Cell-to-cell channels with two independently regulated gates in series: Analysis of junctional conductance modulation by membrane potential, calcium, and pH

Summary

We study cell-to-cell channels, in cell pairs isolated fromChironomus salivary gland, by investigating the dependence of junctional conductance (g _j) on membrane potentials (E ₁,E ₂), 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 measureg _j. We findg _j to depend on membrane potentials whether or not a transjunctional potential is present. The pattern ofg _j 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-stateg _j 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-stateg _j vs.E relationship. Elevation of cytosolic Ca²⁺ or H⁺ at fixedE lowersg _j, but at moderate concentrations of these ions this effect can be completely reversed by clamping to more negativeE. Overall, the effect of a change inpCa _i or pH _i takes the form of a parallel shift of theg _j vs. E curve along theE 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), onpCa _i , and on pH _i ; (3) thatpCa _i and pH _i determine the position of theg _j vs. E curve on theE axis; and (4) that neither Ca²⁺ nor H⁺ at moderate concentrations alters the volrage sensitivity ofg _j.