Ca2+-induced down-regulation of Na+ channels in toad bladder epithelium.
Regulation of epithelial Na+ channels was investigated by measuring the amiloride-blockable 22Na+ fluxes in apical membrane vesicles, derived from cells exposed to various treatments. Maximal amiloride-blockable 22Na+ uptake into vesicles was obtained if the cells were preincubated at 25 degrees C in a Ca2+-free [ethylenebis(oxyethylenenitrilo)]tetraacetic acid (EGTA) solution. Including 10(-5) M Ca2+ in the cell incubating medium blocked nearly all of the amiloride-sensitive flux in vesicles, even though the Ca2+ was removed before homogenization of the cells. This Ca2+-dependent inhibition of Na+ channels could be induced in whole cells only; incubating cell homogenates with Ca2+ had no effect on the transport in vesicles. The dose-response relationships of this effect were measured by equilibrating cell aliquots with various Ca2+-EGTA buffers, preparing membrane vesicles (in the absence of Ca2+ ions), and assaying them for amiloride-sensitive Na+ permeability. It was found that the Ca2+ blockage is highly cooperative (Hill coefficient of nearly 4) and is characterized by an inhibition constant which varies between 6.4 X 10(-8) to 8.15 X 10(-6)M Ca2+. Thus, it is likely that the above process is involved in the physiological control of Na+ transport. The Ca2+-dependent transport changes were not affected by the calmodulin inhibitor trifluoperasine, vanadate (VO3-), phorbol ester, colchicine, cytochalasin B, 3-deazaadenosine, and 8-bromo-cAMP. Vanadyl (VO2+) ions, on the other hand, produced a "Ca2+-like" inhibition of transport.
Garty
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Asher
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D015124:8-Bromo Cyclic Adenosine Monophosphate / Q000494:pharmacology; D000584:Amiloride / Q000494:pharmacology; D000818:Animals; D001692:Biological Transport / Q000187:drug effects; D002024:Bufo marinus; D002118:Calcium / Q000494:pharmacology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D004533:Egtazic Acid / Q000494:pharmacology; D004848:Epithelium / Q000378:metabolism; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism; D012413:Rubidium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism*; D001743:Urinary Bladder / Q000378:metabolism; D014638:Vanadates; D014639:Vanadium / Q000494:pharmacology
D007473:Ion Channels; D014639:Vanadium; D015124:8-Bromo Cyclic Adenosine Monophosphate; D014638:Vanadates; D004533:Egtazic Acid; D000584:Amiloride; D012964:Sodium; D012413:Rubidium; D011188:Potassium; D002118:Calcium