytes in A and/or B.

(D) Percentage of Kir2.1-expressing human iPSC-derived cardiomyocytes treated with control shRNA (filled black bar) or K2P1-targeted shRNA#1 (filled pink bar) that had two levels of resting membrane potential in 2 mM [K⁺]_e; K2P1-like whole-cell inward leak Na⁺ currents recorded in 0 mM [K⁺]_e in Kir2.1-expressing human iPSC-derived cardiomyocytes without (open black bar) and with (open pink bar) treatment withK2P1-targeted shRNA#1, which exhibited the phenomenon (* P = 0.0001, relative to control). (E) Resting membrane potential of K2P1-deficient, Kir2.1-expressing human iPSC-derived cardiomyocytes recorded when Na⁺-based bath solutions were changed from 5.4 mM to 2 mM K⁺, 30 μM ivabradine was reversibly applied (bold teal line), and solution was returned to 5.4 mM K⁺.

(F) Whole-cell ramp currents of K2P1-deficient, Kir2.1-expressing human iPSC-derived cardiomyocytes that show fluctuating resting membrane potentials in 2 mM [K⁺]_e before (pink line, “b” type current) and after sequential application of 0.5 mM Ba²⁺ (purple line) and both 0.5 mM Ba²⁺ and 30 μM ivabradine (teal line) (n=8–35).