title: Permeability of a cell junction and the local cytoplasmic free ionized calcium concentration: a study with aequorin.
abstract: 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 hte 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 in 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 X537A or A23187 produces diffuse elevation of [Ca2+]i associated with depression of junctional 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.
mesh_terms: D000331:Aequorin; D000818:Animals; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D002463:Cell Membrane Permeability / Q000187:drug effects; D002712:Chlorides / Q000494:pharmacology; D003486:Cyanides / Q000494:pharmacology; D004140:Dinitrophenols / Q000494:pharmacology; D004175:Diptera; D004553:Electric Conductivity; D007365:Intercellular Junctions / Q000378:metabolism*; D007832:Lasalocid / Q000494:pharmacology; D008094:Lithium / Q000494:pharmacology; D008564:Membrane Potentials; D011188:Potassium / Q000494:pharmacology; D012469:Salivary Glands / Q000378:metabolism
chemical_list: D002712:Chlorides; D003486:Cyanides; D004140:Dinitrophenols; D000001:Calcimycin; D000331:Aequorin; D008094:Lithium; D011188:Potassium; D002118:Calcium; D007832:Lasalocid