Figure 2. Cx43 HC activation in ventricular cardiomyocytes.

(A–C) Cx43 HC states and the effect of Gap19 and CT9 peptides. (A) Interaction of the CT tail with the CL brings HCs into the “available to open” state. (B) Gap19 interaction with the CT prevents CT-CL interaction, making HCs unavailable for opening. (C) CT9 peptide interaction with the CL brings HCs into the “available to open” state. (D) Bell-shaped [Ca²⁺]_i dependency of HC opening. CT9 removes HC inhibition at above 500 nM [Ca²⁺]_i (dashed line), thereby strongly enhancing HC opening. (E) Caffeine activation of RyRs triggers [Ca²⁺]_i elevation (brown trace) followed by NCX current (black) associated with Ca²⁺ extrusion. Spiking HC opening activity appears superimposed on the NCX current. At the start of caffeine application, simultaneous (stacked spikes) HC openings can be distinguished resulting from up to three HCs. (F) HC opening in ventricular cardiomyocytes requires three conditions: (a) activation of RyR2 by caffeine, adrenergic stress, or rapid electrical pacing; (b) [Ca²⁺]_i elevation; and (c) molecular interaction between RyR2 and Cx43 HCs (inhibited by RyRHCIp peptide). (G) Open HCs carry inward current producing about 1.6 mV depolarization per HC (at –70 mV diastolic potential). HC Ca²⁺ entry triggers microdomain Ca²⁺ elevation that is extruded by NCX activity, adding another 1.3 mV depolarization, yielding almost 3 mV depolarization per HC. HC opening events are brief (~8 milliseconds on average) but sufficiently long to obtain steady-state membrane charge redistribution and achieve the approximately 3 mV depolarization estimate.