Calcium ion produces graded changes in permeability of membrane channels in cell junction

Abstract

THE permeability of the membrane channels in Chironomus salivary gland cell junction depends on the cytoplasmic Ca²⁺ concentration ([Ca²⁺]_i) (refs 1, 2). At the normal [Ca²⁺]_i, < 10⁻⁷ M, the channels are highly permeable to a wide range of molecular sizes; the upper size limit for peptide molecules is about 1,200–1,900 molecular weight (MW) (refs 3,4). When the [Ca²⁺]_i is elevated above 5×10⁻⁵ M in the junctional locale, the permeability falls drastically for all molecular species including the small inorganic ions^2,5,6. With elevations ranging up to about 5×10⁻⁵ M the channel permeability is reduced for the 330-MW fluorescein molecule with little or no detectable reduction of electrical coupling⁷. This suggested the interesting possibility that in this range the permeability change may be selective, that is, the permeability may fall for the larger molecular species but not for the small inorganic molecules carrying the electrical current. An alternative—that a fraction of the junctional channels closed unselectively (that is, completely)—could not be excluded, however, because the sensitivities of the methods for discriminating changes in the junctional permeabilities of the large and small molecular species were not equivalent; the measurements of electrical coupling, while quite sensitive to changes in junctional permeability in the low range of electrical coupling, are relatively insensitive at high levels of coupling⁷. Here we investigate this point by testing junctional permeability with fluorescent molecules of various sizes. In each test we use a pair of molecular species and compare their transit times during simultaneous diffusion in the same direction across the junction. If and only if the non-selective mechanism governs the Ca²⁺-mediated permeability reduction, the transit for both species should be equally retarded, irrespective of size or structural differences between the probe molecules. We show that, at low [Ca²⁺]_i elevation, the junctional permeability reduction is indeed selective and that the molecular size limit for junctional channel permeation changes in a graded manner.