Fig. 6.

Voltage and chemical gating of Cx36 gap junctions. (A–C) Voltage dependence of Cx36 homotypic junctions expressed in Xenopus oocytes and transfected HeLa cells. (A) Graph of initial and steady-state G_j (filled and open symbols, respectively) as a function of V_j. G_j is g_j normalized to its value at V_j = 0. Data represent mean values obtained from 9 Xenopus oocyte cell pairs with maximum gs less than 5 μS. Both initial and steady-state G_j- V_j relations are nearly symmetric about V_j = 0 with initial G_j showing an increase and steady-state G_j a decrease with increasing V_js of either polarity. (B) Representative junctional currents for V_j steps up to ±120 mV in 20 mV increments. A +20 mV V_j step, 200 msec in duration, preceded each long-duration (30 sec) V_j step. Upward and downward currents are elicited by negative and positive V_j 's, respectively. Current calibration, 200 nA. Time calibration, 5 sec. (C) Graph of steady-state G_j as a function of V_j in HeLa-Cx36 cell pairs. Conductance is normalized as described in A. Solid lines are fits of the experimental data with the Boltzmann equation (see Results for details). (D and E) Examples of chemical gating of Cx36 homotypic junctions expressed in transfected HeLa cells. Junctional current, I_j, was measured by applying repeated negative pulses (V_j) of 20 mV to one cell of a pair. Holding potentials of −20 mV were maintained for both cells in each case. (D) Application of arachidonic acid (see horizontal bar) to a cell pair in which g_j≈ 0.8 nS produced full uncoupling. (E) Application of 100% CO₂ (horizontal bar) to a cell pair with g_j ≈ 1 nS produced full uncoupling within ∼1 min.