Cell-to-cell channels with two independently regulated gates in series: analysis of junctional conductance modulation by membrane potential, calcium, and pH.
We study cell-to-cell channels, in cell pairs isolated from Chironomus salivary gland, by investigating the dependence of junctional conductance (gj) on membrane potentials (E1, E2), on Ca2+, and on H+, and we explore the interrelations among these dependencies; we use two separate voltage clamps to set the membrane potentials and to measure gj. We find gj to depend on membrane potentials whether or not a transjunctional potential is present. The pattern of gj dependence on membrane potentials suggests that each channel has two closure mechanisms (gates) in series. These gates pertain, respectively, to the two cell faces of the junction. By treating the steady-state gj as the resultant of two simultaneous but independent voltage-sensitive open/closed equilibria, one within each population of gates (i.e., one on either face of the junction), we develop a model to account for the steady-state gj vs. E relationship. Elevation of cytosolic Ca2+ or H+ at fixed E lowers gj, but at moderate concentrations of these ions this effect can be completely reversed by clamping to more negative E. Overall, the effect of a change in pCai or pHi takes the form of a parallel shift of the gj vs. E curve along the E axis, without change in slope. We conclude (1) that the patency of a cell-to-cell channel is determined by the states of patency of its two gates; (2) that the patency of the gates depends on membrane potentials (not on transjunctional potential), on pCai, and on pHi; (3) that pCai and pHi determine the position of the gj vs. E curve on the E axis; and (4) that neither Ca2+ nor H+ at moderate concentrations alters the voltage sensitivity of gj.