Permeability of a cell junction and the local cytoplasmic free ionized calcium concentration: A study with aequorin

Summary A technique is devised to determine the spatial distribution of the free ionized cytoplasmic calcium concentration ([Ca²⁺]_i) inside a cell:Chironomus salivary gland cells are loaded with aequorin, and the Ca²⁺-dependent light emission of the aequorin is scanned with an image-intensifier/television system. With this technique, the [Ca²⁺]_i is determined simultaneously with junctional electrical coupling when Ca²⁺ is microinjected into the cells, or when the cells are exposed to metabolic inhibitors, Ca-transporting ionophores, or Ca-free medium. Ca microinjections elevating the [Ca²⁺]_i the junctional locale produce depression of junctional membrane conductance. When the [Ca²⁺]_i elevation is confined to the vicinity of one cell junction, the conductance of that junction alone is depressed; other junctions of the same cell are not affected. The depression sets in as the [Ca²⁺]_i rises in the junctional locale, and reverses after the [Ca²⁺]_i falls to baseline. When the [Ca²⁺]_i elevation is diffuse throughout the cell, the conductances of all junctions of the cell are depressed. The Ca injections produce no detectable [Ca²⁺]_i elevations in cells adjacent to the injected one; the Ca-induced change in junctional membrane permeability seems fast enough to block appreciable transjunctional flow of Ca²⁺. Control injections of Cl^– or K⁺ do not affect junctional conductance. The Ca injections that elevate [Ca²⁺]_i sufficiently to depress junctional conductance also produce under the usual conditions an increase in nonjunctional membrane conductance and, hence, depolarization. But injections that elevate [Ca²⁺]_i at the junction while largely avoiding nonjunctional membrane cause depression of junctional conductance with little or no depolarization. Moreover, elevations of [Ca²⁺]_i in cells clamped near resting potential produce the depression, too. On the other hand, complete depolarization in K medium does not produce the depression, unless accompanied by [Ca²⁺]_i elevation. Thus, the depolarization is neither necessary nor sufficient for depression of junctional conductance. Treatment with cyanide, dinitrophenol and ionophores X537 A or A23187 produces diffuse elevation of [Ca²⁺]_i associated with depression of nunctional conductance. Prolonged exposure to Ca-free medium leads to fluctuation in [Ca²⁺]_i where rise and fall of [Ca²⁺]_i correlate respectively with fall and rise in junctional conductance.