Junctional membrane permeability : Effects of divalent cations

Junctional membranes ofChironomus salivary gland cells were exposed to test media of varying divalent cation concentration through a hole (estimated diameter ∼10 μ) in a cell's nonjunctional surface membrane. Junctional conductance is markedly depressed by Ca(++), Mg(++), Sr(++), Ba(++) and Mn(++). The order of potency is Ca(++)>Mg(++)>Sr(++)>Ba(++); the minimal effective concentration for Ca is 4 to 8×10(-5) M. Tests with Ca(++) show that, at least, this ion also depresses junctional permeability to fluorescein (mol. wt. 330). The permeability depression is confined to the junctional membranes to which (exogenous) Ca(++) has direct access via the hole. The permeability change produced by Ca(++) is apparently fast enough to limit transjunctional flux of this ion. The depression is reversed by repolarization of the nonjunctional membrane with inward current when the junctional membrane is exposed to divalent cation-free medium, but not when it is exposed to medium containing 10(-3) M Ca.Perforation of the nonjunctional membrane in divalent cation-free medium leads to transient depression of junctional permeability when the membrane hole is large enough to cause nearly complete cell depolarization. This depression can be prevented by clamping the membrane potential with inward current. Smaller holes (estimated diameter ∼2 μ) seal in the presence of divalent cations; the ion diffusion barrier is restored within 14 to 30 min of divalent cation application.