Figure 3.

V_j dependence of Cx45/Cx43-EGFP heterotypic junctions. (A) Phase contrast (Left) and fluorescence (Right) images of a Cx45/Cx43-EGFP cell pair. The Cx43-EGFP cell is identified by its fluorescence. The arrow indicates a junctional plaque. (B) Changes in I_j in response to positive and negative 60-mV V_j steps (positive V_j is defined as relatively positive on the Cx43-EGFP side). I_j increased slowly from an initial level of ≈13 pA during the negative V_j step and decreased rapidly during the positive V_j step. g_j decreased by about 30% between the steps. (C) Normalized g_j–V relation for Cx45/Cx43-EGFP heterotypic junctions in 14 cell pairs. The thin black line is a fit of a 4-state contingent gating model with one gate in each hemichannel to all data points (open and center-dotted circles). The parameters are A= 0.18 mV⁻¹ and V₀ = 1 mV for the Cx45 hemichannel and A = 0.03 mV⁻¹ and V₀ = 26 mV for the Cx43-EGFP hemichannel. The thick gray line is a fit of the data points indicated by dotted circles with a four-state contingent gating model of the channel containing fast and slow gates only in the Cx45 hemichannel. The parameters are A = 0.27 mV⁻¹ and V₀ = 3 mV for the slow gate and A = 0.46 mV⁻¹ and V₀ = 10 mV for the fast gate. Normalized g_j decreased rapidly with voltage for relative positivity on the Cx43-EGFP side; the relatively high sensitivity indicates that this decrease is mediated by the Cx45 hemichannel and that its gating polarity is negative. An expanded view (Inset) demonstrates that the contingent model can explain the secondary increase in g_j at large V_j (see Discussion and Fig. 6).