Junctional Membrane Uncoupling : Permeability transformations at a cell membrane junction

The permeability of the membrane surfaces where cells are in contact (junctional membranes) in Chironomus salivary glands depends on Ca⁺⁺ and Mg⁺⁺. When the concentration of these ions at the junctional membranes is raised sufficiently, these normally highly permeable membranes seal off; their permeability falls one to three orders, as they approach the nonjunctional membranes in conductance. This permeability transformation is achieved in three ways: (a) by iontophoresis of Ca⁺⁺ into the cell; (b) by entry of Ca⁺⁺ and/or Mg⁺⁺ from the extracellular fluid into the cell through leaks in the cell surface membrane (e.g., injury); or (c) by entry of these ions through leaks arising, probably primarily in the perijunctional insulation, due to trypsin digestion, anisotonicity, alkalinity, or chelation. Ca⁺⁺ and Mg⁺⁺ appear to have three roles in the junctional coupling processes: (a) in the permeability of the junctional membranes; (b) in the permeability of the perijunctional insulation; and (c) a role long known— in the mechanical stability of the cell junction. The two latter roles may well be closely interdependent, but the first is clearly independent of the others.