Figure 2. Representative current traces and average normalized junctional conductance-junctional voltage (G_j-V_j) relations.

Panels a, c, and e show the junctional current traces for wild type Cx32, R15W and H94Y paired heterotypically with wild type Cx26; all expressed in Xenopus oocytes. For clarity, only traces in 20 mV increments from +/−20 to +/−120 (or +/−10 to +/−110 for R15W) are shown. Panels b, d, and f show the average G_j-V_j relations for these pairs, averaged and plotted as mean +/− SEM as previously described (Abrams et al., 2001). The smooth curves approximating the steady state data correspond to the curves generated by the Boltzmann fits. Junctional voltages (V_j) were applied by varying the voltage applied to the cell expressing Cx26, with the cell expressing the wild type or mutant form of Cx32 held at V=0. Filled squares represent steady state conductances; hollow triangles represent instantaneous conductances. As show in 2b, the heterotypic Cx32/Cx26 conductance shows substantial instantaneous rectification between −120 and +120 mV, with instantaneous current increasing about 3 fold as the cell expressing Cx26 is made more positive with respect to the cell expressing Cx32; however, because the Cx26 hemichannel closes only at positive V_j and the Cx32 hemichannel closes on negative V_j, closure of these channels is seen only when the cell expressing Cx26 is made more positive than 50 mV with respect to Cx32 (and in turn the cell expressing Cx32 is 50 mV negative with respect to the cell expressing Cx26). In the case of the R15W and H94Y mutants paired with Cx26, channels remain predominantly closed until junctional voltage is more negative than +20 mV with respect to the cell expressing Cx26 (and more positive than −20 mV with respect to the cell expressing Cx32). Since the Cx26 hemichannel is predicted to be fully open at these voltages, the G_j-V_j curves in figures 2d and 2f reflect the relationships between open probability and junctional voltage for the mutant hemichannel. If two such hemichannels are placed head to head, as the case in a homotypic channel, the resulting open probability of that channel would be very low at all voltages.